Ultimate Guide for Slip Ring Induction Motor

by | Jul 4, 2021

Have you ever wondered how a slip ring motor can help solve your design requirements? For some decades now, induction motors have been notable in the industrial world. You will get to know everything you need to know about the slip ring induction motors in this article.

What is a slip ring induction motor?

Slip ring Induction Motor

A slip ring induction motor is also known as a wound-rotor motor. And by definition, it is a type of motor where the external resistance is connected to the rotor windings. And this is done through the slip rings.

It is referred to as an asynchronous motor because of the imbalance of its speed with the synchronous speed. The slip ring induction motor is made of a cylindrical metal collar and a semi-closed hole at the external boundary to hold on to an insulated 3-phase winding circuit.

Why are slip ring induction motors used?

They are used to start the torque of a motor when a high current is needed. When you add these slip rings, the high external resistance reduces the inductive reactance and increases the resistance. When you reduced inductive reactance, there will be a reduction in the phase difference between the EMF induced and the current. And with this, a sufficient unidirectional torque that can develop high starting torque is generated.

It is used for regulating the speed of a rotor or device. And this is due to the other end of the slip ring linked to the variable resistor.

How does Slip ring induction work?

The working of a slip ring induction is identical to that of a typical motor. Only that slip ring has more functions which add to its advantages. The slip-ring induction motor operates on the standard of Faraday’s law of electromagnetic induction. The stator windings a magnetic flux when a stator winding is fascinated with an AC supply. Then the induced EMF generates torque that enables the rotation of the rotor.

The torque that is developed is not unidirectional for the rotation. But, the phase difference between the current and voltage do not fulfill the requirements. To improve the difference in the phase, the external resistance of a vast value is connected with the circuit. And thereby, this results in phase difference between current and voltage and the inductive reactance.

Construction of Slip Ring Induction Motor

The construction of a slip ring induction motor is identical to a squirrel cage motor. But there is a slight variation in its rotor construction. We make use of rotor bars in squirrel-cage motor, but in-ring type makes use of windings that are connected in star. And the windings are linked up to the rings. High resistance is then connected to these rings in series, which then act as an outer circuit.

slip-ring induction motor

Stator:

The construction of a slip ring induction motor is identical to a squirrel cage motor. But there is a slight variation in its rotor construction. We make use of rotor bars in squirrel-cage motor, but in-ring type makes use of windings that are connected in star. And the windings are linked up to the rings. High resistance is then connected to these rings in series, which then act as an outer circuit.

Rotor:

The slip ring induction motors always have a “Phase-Wound” rotor. This rotor type is furnished with a double-layer, 3-phase, and distributed winding consisting of coils used in alternators. The core of this rotor is made of a steel collar that has holes on 3-single phase windings. And these windings are positioned about 120 degrees apart electrically.

As the name “wound-rotor” implies, the rotor is wound with the number of poles in the stator in mind. It is often 3-phase, even if the stator is wound for 2-phase. The interior of these three windings is then starred. And then, the other end of the windings are then connected to 3 insulated slip-rings that are mounted on the rotor shaft. The 3-terminal tips touch these three slip-rings with the aid of carbon brushes held against the rings. In which, this is done with the help of a spring assembly.

These 3-carbon brushes are then connected to a 3-phase externally to start the connected rheostat. The external rheostat and the slip rings make it feasible to add outer friction to the rotor circuit. This process enables them to have an increased resistance when starting and hence, increased starting torque.

The slip rings are short-circuited automatically using a metal collar when running under normal conditions. Then pushes the shaft along, hence, results in the three rings touching each other. Also, to avoid loss of resistance, the brushes are automatically raised from the slip-rings, wear and tear.

Pros and Cons of Slip Ring Induction Motor

The construction of a slip ring induction motor is identical to a squirrel cage motor. But there is a slight variation in its rotor construction. We make use of rotor bars in squirrel-cage motor, but in-ring type makes use of windings that are connected in star. And the windings are linked up to the rings. High resistance is then connected to these rings in series, which then act as an outer circuit.

Pros:

  1. A good level of starting torque to support loads with high inertia.
  2. It has a low starting current due to the effect of the external resistance.
  3. It can take a total load current that is about 6 – 7 times higher
  4. there is the possibility of adding more resistance to regulate the speed.

Cons:

  1. Sophisticated construction
  2. Higher cost of maintenance due to slip rings and brushes compared to other motors.
  3. High rate of copper loss
  4. Low efficiency and low power factor
  5. More costly than 3-phase squirrel cage induction motor

Conclusion

A slip ring motor is a type of induction motor that big power engines uses. It is used in solving problems that the alternative can hardly do. Also, It is an incredible tool with a 3-phase winding on the phase wound rotor. And that is it, everything you need to know about slip rings induction motors. Or better still, all you will need when considering slip ring motors.

Is there any other thing you will like to know about induction motors? We will be looking forward to your response in the comment box.