Here is a list of three-phase ac motors: 1. Three-Phase Induction Motors 2. Three-Phase Synchronous Motors 3. Synchronous Induction Motors 4. Variable Speed Commutator Type 3-Phase Induction Motor 5. Compensated Induction Motor 6. Three-Phase AC Series Commutator Motor 7. Three-Phase AC Shunt Commutator Motor.
1. Three-Phase Induction Motors:
3-phase induction motors have gained extremely wide application in industry by virtue of the advantages they have over other types of motors. Induction motors are simple in design, rugged in construction and reliable in service, as they have no commutator. Besides this, they have low initial cost, easy operation and simple maintenance, high efficiency and simple control gear for starting and speed control.
The speed-torque characteristics of induction motors are quite important in the selection of an induction motor drive. In addition, the ratio of maximum torque to rated torque, ratio of starting current to rated current, ratio of starting torque to rated torque and the ratio of no-load current to rated current are equally significant. The above characteristics can be conveniently determined by means of the equivalent circuit of the induction motor.
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The energy is transferred from primary (stator) winding to secondary (rotor) winding entirely by induction, therefore, induction motor is essentially a transformer. At standstill, the induction motor is actually a static transformer having its secondary (rotor) winding short circuited.
When the motor operates at slip s, the frequency of rotor currents is .v times the frequency of the stator currents, therefore, the revolving field produced by the rotor currents revolves with respect to rotor itself at speed.
Mechanical speed of the motor, N = Ns (1 – s). The speed of the revolving field of the rotor with respect to stator or space is obtained by combining the rotational speed of rotor field with respect to rotor with mechanical speed of the rotor. Hence speed of rotor revolving field with respect to stationary stator or space = sNs + Ns (1 – s) – Ns.
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Hence from the point of view of stator, the induction motor still can be considered as a static transformer, even when its rotor is rotating, and it is possible to represent the performance of an induction motor by a transformer phasor diagram. Actually the rotor field does not exist alone but combines with the revolving field of stator to produce a resultant field, just as in the transformer, the resultant field is produced by the combination of primary and secondary ampere-turns.
In the transformer the load on secondary is electrical and in an induction motor the load is mechanical which can be replaced by an equivalent electrical load of resistance R’L given by-
Where, R2 is the resistance per phase of rotor and K is the turn-ratio of the secondary (rotor) to primary (stator).
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If R’2, the effective rotor resistance referred to stator, is combined with R’L, the fictitious resistance representing electrically the mechanical power developed in rotor, we have-
The simplified equivalent circuit for an induction motor is shown in Fig. 1.22 where V is applied voltage per phase, R1 and X1 are stator resistance and leakage reactance per phase, R’2 and X’2 are rotor resistance and standstill leakage reactance per phase referred to stator, R0 and X0 are the resistance and inductive reactance per phase of the magnetizing branch, I’2 is the rotor current per phase referred to stator and s is the slip.
In accordance with the equivalent circuit, rotor current referred to stator comes out to be-
Using the expression for losses,
We have the expression for the torque of the induction motor. The expression for the torque is, therefore, given as-
Substituting the value of I’2, from eq. (1.34) in Eq. (1.35), we have-
Typical speed-torque curves are illustrated in Fig. 1.23. The two curves are identical; one being drawn for the forward direction of rotation and the other for the reverse. Extension of motoring curve from the first quadrant into the fourth quadrant indicates that the positive torque is developed even though the motor is rotating in the reverse direction.
The extension of the same curve into the second quadrant indicates that a negative torque is developed when the machine operates at speeds above synchronous one. Thus regeneration is possible if the system to which the motor is connected is capable of supplying required reactive power for excitation.
By differentiating the torque Eq. (1.36) w.r.t. slip s and equating to zero we get slip corresponding to maximum torque, as-
Substituting the value of slip corresponding to maximum torque from Eq. (1.37) in Eq. (1.36), the maximum torque is obtained as-
Plus and minus sign in Eqs. (1.37) and (1.38) corresponds to motoring (or braking) and generating operation respectively. Dividing Eq. (1.36) by Eq. (1.38), we have-
For motor operation, R2’ in the above Eq. (1.39) can be replaced by its value in terms of smaxT from Eq. (1.37). After simplification, we have-
Neglecting stator resistance R1, the above expression becomes simpler and approximate one, which is as-
From above Eq. (1.41) it is obvious that if the maximum torque Tmax and the slip corresponding to maximum torque, smax T are specified, the speed-torque characteristic is almost fixed throughout the entire speed range provided that the motor parameters are constant. The above statement is not true for motors with variable rotor resistance.
Double Cage Rotor Motor:
There are three types of 3- phase induction motors namely plain or squirrel cage induction motor, slip-ring or wound rotor induction motor and double cage rotor induction motor.
The simple squirrel cage induction motor is superior to slip-ring induction motor on account of motor output, frame size, initial, maintenance and repair costs, weight, life, reliability, efficiency and power factor. The only drawback of the squirrel cage induction motor is that its speed cannot be controlled by inserting resistance in the rotor circuit as in case of slip-ring induction motor. Maximum torque developed, which is also called the pull-out torque, by the squirrel cage motor, is more than that developed by slip-ring induction motor.
The double cage rotor motor is designed to provide a high starting torque with a low starting current. The rotor is so designed that the motor operates with the advantages of high- resistance rotor circuit during starting and a low-resistance rotor circuit under running conditions. The starting torque of a double cage rotor motor ranges from 200 to 250 per cent of full-load torque with a starting current of 400 to 600 per cent of full-load value.
The rotor of a double squirrel cage motor carries two squirrel cage windings embedded in two rows of slots. The outer slots contain a high resistance and low leakage reactance winding and the inner slots contain a low resistance and high leakage reactance winding. At starting, the rotor current has the same frequency as the supply current, and the high reactance winding carries very little current, so that the performance approximates to that of high-resistance low-reactance cage alone.
At normal speed, however, the rotor current frequency is quite low so that the reactance of the two rotor cages are insignificant compared with the resistances, and the performance approximates to that of the low-resistance cage. The cost of double squirrel motor is 20 to 30% higher than that of an ordinary squirrel cage motor.
The choice of an induction motor of any type depends on the requirements of starting torque. Squirrel cage induction motor is suitable for constant speed industrial drives of small power where speed control is not required and where starting torque requirements are of medium or low value, such as for printing machinery, flour mills and other shaft drives of small power. Wound rotor or slip-ring induction motors are used for loads requiring severe starting conditions or for load requiring speed control such as for driving line shafts, lifts, pumps, generators, winding machines, cranes, hoists, elevators, compressors, small electric excavators, printing presses, turntables, strokers, large ventilating fans, crushers etc. Double squirrel cage motor is particularly suited where both high starting torque and a small slip on full load are required.
2. Three-Phase Synchronous Motors:
The synchronous motor is a constant speed motor whose speed is fixed by the supply frequency and number of poles and, therefore, is independent of load. Since this motor is not self-starting, therefore, some special arrangement is required to be made for making itself starting. The special arrangement may be such as embedding of squirrel cage winding in the pole faces (as used in case of plain synchronous motor) or by arranging the field winding in the form of an ordinary induction motor rotor winding (as used in case of synchronous-induction motor). In both cases the motor starts as a plain induction motor and when it attains speed near the synchronous speed the dc excitation is switched on and the motor pulls into synchronism.
On no load synchronous motor draws very little current from the supply to meet the internal losses. With fixed excitation the input current increases with the increase in load. After the input current reaches maximum no further increase in load is possible. If the motor is further loaded, the motor will stop.
It can operate under a wide range of power factor both lagging and leading. This enables the motor to perform phase compensation action in addition to driving the load.
Torque developed by the synchronous motor varies directly as the voltage where as in an induction motor it varies as square of the applied voltage, therefore, synchronous motors are best suited to withstand large voltage variations.
These motors can be constructed with wider air gaps than induction motors, which make them better mechanically. These motors usually operate at higher efficiencies (92 to 96% as against 87 to 90% for induction motors). Such motors can be made to take large overloads.
Synchronous motors are rarely used below 40 kW output in the medium speed range because of their much higher initial cost in comparison to that of induction motors. In addition they need a dc excitation source, and the starting and control devices which are usually more expensive. Where low speeds and high kW outputs are involved the induction motors are no long cheaper.
The various classes of services for which synchronous motors are employed may be classified as:
(i) Power factor correction
(ii) Voltage regulation and
(iii) Constant-speed, constant-load drives.
Because of the higher efficiency possible with synchronous motors, they can be employed advantageously for the loads where constant speed is required. Typical applications of high-speed synchronous motors (above 500 rpm) are drives such as fans, blowers, dc generators, line shafts, centrifugal pumps and compressors, reciprocating pumps and compressors, constant speed frequency changers, rubber and paper mills etc.
The fields of applications of low-speed synchronous motors (below 500 rpm) are drives such as reciprocating compressors when started unloaded, dc generators, centrifugal and screw type pumps, vacuum pumps, electroplating generators, line shafts, rubber and band mills, ball and tube mills, chippers, metal rolling mills etc. Flywheel is used for pulsating loads.
High power electronic convertors generating very low frequencies made the operation of synchronous motors at ultra-low speeds. Synchronous motors in very large sizes (as high as 10 MW size) operating at ultra-low speeds are employed to drive crushers, rotary kilns and variable speed ball mills.
3. Synchronous Induction Motors:
The synchronous induction motor, as its name implies, is a machine which is capable of running both as an induction motor and as a synchronous motor, the former being its mode of operation during the starting period, and the latter its mode of operation during normal running.
The secondary winding of a synchronous induction motor consists of a polyphase winding, almost invariably of the three phase type. Since the rotor is uniformly slotted around the whole periphery, there is no saliency, and thus it is a true round rotor machine.
For the synchronous mode of operation it is necessary to supply the rotor winding with direct current. When the rotor windings of a wound rotor induction motor are excited by a direct current, the mmf distribution develops alternate N and S poles in the same way as does a 3-phase current, but the essential difference is that the dc excitation being fixed, the pole axes due to dc excitation are fixed in space and do not shift as in case when the rotor winding carries ac. These fixed rotor poles get magnetically locked with the rotating magnetic field developed by the 3-phase stator windings carrying ac and the motor runs at a constant speed equal to the synchronous speed.
The synchronous induction motor is provided with a large air gap as provided in a plain synchronous motor. A long air gap gives a stiffer machine with a larger overload capacity. The rotor slots are made fewer in number and larger in size.
These machines are provided with a heavy rotor winding in order to have a low slip, which facilitates in pulling it into synchronism. Also in order that the induced emf in the field at the starting may not be too high, the field turns provided are few in number and the excitation voltage is kept low. As the exciter winding serves the purpose of damper winding, so there is no need of providing separate damper winding with synchronous induction motors.
The synchronous induction motor is started as a slip-ring induction motor by inserting resistances in the rotor circuit. When the additional resistances are completely cut out and the motor attains the normal induction motor speed, the rotor is disconnected from the starting resistances and connected to the exciter which is usually mounted on the same shaft. Motor will now be operating as a synchronous motor.
With secondary rheostats for starting, such a motor gives low starting current and high starting torque of the wound rotor induction motor and an improved power factor under load. Hence the synchronous induction motor is essentially a motor, having the induction motor features like high starting torque with low starting current combined with the synchronous motor features like constant speed and power factor control.
The interesting point about this motor is that its peak torque as an induction motor exceeds that as a synchronous motor. If it is momentarily overloaded as a synchronous motor, it may continue to operate as an induction motor-albeit with considerable pulsations of current and torque due to dc until the load falls and it is able to re-synchronise.
However, the extra cost, low efficiency as compared to that of standard types, the small copper space available with a distributed rotor winding, and the necessary compromise between excessive ring voltage at start, or excessive rotor current in operation are sever handicaps.
The synchronous induction motor is rarely built for ratings below 25 kW because of the relatively higher cost of the exciter. Synchronous induction motors are used where a high starting torque and constant speed operation are required. Typical applications of synchronous induction motors include fans, pumps, blowers, generators, air-compressors, ammonia compressors, machinery and line shafting in industrial works such as cement mills, rolling mills, flour mills, paper mills, rubber works and textile mills.
Synchronous induction motors are very often installed alongwith other induction motors, so that they may improve the overall power factor of the system. Their leading kVAR capacity is designed to offset the lagging kVAR demand from the induction motors. They have been made for ratings as high as 30 MW.
4. Variable Speed Commutator Type 3-Phase Induction Motor (Schrage Motor):
This is also an improvement of the plain induction motor designed to provide a variable speed. It also consists of a commutator winding placed on the rotor in addition to primary winding, whose emf is collected and injected into the secondary winding, placed on the stator. It is provided with two sets of brushes.
The speed can be controlled over 3 to 1 range by moving the brush sets relatively to one another in opposite directions. The speed which depends upon the total emf induced in the secondary winding, will be affected by the injected emf. The speed will decrease if the injected emf is in phase opposition to the induced emf and will increase if the injected emf is in phase with the induced emf. Some power factor control can be had by moving both the brush sets together in the same direction. The operating power factor is high.
Speed-torque characteristic curves are shown in Fig. 1.34. The output power available from the machine is directly proportional to the operating speed. Maximum operating torque ranges from 140 to 250% of full-load speed.
The main advantages of the Schrage motor are:
(i) Continuous speed regulation within the required range
(ii) High power factor for high speed settings and
(iii) High efficiency at all speeds except synchronous speed and very much high at lower speeds.
One major drawback of the Schrage motor is that its operating voltage is limited about to 700 V, because the power is to be supplied to the motor through the slip rings. Other disadvantages are its low power factor at low speed settings and its poor commutation and high cost.
Schrage motor is used only where adjustable speed is required as in bakery, machinery, stokers, printing machines, calendars etc.
One important application of the Schrage motor is to drive the hosiery-knitting and ring-spinning machines, where an automatically controlled speed variation of 10 to 30% is often required. They are also used for a wide range of industrial services, driving fans, pumps, conveyors, packaging machinery, paper mills etc. Sizes up to several hundred kW have been built. They are normally rated on a constant torque basis, the kW output varying in direct proportion to the speed.
5. Compensated Induction Motor:
This is an improvement of the plain induction motor designed to operate at approximately unity or slightly leading power factor over the whole range of loads. The usual type of motor is no-lag type motor, which consists of a primary winding placed on the rotor and the secondary winding on the stator. The rotor consists of an additional winding, known as commutator winding, whose emfs are collected by the brushes from the commutator and injected into the secondary winding in such a way that power factor improvement is accomplished.
No-lag motor was developed earlier and Schrage motor is actually modification form of the no-lag motor. In this motor phase of the emf can be varied but not the magnitude.
6. Three-Phase AC Series Commutator Motor:
This machine has a 3-phase stator winding similar to that of an ordinary induction motor and may be wound for high voltage if required. The rotor carries a winding similar to that of a dc machine and its commutator is provided with three brush sets per pair of poles spaced 120 electrical degrees apart. In effect, the stator and rotor windings are in series, so that the working flux is dependent on the current providing series speed characteristic. The speed can be controlled by moving the brush gear—the backward movement (i.e., in a direction opposite to that of rotation of the rotor) of brushgear increases the speed and vice versa.
For any given position of brushes, the speed falls with the increase in load, as in case of a dc series motor. The power factor is high approaching unity for speeds near and above synchronous speeds. The speed range is limited between 40 and 150 per cent of synchronous speed owing to commutation difficulties. Such a motor is useful when large starting torque is required such as in haulage and hoisting work if only ac supply is available.
7. Three-Phase AC Shunt Commutator Motor:
In a 3-phase ac shunt commutator motor, the stator is connected directly to the supply while the rotor brushes are connected to the supply through a transformer. Speed variation is obtained by changing the tapping points on the transformer. It is to be noted here that brushes are fixed in position in the motor of this type. Such a motor provides more or less constant speed at all loads, for a given tapping on the transformer.