Slip ring induction motor, how it works?

Induction motors have been ruling the industrial world for many decades. In the induction motors used in lift and hoists, you will see a type of rotor called a slip ring rotor, whereas in most of the other applications you will see a simpler, squirrel cage type of rotor. In this article I will explain to you, why are there two different designs of rotor construction for induction motors? and also explain to you what is slip ring induction motors and its working.

Why slip ring induction motors are used?

Your answer is here, normal induction motors, or squirrel cage type motors, produce a very low starting torque, and for some applications, this low starting torque causes a huge problem. It is in these circumstances that slip ring induction motors are used, as they produce high starting torque(refer Fig 1), Let’s see this in detail. Start with the working of squirrel cage induction motor.

Fig 1 : Slip ring rotor and squirrel cage rotor

Working of squirrel cage induction motor

First, let me explain to you how a squirrel cage induction motor works? (refer Fig 2a). You can see in the Fig:2a the three-phase AC supply is connected to the stator winding, and its produces a rotating magnetic field in the air gap between the stator and rotor. You can see this RMF cuts the rotor bars, I have illustrated in Fig below (refer Fig 2b).

Fig 2a : Squirrel cage induction motor
Fig 2b : Rotating magnetic field produced by the stator

Now I am going to explain two laws here. First, according to Faraday's law of electromagnetic induction, an electromotive force is induced in the bars. Because the rotor bars are short-circuited by the end rings, this induced EMF generates a current to flow through the rotor bars. According to Lorentz’s law, when a current carrying conductor is placed in a magnetic field it will experience a force. You can see the force distribution on the different bars at a particular moment in time (refer Fig 3). These collective forces make the rotor turn. This explanation of the way an induction motor works won’t be complete without an understanding of the concept of inductance. Let’s see next: what is an inductance?

Fig 3 : Force distribution on different bars

What is an inductance?

Let me take you deeper about an inductance. You can see in Fig 4, there is a simple circuit combination of a resistor and an inductor in series, which is connected to the AC sinusoidal voltage. Let us connect a phase angle meter to the circuit, to measure the phase difference between the applied voltage and the current. You can see that the current flowing through the circuit is not in phase with the applied voltage. This is because of the presence of inductive reactance in the circuit. The higher the frequency of the electricity, the greater will be the inductive reactance and the phase difference. A higher resistance value reduces this phase difference. That’s it about an inductance.

Fig 4 : Combination of resistor and inductor

What are the common faults that occur in an induction motors?

The main thing is, you have to know what the issues of the induction motor are, when we use it. The exact same thing is also happening in the rotor. The rotor is a combination of resistance and inductive reactance. Due to the same phase lag phenomenon, if the maximum EMF is on one bar, then the maximum current is on another bar, this is what I have illustrated in Fig 5.

Fig 5 : Maximum EMF and maximum current are on different bars

The maximum torque condition

Now, I will explain one interesting fact about an induction motors. An induction motor produces maximum torque when the maximum current is induced on the rotor, is near to the maximum magnetic flux. This fact is clear from the Fig:6. Let us call it the “Maximum torque condition”. Throughout this article you have to keep this fact in mind. As the current induced does not meet the ‘maximum torque condition’, this will definitely reduce the amount of torque produced by the induction motor. This phase difference will be high as the motor starts. We will see the next section below.

Fig 6 : Maximum force is produced when the maximum current is near to the maximum flux area

When starting the motor, the rotor speed is zero. Due to this, the magnetic field cut through the rotor at a very high rate and the frequency of the induced EMF will be high. This leads to the high inductance and phase difference, which causes the very low starting torque of a normal induction motor (refer Fig 7).

Fig 7 : Rotor at stand still

Production of high starting torque with slip ring induction motor

To overcome this problem the slip ring induction motor comes into the picture. The working principles and stator construction of the slip ring induction motor are exactly the same as that of the squirrel cage motor. However, the rotor construction of the slip ring motor is quite interesting. Instead of bars, three windings are used in the motor(refer Fig 8a). This construction of the rotor is aimed at reducing the phase difference. Let me explain how the slip ring rotor does it. For your better understanding instead of the current 24 slots winding let’s use a 12 slot winding(refer Fig 8b and 8c).

Fig 8a : 3 Phase windings
Fig 8b : 24-Slots windings
Fig 8c : 12-Slots windings

Here again, the RMF induces EMF across the terminals of the windings. Let’s join the winding ends in a star connection and again assume that the inductive reactance is zero. The current flow established in the winding as shown in fig below. However, in practice, the current flow will be lagging behind the induced EMF. Here again the ‘the maximum torque condition’ is not met (refer Fig 9).

Fig 9 : Production of maximum torque in ideal and practical condition

Why do we use an external resistance?

In the slip ring induction motor, there is an option to reduce this EMF current phase difference, by use of external resistance. The other ends of the coils are connected to an external resistance via the slip rings (refer Fig 10). We saw in the simple circuit that by increasing the resistance value we can decrease that phase difference. As a slip ring induction motor starts, the external resistance value is increased (refer Fig 11).

Fig 10 : Three phase winding is connected to an external resistance
Fig 11 : Increase in external resistance makes a reduction in phase angle

This reduces the phase difference angle and the current induced approaches the ‘maximum torque condition’. This way slip ring induction motors are able to produce high torque even as they are starting. These graphs(refer Fig 12) clearly show the higher starting torque produced by slip ring motors in comparison to squirrel cage motors. Apart from the high starting torque, it also has some other advantages, and although slip ring induction motors have some disadvantages, they play a very important role in elevators, cranes, hoists and in industrial uses such as printing presses.

Fig 12 : Torque speed characteristics of slip ring and squirrel cage

That’s all about the slip ring induction motor and its working. I hope you have enjoyed reading.


Mayuri Baradkar

This article is written by Mayuri Baradkar, M.E.(Power Systems) electrical engineering. She is a experienced product developer with extensive knowledge of engineering principles, specifications, and standards. Her areas of interest are power system, power electronics, electrical machines. To know more about the author check this link.