Induction Motor Design and Function

Squirrel Cage and Slip Ring Rotor

The induc­tion motor is also referred to as the asyn­chro­nous motor, because asyn­chro­nous motors oper­ate via the induc­tion prin­ci­ple. The induc­tion motor is short­ened with ASM or IM. In motor oper­a­tion, the rotor of an induc­tion motor runs slow­er than the mag­net­ic rotat­ing field of the sta­tor, i.e. asyn­chro­nous to the sta­tor. The dif­fer­ence between sta­tor speed and rotor speed is also called slip. When the rotor speed is equal to the sta­tor speed, the slip is zero and the induc­tion motor does not pro­vide pos­i­tive torque. In gen­er­a­tor mode, the rotor is rotat­ed faster than the rotat­ing field from the sta­tor. The speed dif­fer­ence pro­duces a neg­a­tive torque which attempts to decel­er­ate the rotor. Induc­tion motors which are oper­at­ed direct­ly on two-phase alter­nat­ing cur­rent or three-phase three-phase cur­rent with­out an invert­er have a low­er effi­cien­cy than syn­chro­nous motors with per­ma­nent mag­nets. How­ev­er, induc­tion motors oper­at­ed with an invert­er can achieve sim­i­lar high efficiencies. 

Induction Motor Design

There are two dif­fer­ent types of induc­tion motors, squir­rel cage and slip ring. The struc­ture of the sta­tor is the same for both and is sim­i­lar to that of a syn­chro­nous motor. To con­duct the mag­net­ic flux in the elec­tric motor, the sta­tor and rotor con­sist of sev­er­al lay­ers of elec­tri­cal sheet, which is usu­al­ly 0.5 mm thick. The thin­ner the elec­tri­cal sheet is made, the small­er the eddy cur­rent loss­es in the elec­tric motor and the high­er its effi­cien­cy. The sta­tor car­ries the wind­ings in which the three-phase cur­rent flows. Usu­al­ly, the sta­tor has three motor phas­es, which can be con­nect­ed in a star or delta con­fig­u­ra­tion. How­ev­er, there are also motors with more and also few­er phas­es, which depends pri­mar­i­ly on the intend­ed use and sup­ply volt­age. The rotor con­tains short-cir­cuit­ed con­duc­tor bars or wind­ings depend­ing on the type of induc­tion motor. 
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Induction Motor with Squirrel Cage Rotor

Asynchronmotor mit Kurzschlussläufer

The rotor of an IM with squir­rel cage rotor con­sists of a cage of bars, which are made of alu­minum or cop­per. The bars are short-cir­cuit­ed at the upper and low­er ends with rings made of the same mate­r­i­al. The squir­rel cage rotor is most com­mon­ly used because it has no slip rings and there­fore has a longer life. In addi­tion, the pro­duc­tion of the rotor is much cheaper.

Induction Motor with Slip Ring Rotor

In a slip ring rotor, the rotor con­sists of wind­ings instead of bars. The wind­ings are not short-cir­cuit­ed in the rotor, but are led to the out­side via slip rings and short-cir­cuit­ed via addi­tion­al resis­tors. The cur­rent flow in the rotor can be con­trolled via the resis­tors out­side the elec­tric motor.

Asynchronmotor mit Schleifringläufer

Induction Motor Function

A rotary mag­net­ic field is gen­er­at­ed by a three-phase cur­rent in the wind­ings of the sta­tor. The rotat­ing mag­net­ic field from the sta­tor also flows through the rotor via the air gap. If there is a speed dif­fer­ence between the rotor speed and the sta­tor speed, a volt­age is induced in the con­duc­tor bars of the rotor by the rotat­ing mag­net­ic field. As the con­duc­tor bars are short-cir­cuit­ed to each oth­er at the low­er and upper ends, the induced volt­age gen­er­ates a cur­rent flow in the bars. The short-cir­cuit cur­rent in the bars in itself gen­er­ates a mag­net­ic field in the rotor, which fol­lows the mag­net­ic field of the sta­tor. In con­trast to syn­chro­nous motors with per­ma­nent mag­nets, the mag­net­ic field of the rotor is not sta­tion­ary, but rotates across the rotor. When the rotor rotates at the same speed as the sta­tor, no more cur­rent is induced in the con­duc­tor bars and, there­fore, no more torque is gen­er­at­ed. In the case of a short cir­cuit of the sta­tor, no more volt­age is induced by the rotor. This makes the induc­tion motor a very safe elec­tric motor, and this is why major car man­u­fac­tur­ers such as Tes­la and Audi, for exam­ple, use the induc­tion motor in their elec­tric vehicles. 

Advantages and Disadvantages

Take a look at the advan­tages and dis­ad­van­tages of induc­tion motors with slip ring rotor and squir­rel cage rotor with and with­out an inverter.

Induction Motor Advantages and Disadvantages

IM Slip Ring Rotor

The main advan­tage of an induc­tion motor with slip rings is the high­er torque in the low­er speed range and the low­er start­ing cur­rent. How­ev­er, the addi­tion­al cost of slip rings and also the pro­duc­tion of the rotor with the wind­ings is sig­nif­i­cant. There­fore, today the slip ring rotor is only used for very large elec­tric motors where an invert­er would be too expensive. 

IM Squirrel Cage without Inverter

The main advan­tage of a squir­rel cage induc­tion motors are the low pro­duc­tion costs for the rotor com­pared to a per­ma­nent mag­net syn­chro­nous motor short PMSM. The induc­tion motor is very robust against high tem­per­a­tures. You don’t have to wor­ry about demag­ne­ti­za­tion of mag­nets, like with a PMSM. The main dis­ad­van­tage is the low start­ing torque and a low effi­cien­cy, if you don’t use an inverter. 

IM Squirrel Cage with Inverter

With an invert­er, the induc­tion motor can achieve sim­i­lar high effi­cien­cy as a PMSM at high speeds. The peak pow­er and peak torque are also very good, since you do not have to wor­ry about demag­ne­ti­za­tion of mag­nets. The con­tin­u­ous pow­er can be a prob­lem if the gen­er­at­ed heat in the rotor can­not be removed properly.