Starting torque of slip ring motor and induction motor with VFD

The torque speed curves are completely different between an induction motor and slip ring motor. You need to understand the differences. If you have selected your slip ring motor to limit starting currents you can short out the slip rings and run your motor but the starting torque is reduced unless you use some features on the VFD to boost starting torque.

If you are using it as torque control on a crane then you have a very different challenge and you will need an expert to de-rate your motor. The torque will be way less and the boost feature that the VFD has raises the voltage to raise the torque. Old motors don't like this as if the insulation is not rated for a VFD, particularly if it is an old motor. Also if you change a 22kW Slip ring motor for a 22kW induction motor it will not work as the starting (breakout torque) may not be sufficient.

To ensure our crane had the required breakout torque, we went to a 45kW motor. This motor is much heavier than the 22kW slip ring motor and resistor bank so the crane had to be re-rated as well.

Short out the slip rings and the motor will run just like a standard induction motor. The next thing to worry about is the load. Slip ring motors are normally used where additional torque is required to overcome the inertia of the load. The rotor is normally connected over a 3 stage resistor bank. It goes through the three, (sometimes 4), stages. Once the inertia of the load is overcome the rotor is shorted and at that time it is running like an induction motor. A VFD will run the motor from Zero to full speed but without the resistor banks, the torque will be reduced. In some instances such as centrifugal pumps where the resistors are used to hold the motor at a slower speed but where the added torque is required you should do some engineering.

If money is tight and you don't have funds to change the motor, you could always use the starting torque of the slip ring motor and bridge it out once moving. Complex and expensive to maintain but it is a solution. Also, if you require an induction motor to produce the same starting torque you will require a larger motor. If the motor is on a crane or platform, you will have to de-rate the load rating of the crane or check the loading of the beam on which it sits. This may not be available to smaller companies.