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.
Induction Motor with Squirrel Cage Rotor
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.
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.
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.