Slip Ring or Wound Rotor Motors

The slip ring or wound rotor motor is an induction machine where the rotor comprises a set of coils that are terminated in sliprings to which external impedances can be connected. The stator is the same as is used with a standard squirrel cage motor.

When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the lad will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation is also very poor.

Motor Characteristics.

The Slip Ring motor has two distinctly separate parts, the stator and the rotor. The stator circuit is rated as with a standard squirrel cage motor and the rotor is rated in frame voltage and short circuit current. The frame voltage is the open circuit voltage when the rotor is not rotating and gives a measure of the turns ratio between the rotor and the stator. The short circuit current is the current flowing when the motor is operating at full speed with the slip rings (rotor) shorted and full load is applied to the motor shaft.

Secondary Resistance Starters.

The secondary resistance starter comprises a contactor to switch the stator and a series of resistors that are applied to the rotor circuit and gradually reduced in value as the motor accelerates to full speed. The rotor would normally be shorted out once the motor is at full speed. The resistor values are selected to provide the torque profile required and are sized to dissipate the slip power during start. The secondary resistors can be metalic resistors such as wound resistors, plate resistors or cast resistors, or they can be liquid resistors made up of saline solution or caustic soda or similar, provided there is sufficient thermal mass to absorb the total slip loss during start.

To select the values of the resistors, you need to know the frame voltage and the short circuit current. The maximum torque occurs approximately at the point where the rotor reactance equals the termination resistance. The final stage of the resistance should always be designed for a maximum torque close to full speed to prevent a very large step in current when shorting the final stage of resistance. If a single stage was used and the maximum torque occured at 50% speed, then motor may accelerate to 60% speed, depending on the load. If the rotor was shorted at this speed, the motor would draw a very high current (typically around 1400% FLC) and produce very little torque, and would most probably stall!

High Inertia Loads

Slip ring motors are commonly used on high inertia loads because of their superior start efficiencies and their ability to withstand the inertia of the loads
When a load is started, the full speed kinetic energy of that load is dissipated in the rotor circuit. With a standard cage type induction motor, there only some motors that can be used on high inertia loads. Most will suffer rotor damage due to the power dissipated by the rotor. With the slip ring motor, the secondary resistors can be selected to provide the optimum torque curves and they can be sized to withstand the load energy without failure.
Starting a high inertia load with a standard cage motor would require between 400% and 550% start current for up to 60 seconds. Starting the same machine with a wound rotor motor (slip ring motor) would require around 200% current for around 20 seconds. A much more efficient solution.
Shorting the rings out on a slip ring motor with a high inertia load is not an option as the load energy must be dissipated in the rotor winding during start. This will cause insulation failure in the rotor circuit.

Can I Bridge out the slip rings and use a soft starter on a slip ring motor?

A slip ring motor uses resistors in the rotor circuit to modify the starting characteristics of the slip ring motor. Increasing the resistance in the rotor circuit has two effects:
1. It reduces the start current
2. It increases the slip at which maximum torque occurs.

If the slip ring motor has been employed to provide a very high starting torque across the entire speed range during start, then the slip ring or secondary resistance starter can not be replaced. In this case, the first stage of the resistors would be selected to provide a high torque at 100% slip (zero speed) and a number of stages are then employed, each with reducing resistance to move the Slip point in steps from 100% towards 0%. The effect of this is to provide maximum torque at all speeds and at a reduced start current. (typically 200 - 300%)

Shorting out the slip rings and attempting any form of reduced voltage start in the stator supply, will result in a much reduced start torque at a much higher start current. Effectively, the motor could exhibit a Locked Rotor Current in excess of 1000% and a Locked Rotor Current less than 100%. If we reduce the start current down to say 400%, then the start torque would be less than 100 x (400/1000) x (400/1000) or less than 16%!

If the driven load does not require a high start torque, then the slip ring motor can be set up to emulate a standard cage motor by applying rotor resistance that will cause a full voltage start current of about 550%. A reduced voltage starter can now be applied, and the rings should be shorted out once the machine reaches full speed. If you do not short the rings at full speed, the slip will be higher than ideal and the motro efficiency will be reduced. There will be a high power dissipation in the resistors.

Can I use a slip ring motor on a variable speed drive?

Yes, a slip ring motor will perform well on the output of a variable speed drive.
Short the rotor out, preferably on the rotor itself rather than on the output of the brushgear. There can be problems if the brushgear arc while in circuit.
A motor operating on the output of a variable speed drive does not operate under high slip conditions so the high slip characteristics of the slipring motor with a shorted rotor are not relevent.

Calculating Secondary Resistors.

There are two parts in determining the size of secondary resistors for a slip reing motor starter. The resistnce of the resistors is selected to provide the correct torque characteristics. The dissipation of the resistors is selected for the inertia of the load and short term overload requirements.
The resistance can be determined using our Electrical Calculations software.
1. Determine the rotor voltage (often referred to as the frame voltage). This would normally be on the nameplate of the motor.
2. Determine the short circuit current of the rotor from the motor nameplate.
3. Decide on the number of steps required. More steps will result in a higher average torque.
4. Use the Electrical Calculations software to calculate the resistances required.
5. Determine the full speed kinetic energy of the load. This will determine the total power dissipation of the starter during start.