VVVf Drives and Slip Ring Motors

Starting torque will be a concern to use the existing slipring motors with VSD's.

The slipring motors deliver a starting torque of between 2 and 3 times rated torque (depending on the resistance value incorporated in the rotor circuit during start up).

With the motor connected in a squirrel cage configuration you need to size the VSD in order to get the same starting torque in this configuration. Most VSD's can deliver 1.5 X Full load current (that is vsd full load current) for a short period (for starting purposes). If you select a vsd the same size as your motor (i.e same rated full load current) the motor will be supplied by 1.5 x rated current, which equates to 1.5 x rated torque of the motor. This means in this configuration you will only have half the starting torque compared to the original design of the drive train (assuming 3 times starting torque)

This means you need to double the size of the VSD to give 3 x rated current when compared to the motor full load current. This theoretical then equates to 3 times motor torque. The problem with this no motor in standard squirrel cage configuration will be able to produce more than 2 times rated torque (I have not seen one). When connecting the slipring in squirrel cage configuration the motor has the characteristics of a squirrel cage machine and therefore will not be able to produce more than 2 times starting torque and you may not move larger loads with the crane (what the crane is designed for).

The second concern is the motor insulation system. The slipring motor is not designed for the spikes generated by the VSD and the insulation will definitely breakdown very quickly. An output choke (inductor) fitted between the VSD and motor will take care of this problem.