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Slip, s {\displaystyle s} , is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed. Thus where f {\displaystyle f} is the frequency of the power supply, p {\displaystyle p} is the number of magnetic poles, and f s {\displaystyle f_{s}} is the synchronous speed of the machine. For f {\displaystyle f} in hertz and n s {\displaystyle n_{s}} synchronous speed in RPM, the formula becomes: In certain smaller single-phase motors, starting is done by means of a copper wire turn around part of a pole; such a pole is referred to as a shaded pole. The current induced in this turn lags behind the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational field energy to start the motor. These motors are typically used in applications such as desk fans and record players, as the required starting torque is low, and the low efficiency is tolerable relative to the reduced cost of the motor and starting method compared to other AC motor designs. Before the development of semiconductor power electronics, it was difficult to vary the frequency, and cage induction motors were mainly used in fixed speed applications. Applications such as electric overhead cranes used DC drives or wound rotor motors (WRIM) with slip rings for rotor circuit connection to variable external resistance allowing considerable range of speed control. However, resistor losses associated with low speed operation of WRIMs is a major cost disadvantage, especially for constant loads. [40] Large slip ring motor drives, termed slip energy recovery systems, some still in use, recover energy from the rotor circuit, rectify it, and return it to the power system using a VFD.
Induction motor improvements flowing from these inventions and innovations were such that a modern 100- horsepower induction motor has the same mounting dimensions as a 7.5-horsepower motor in 1897. [12] Principle [ edit ] 3-phase motor [ edit ] A three-phase power supply provides a rotating magnetic field in an induction motor. Inherent slip – unequal rotation frequency of stator field and the rotor For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field ( n s {\displaystyle n_{s}} ); otherwise the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field induced in the rotor and the rotation rate of the stator's rotating field is called "slip". Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as "asynchronous motors". [31]
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For example, for a four-pole, three-phase motor, p {\displaystyle p} = 4 and n s = 120 f 4 {\displaystyle n_{s}={120f \over 4}} = 1,500RPM (for f {\displaystyle f} = 50Hz) and 1,800RPM (for f {\displaystyle f} = 60Hz) synchronous speed. Regulatory authorities in many countries have implemented legislation to encourage the manufacture and use of higher efficiency electric motors. Some legislation mandates the future use of premium-efficiency induction motors in certain equipment. For more information, see: Premium efficiency. Steinmetz equivalent circuit [ edit ] Paraphrasing from Alger in Knowlton, an induction motor is simply an electrical transformer the magnetic circuit of which is separated by an air gap between the stator winding and the moving rotor winding. [28] The equivalent circuit can accordingly be shown either with equivalent circuit components of respective windings separated by an ideal transformer or with rotor components referred to the stator side as shown in the following circuit and associated equation and parameter definition tables. [39] [46] [51] [52] [53] [54] Steinmetz equivalent circuit Vector control allows independent control of the speed and torque of the motor, making it possible to maintain a constant rotation speed at varying load torque. But vector control is more expensive because of the cost of the sensor (not always) and the requirement for a more powerful controller. [43] Construction [ edit ] Typical winding pattern for a three-phase (U, W, V), four-pole motor. Note the interleaving of the pole windings and the resulting quadrupole field.
Speed control [ edit ] Resistance [ edit ] Typical speed-torque curves for different motor input frequencies as for example used with variable-frequency drives O u t p u t M e c h a n i c a l P o w e r ÷ I n p u t E l e c t r i c a l P o w e r {\displaystyle \eta =OutputMechanicalPower\div InputElectricalPower} Over a motor's normal load range, the torque's slope is approximately linear or proportional to slip because the value of rotor resistance divided by slip, R r ′ / s {\displaystyle R_{r}'/s} , dominates torque in a linear manner. [38] As load increases above rated load, stator and rotor leakage reactance factors gradually become more significant in relation to R r ′ / s {\displaystyle R_{r}'/s} such that torque gradually curves towards breakdown torque. As the load torque increases beyond breakdown torque the motor stalls.An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding. [1] An induction motor therefore needs no electrical connections to the rotor. [a] An induction motor's rotor can be either wound type or squirrel-cage type. In wound rotor motors, rotor circuit connection through slip rings to external resistances allows change of speed-torque characteristics for acceleration control and speed control purposes. The two figures at right and left above each illustrate a 2-pole 3-phase machine consisting of three pole-pairs with each pole set 60° apart.
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