Version 18.01 (30-Jan-18)
It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable. Why we do 2 types of earthing on transformer ie: The two types of earthing are Familiar as Equipment earthing and system earthing. In this neutral of the supply source Transformer or Generator should be grounded. With this,in case of unbalanced loading neutral will not be shifted.
We can protect the equipment also. With size of the equipment transformer or alternator and selection of relying system earthing will be further classified into directly earthed,Impedance earthing, resistive NGRs earthing. MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit.
MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Normally it is used where normal current is more than A. Where should the lighting arrestor be placed in distribution lines? Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and near power transformers in sub-stations.
It is an inverse definite minimum time relay. In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates. It is inverse in the sense ,the tripping time will decrease as the magnitude of fault current increase. What are the transformer losses? Copper losses are caused by the resistance of the wire I2R.
Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer. What is the count of hvdc transmission lines in India? At present there are three hvdc transmission lines in india. What is meant by regenerative braking?
When the supply is cutt off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load resitor across the armature winding and the motor should have maintained continuous field supply.
Why is the starting current high in a DC motor? At starting, Eb is zero. What are the advantages of star-delta starter with induction motor? The main advantage of using the star delta starter is reduction of current during the starting of the motor.
Starting current is reduced to times Of current of Direct online starting. Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced. For lighting loads, neutral conductor is must and hence the secondary must be star winding. To minimize the current unbalance in the primary we use delta winding in the primary.
Why in a three pin plug the earth pin is thicker and longer than the other pins? It is longer because the The First to make the connection and Last to disconnnect should be earth Pin.
This assures Safety for the person who uses the electrical instrument. Why series motor cannot be started on no-load? Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc. ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero.
Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB wil trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.
How electrical power is generated by an A. For the generation of elect power we need a prime mover which supplies mechanical power input to the alternator, can be steam turbines,or hydro turbines. When poles of the rotor moves under the armature conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated and is of sinusoidal in nature…due to polarity change of rotor poles i,e N-S-N-S.
Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes? Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC voltage is continuously changing for every half cycle. The current currying of cables will change depending upon the site temperature location of site , type of run it will run through duct, trench, buried etc. Considering this condition actual current currying capacity of cable reduce than current currying capacity which given to cable Catalogue this is called derating.
Why temperature rise is conducted in bus bars and isolators? Bus bars and isolators are rated for continuous power flow, that means they carry heavy currents which rises their temperature. When voltage increases then current also increases then what is the need of over voltage relay and over current relay?
Can we measure over voltage and over current by measuring current only? So, for a better protection of the system. If one lamp connects between two phases it will glow or not? If the voltage between the two phase is equal to the lamp voltage then the lamp will glow.
When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp. At first calculate the electrical current of the load, after that derate the electrical current considering derating factor depending on site condition and laying of cable after choose the cable size from cable catalog considering derating electrical current.
After that measure the length of cable required from supply point of load to load poin. What are HRC fuses and where it is used? Which power plant has high load factor? All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load,we can reduce the cost of generation.
Mention the methods for starting an induction motor? The different methods of starting an induction motor. What is the difference between earth resistance and earth electrode resistance? Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.
The resistance of the electrode has the following components: A the resistance of the metal and that of the connection to it. B the contact resistance of the surrounding earth to the electrode. Explain What is use of lockout relay in ht voltage?
A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself.
This allows the control power for multiple machines to be locked out by the turn of a single key switch. What is the power factor of an alternator at no load? At no load Synchronous Impedance of the alternator is responsible for creating angle difference.
So it should be zero lagging like inductor. Explain How to determine capacitor tolerance codes? In electronic circuits, the capacitor tolerance can be determined by a code that appears on the casing. The code is a letter that often follows a three-digit number such as Z.
The first two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last digit tells you how many zeros to write after the first two digits and these are read as Pico-Farads. The reason that 4ma is chosen instead of 0 mA is for fail safe operation.
For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate psi, or full scale. Due to any problem in instrument i. So if range is mA then we can differentiate whether it is due to broken wire or due to 0 psi. Two bulbs of w and 40w respectively connected in series across a v supply which bulb will glow bright and why?
What is meant by knee point voltage? Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated. What is reverse power relay? A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid.
To stop the flow of power from grid to generator we use reverse power relay. What will happen if DC supply is given on the primary of a transformer? Mainly transformer has high inductance and low resistance. So high electrical current will flow through primary side of the transformer.
So for this reason coil and insulation will burn out. Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions. Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected. Bus bar is nothing but a junction where the power is getting distributed for independent loads.
What are the advantage of free wheeling diode in a Full Wave rectifier? It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. What is the function of interposing current transformer? The main function of an interposing current transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers.
Interposing current transformer are equipped with a wide range of taps that can be selected by the user to achieve the balance required. What are Motor Generator Sets and explain the different ways the motor generator set can be used? Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment.
Motor generator set is also referred to as a genset, or more commonly, a generator. The motor generator set can used in the following different ways:. Define what is power quality meter? Power Quality meters are common in many industrial environment. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance.
In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem, or highspeed communication lines. So we can understand the importance of power measurement through power quality meters.
What is the different between digital phase converter and ordinary phase converter? Digital phase converter are a recent development in phase converter technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components.
This microprocessor, called a digital signal processor DSP , monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions. Explain the operation of variable frequency transformer? A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains.
A variable frequency transformer is a doubly-fed electric machine resembling a vertical shaft hydroelectric generator with a three-phase wound rotor, connected by slip rings to one external ac power circuit. A direct-current torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator.
The variable frequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two networks. What is the main use of rotary phase converter? Rotary phase converter will be converting single phase power into true balanced 3 phase power,so it is often called as single phase to three phase converter.
Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.
Use of switch mode power converter in real-time basis? Switch mode power converter can be used in the following 5 different ways. Which type of oil is used as a transformer oil? Transformer oil, or insulating oil, is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers.
Its functions are to insulate, suppress corona and arcing, and to serve as a coolant. Well into the s, polychlorinated biphenyls PCB s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan.
Starting in the early s, concerns about the toxicity of PCBs have led to their banning in many countries. Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used, where the added expense of a fireresistant liquid offsets additional building cost for a transformer vault.
Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid C7, C8 esters are also becoming increasingly common as alternatives to naphthenic mineral oil.
Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil. If we give A, V on Primary side of 1. Mcb specification are done on maximum current flow in circuit. What is the full form of KVAR? Excitation is applying an external voltage to DC shunt coil in DC motors. In three pin plug 6 Amp. Because Current flow in the conductor is inversely proportional to the conductor diameter.
So if any short circuits occur in the system first high currents bypassed in the Earthling terminal. Difference between megger test equipment and contact resistance meter test instruments?
Megger test equipment used to measure cable electric resistance, conductor continuity, phase identification where as contact resistance meter test instruments used to measure low resistance like relays ,contactors. When we connect the large capacitor bank in series? So in order to bring the voltage at the load terminals within its limits i. What is electrical diversity factor in electrical installations? Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration.
Electrical diversity factor is usually more than one. Why field rheostat is kept in minimum position while armature rheostat at maximum position? In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting torque.
Why computer humming sound occurred in HT transmission line? This computer humming sound is coming due to ionization breakdown of air into charged particles of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss. Explain What is rated speed?
At the time of motor taking normal current rated current the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency. What is different between resistance grounding system and resistance earthing system? Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in an electric equipment in order to protect he equipment in occurrence of fault in the system.
Why should be the frequency 50 Hz 60Hz only why not others like 45, 95 56 or anything , why should we maintain the frequency constant if so why it is only 50 Hz 60Hz? We can have the frequency at any frequency you like, but than you must also make your own motors,high voltage transformers or any other equipment you want to use.
How to determine alternating current frequency? Zero crossings of the sine wave to trigger a monostable pulse generator is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. The more pulses there are per second, the more the energy. The pulses are integrated filtered or averaged to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency.
This method is more suitable than a direct counter, as it can get good accuracy in a second or so. Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv? Transformer Induced voltage equation contains 4.
So always transmission voltage is multiple of Why we use ac system in India why not dc? Secondly it is much easier to change the voltage of AC electricity for transmission and distribution.
When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
Which type of motor is used in trains, what is the rating of supply used explain Working principal? Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is v dc. Battery banks are in connected in series or parallel and why? Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same.
What is inrush current? Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages. In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?
Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current. Why transformer ratings are in kva? Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor. In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor. Define what is difference between fuse and breaker?
Fuses are burned at the time of over current flows in the circuit but breakers are just open not burn at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times. What is the difference between delta-delta, delta-star transformer?
Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena. Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?
As we know that Electrical is having two type of load, Active and Reactive. Traction implies with the electric power for traction system i.
Now a day, magnetic traction is also utilised for bullet trains. Essentially dc motors are utilized for electric traction systems. Pu stands for per unit in power system. What is the use of stepper motor? The motor which work or act on the applied input pulse in it, is called as stepper motor. This stepper motor is under the category of synchronous motor, which often does not fully depend of complete cycle. It likes to works in either direction related to steps. What is a differential amplifier?
The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise which is injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal. It can be defined as the ratio of differential voltage-gain to common made voltage gain. The current, therefore, lags the voltage by a large angle.
The starting winding is designed to have a fairly low inductance and a high resistance. Here the current lags the voltage by a smaller angle. The magnetic fields are out of phase by the same amount. This supplies enough torque to start the motor. When the motor comes up to speed, a speed-controlled switch disconnects the starting winding from the line, and the motor continues to run as an induction motor.
The starting torque is not as great as it is in the capacitor-start. Applications, Advantages and disadvantages: The resistance start motor is used in applications where the starting torque requirement is less than that provided by the capacitor start motor. Apart from the cost, this motor does not offer any major advantage over the capacitor start motor.
A comparison for the popular types of a split phase motors is shown in the below image. Tools and appliances are among the most frequent applications. Here, the rotor is of the squirrel cage type and it works as explained earlier. The power ratings range from one-third to several hundred horsepower in the three-phase motors.
Motors of this type rated one horsepower or larger, cost less and can start heavier loads than their single-phase counterparts. Three phase Squirrel cage Induction motors are classified by application with a design letter which gives an indication of key performance characteristics of the motor, these classification are made by NEMA and IEC.
The main Classifications of Three phase Squirrel cage Induction motors are shown in the below image. The motor has a stator and a rotor but there is no electrical connection between the two and the rotor current is generated by induction.
The rotor winding is connected to a commutator which is in contact with a pair of short-circuited brushes which can be moved to change their angular position relative to an imaginary line drawn through the axis of the stator. The motor can be started, stopped and reversed, and the speed can be varied, simply by changing the angular position of the brushes.
Because of excellent starting and accelerating characteristics, repulsion-induction motors are ideal for: The various types of motors which works under the repulsion principle are: A repulsion-start induction motor is a single phase motor having the same windings as a repulsion motor , When an induction motor drives a hard starting load like a compressor, the high starting torque of the repulsion motor may be put to use.
How large this current is, and how quickly it diminishes depends on the motor type, how its controlled, and what its mechnical load is. A 5HP v 3ph motor have a full load rating of about 15 amps but on initial start it may draw 90 amps or more for the brief period of starting time. The mains connector blades have to be able to handle this brief current surge without arcing. To get this working installers sometimes need to goto larger rated plugs which have better contact mating characteristics - hence the HP rating in some mains plugs.
Mains plug ratings are really based on current handling capability,a nd the horsepower rating is derived from that, based on the current required by a given size motor to start.
In some applications where large current and mechanical shock cause by the motor starting is not allowed, soft starters are used to start the motor softly more expensive alternative to soft starting is using frequncy converter to speed up and control the motor.
In soft starter motor terminal voltage is reduced. This will reduce the starting torque, taken current and mechanical shock. Electronics soft starters is are generally implemented so that thyristors reduce the motor terminal voltage in the device by usingphase control principles. This this can achieved by using a pair of thyristors per motor phase. Alternatively for 3 phase motors in some cases spft starting can be made to work with controlling only two of the phases. Siemens claims in their web pages that from a technical point of view, 2 and 3-phase control is almost identical.
A Controlled soft-start limits the inrush of current, prevents unnecessary excess torque and reduces line disturbances on the power distribution system. Soft-starting AC induction motors on power distribution systems with low voltage, or weak capacity, will substantially reduce nuisance trips of circuit breakers and contactors. Across-the-line industrial motor starters are made in sizes up to those capable of carrying amperes. Contacts of power relays used for motor control must be capable of opening at six to eight times the rated steady current in case a motor should stall.
The running of the motor can be controlled with a contactor or an electrical circuit. As far as standards are concerned, contactors and soft starters are equivalent. Both are switching devices that enable functional switching. Furthermore, a switching device is required that implements a break function. This is usually a circuit-breaker. A motor draws well above run current during start-up, and this needs to be taken into aacount when desiging over current protection for the motor.
A fuse or breaker will tolerate overcurrent of a limited amount and duration only before it trips. Note also that breaker characteristics vary a lot between eg US and European countries. European domestic breakers are normally type B, whereas US breakers are closer to our type D, which are a lot more tolerant of overload than type B.
The typical US breaker tolerates quite a bit of short-term overload, but for long-term, the overload limit is a lot closer. Motor protection circuits for three phase motors usually include also protections against overvoltage, undervoltage and loss of phase.
Loss of one mains power phase easily stops the motor and causes it to heat up very much. Small electrical motors few kW or less are very sensitive to overvoltage, which easily heats up the motor heavily this kind of motor is resistance dominating. For small motors a slight undervoltage does not usually cause major problems as long as the voltage is enough to keep it running normally.
Some very large tens to hundreds of kilowatts of power motors are very sensitive to undervoltage, which heats them heavily. In addition to protection and control, some form of reliable method of isolation between the mains network and the motor, to be able to disconnect the power in case of problems exist or the device needs to be repaired. There are many specific safety related regulations on this kind of switching devices. The disconnecting device can be a detecheable plug like in normal mains equipment or a special dafety switch of some form.
No electronic switching device fulfils this safety requirement, because with them there may also be a measurable and tangible voltage at the motor terminal even if the switching device is switched off and the electronic switchign components generally do not fullfill the strict insulation requirements demanded of safety switches mechanical switches use many millimeters of isolation between contacts.
Sometimes it comes need to measure ho much the electrical motor is loaded. To know this, you need to measure the current taken by the motor.
This can be made in many ways. But in may applications where load varies somewhat don't think ofusing a digital meter, because when the current fluctuates wildly youwon't be able to read the numbers.
This is a case where a pointer instrument scores. It is possible to 'fix' a digital meter to get astable display you need a special instument or knowledge how to modify a commercial meter. General information articles AC Motors - A synchronous and synchronous motors are the two main categories of ac motors. The induction motor is a common form of asynchronous motor and is basically an ac transformer with a rotating secondary. The primary winding stator is connected to the power source and the shorted secondary rotor carries the induced secondary current.
Torque is produced by the action of the rotor secondary currents on the air-gap flux. The synchronous motor differs greatly in design and operational characteristics, and is considered a separate class of motor.
Rate this link 3 phase A. These designs are based on standardizing certain motor characteristics such as starting current, slip, and specified torque points. This article is a brief rundown on NEMA motor types. Essentially, the dimmer works by chopping bits out of the AC waveform. Inductors, like the coils in a motor, dislike having the current through them changed abruptly, and produce large voltage spikes which can cause the triac to self-trigger or burn if not properly taken care of.
This article tells how to do this. Rate this link Generate advanced PWM signals using 8-bit? It is a measure of how effectively the current is being converted into useful work output and more particularly is a good indicator of the effect of the load current on the efficiency of the supply system. A poor power factor due to an inductive load can be improved by the addition of power factor correction, but, a poor power factor due to a distorted current waveform requires an change in equipment design or expensive harmonic filters to gain an appreciable improvement.
They are the most common type of industrial AC electric motor, being rugged and requiring neither a separate DC power source nor slip-rings. They are essentially constant speed devices when energised by a constant frequency AC supply, however electronic speed control is available.
There are several types of single phase AC motors in current production in the U. Rate this link Troubleshooting Adjustable Speed Drives - There are lots of things to look out for when troubleshooting adjustable speed drives, from current and voltage imbalance to single phasing and overvoltages. Here is just a small sampling of what you need to look at, and the test tools you should be using, when troubleshooting ASDs.
Rate this link AC Motor Loads - Table listing electrical operating and full load current at different horse power and voltage levels. Here and Now - Electric motors convert electrical energy into mechanical motion and are broadly classified into two different categories: Within these categories are numerous types, each offering unique abilities that suit them well for specific applications.
Rate this link Controlling three phase motors The actual rotation speed of AC induction motor depends on the motor design. If you want to change that you need to change the frequency of mains voltage you need special variable speed drive electronics for this or you need to change the motor. The rotation direction of a three phase motor is controlled on the order of the phases applied to the motor. Modern motor protection devices usually called electronic thermal overload relays protect against locked rotor conditions, running overload, high ambient temperature, lost phase and low voltage.
Three-phase power regulator - This three-phase linear power regulator can control resistive loads or induction motors. Drive outputs are optoisolated and regulation from zero to full load is via a single potentiometer so interfacing to a PC should be easy, assuming all mains isolation is properly implemented. This results in the voltage rising from zero to peak in only one-tenth of a microsecond.
Unfortunately, there are many motors in existence that do not have sufficient insulation to operate under these conditions. When the rise time is very fast the motor insulation system becomes stressed. Inverter duty motors typically have more phase-to-phase and slot insulation than standard duty motors NEMA design B.
Motion-control-system development poses many challenges for conventional simulation tools. In addition, computers or DSPs control most of these systems in real time, requiring modeling capabilities that encompass both Z-domain transfer functions and software algorithms. Rate this link Using three phase motor on single phase supply Because in most places three phase power is expensive to get, and most homes only have single phase or power. You can drive the circuit with a single CMOS gate.
This circuit is probably the simplest high-voltage inverter you can design. It has served in thousands of three-phase motor drives from 0. Rate this link DC motors The direct current DC motor is one of the first machines devised to convert electrical power into mechanical power.
Permanent magnet PM direct current convert electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly, the other field is produced by an electrical current flowing in the motor windings. These two fields result in a torque which tends to rotate the rotor. As the rotor turns, the current in the windings is commutated to produce a continuous torque output.
The stationary electromagnetic field of the motor can also be wire-wound like the armature called a wound-field motor or can be made up of permanent magnets called a permanent magnet motor. In either style wound-field or permanent magnet the commutator. The commutator is composed of conductive segments called bars , usually made of copper, which represent the termination of individual coils of wire distributed around the armature.
The second half of the mechanical switch is completed by the brushes. These brushes typically remain stationary with the motor's housing but ride or brush on the rotating commutator. As electrical energy is passed through the brushes and consequently through the armature a torsional force is generated as a reaction between the motor's field and the armature causing the motor's armature to turn. As the armature turns, the brushes switch to adjacent bars on the commutator. This switching action transfers the electrical energy to an adjacent winding on the armature which in turn perpetuates the torsional motion of the armature.
Permanent magnet PM motors are propably the most commonly usedDC motors, but there are also some other type of DC motors types which use coils to make the permanent magentic field also. DC motors operate from a direct current power source. Movement of the magnetic field is achieved by switching current between coils within the motor.
This action is called "commutation". Very many DC motors brush-type have built-in commutation, meaning that as the motor rotates, mechanical brushes automatically commutate coils on the rotor. You can use dc-brush motors in a variety of applications. A simple, permanent-magnet dc motor is an essential element in a variety of products, such as toys, servo mechanisms, valve actuators, robots, and automotive electronics.
There are several typical advantages of a PM motor. When compared to AC or wound field DC motors, PM motors are usually physically smaller in overall size and lighter for a given power rating.
Furthermore, since the motor's field, created by the permanent magnet, is constant, the relationship between torque and speed is very linear. A PM motor can provide relatively high torque at low speeds and PM field provides some inherent self-braking when power to the motor is shutoff. There are several disadvanges through, those being mostly being high current during a stall condition and during instantaneous reversal.
Those can damage some motors or be problematic to control circuitry. Furthermore, some magnet materials can be damaged when subjected to excessive heat and some loose field strength if the motor is disassembled. High-volume everyday items, such as hand drills and kitchen appliances, use a dc servomotor known as a universal motor. Those unisversal motors are series-wound DC motors, where the stationary and rotating coils are wires in series. Those motors can work well on both AC and DC power.
Some can literally tear themselves apart if run unloaded. The term gearmotor is used to define a motor that has a gear reduction system or gearbox integrally built into the motor. The gearbox increases the torque generating ability of the motor while simultaneously reducing it's output speed. A brushless motor operates much in the same way as a traditional brush motor. However, as the name implies there are no brushes and no commutator.
The mechanical switching function, implemented by the brush and commutator combination in a brush-type motor, is replaced by electronic switching in a brushless motor. In a typical brushless motor the electromagnetic field, created by permanent magnets, is the rotating member of the motor and is called a rotor. The rotatign magnetic field is generated with a number of electromagnets commutatated with electronics switches typically transistors or FETs in a right order at right speed.
In a brushless motor, the trick becomes to know when to switch the electrical energy in the windings to perpetuate the rotating motion. This is typically accomplished in a brushless-type motor by some feedback means designed to provide an indication of the position of the magnet poles on the rotor relative to the windings.
A hall effect device HED is a commonly used means for providing this positional feedback. In some applications brushless motors are commutated without sensors or with the use of an encoder for positional feedback. A brushless motor is often used when high reliability, long life and high speeds are required. The bearings in a brushless motor usually become the only parts to wear out.
In applications where high speeds are required usually above 30, RPM a brushless motor is considered a better choice because as motor speed increases so does the wear of the brushes on traditional motors.
A brushless motor package motor and commutation controller will usually cost more than a brush-type, yet the cost can often be made up in other advantages. For example, in applications where sophisticated control of the motor's operation is required.
Brushless motors are seen nowadays in very many computer application, they for example rotate normal PC fans, hard disks and disk drives. Sometimes the rotation direction needs to be changed. In normal permanent magnet motors, this rotation is changedby changing the polarity of operating power for example byswitching from negative power supply topositive or by interchangingthe power terminals going to power supply.
This directrion chaning is typicaly implemented using relay or a circuit called an H bridge. There are some typical characteristics on "brush-type" DC motors. When a DC motor is straight to a battery with no controller , it draws a large surge current when connected up.
The surge is caused because the motor, when it is turning, acts as a generator. The generated voltage is directly proportional to the speed of the motor. The current through the motor is controlled by the difference between the battery voltage and the motor's generated voltage otherwise called back EMF.
When the motor is first connected up to the battery with no motor speed controller there is no back EMF. So the current is controlled only by the battery voltage, motor resistance and inductance and the battery leads.
Without any back emf the motor, before it starts to turn, therefore draws the large surge current. When a motor speed controller is used, it varies the voltage fed to the motor. Initially, at zero speed, the controller will feed no voltage to the motor, so no current flows.
As the motor speed controller's output voltage increases, the motor will start to turn. At first the voltage fed to the motor is small, so the current is also small, and as the motor speed controller's voltage rises, so too does the motor's back EMF. The result is that the initial current surge is removed, acceleration is smooth and fully under control. Motor speed control of DC motor is nothing new. A simplest method to control the rotation speed of a DC motor is to control it's driving voltage.
The higher the voltage is,the higher speed the motor tries to reach. In many applicationsa simple votlage regulation would cause lots of power losson control circuit, so a pulse width modulation method PWM is used in many DC motor controlling applications.
In the basic Pulse Width Modulation PWM method, the operating power to the motors is turned on and off to modulate the current to the motor. The ratio of "on" time to "off" time is what determines the speed of the motor. When doing PWM controlling, keep in mind that a motor is a low pass device. The reason is that a motor is mainly a large inductor. It is not capable of passing high frequency energy, and hence will not perform well using high frequencies.
Reasonably low frequencies are required, and then PWM techniques will work. Lower frequencies are generally better than higher frequencies, but PWM stops being effective at too low a frequency. The idea that a lower frequency PWM works better simply reflects that the "on" cycle needs to be pretty wide before the motor will draw any current because of moter inductance. A higher PWM frequency will work fine if you hang a large capacitor across the motor or short the motor out on the "off" cycle e.
Then the current that did flow is dissipated as an inductive kick - probably as heat through the flyback diodes. The capacitor integrates the pulse and provides a longer, but lower, current flow through the motor after the driver is cut off.
There is not inductive kick either, since the current flow isn't being cut off. Knowing the low pass roll-off frequency of the motor helps to determine an optimum frequency for operating PWM. Try testing your motor with a square duty cycle using a variable frequency, and then observe the drop in torque as the frequency is increased. This technique can help determine the roll off point as far as power efficiency is concerned. There are also high frequency PWM systems that work.
The low versus high frequency for PWM of dc motors describes two totally different approaches. At low frequencies you get a mechanical averaging. When the drive is turned off, there is a momentary spike of voltage that the catch diodes clamp but after that transient dies out the motor is left to freewheel. You will typically hear the motor buzzing. But this is usually pretty simple to implement with small motors and low voltages remember the motor must freewheel when the drive is off.
At high frequencies the inductance of the motor armature does the current averaging. This is similar to a switching power supply or a chopper drive. The catch diodes are more critical here because they carry full motor current a substantial amount of time not so if you are driving the motor locked anti-phase. High frequency PWM is quite sensitive to the motor properties inductance. For medium size motors usually 20 kHz or higher frequency works.
There are also applications where you need PWM controllign for two directions. In those cases you usually combine PWM controlling with H bridge. There are many ways to do this In locked anti-phase system the motor is always driven either forward or backwards, but always connected to the power. Because the motor is always being driven, it always has a low impedance across it's terminals.