VFDs and Slip-Ring Induction Motors

Yes to operate with a VFD, but you will have to short out the rotor circuit on the motor. The motor will then operate as a standard squirrel cage motor driven by a vfd.

I will not recommend driving it with a soft starter. Chances are that the insulation of the motor will not be able to withstand the stresses placed on the winding by the soft starter during start up. These stresses will be more severe if the load requires a high starting torque motor.

Remember a slipring motor was initially installed in the application, because of the higher starting torque required (slipring motor = higher starting torque with lower starting current compared to a squirrel cage motor). With a soft starter the starting torque reduces with the reduction in voltage during start up (see other threads in this forum for this explanation). The motor driven by a soft starter might not accelerate the load, resulting in a trip during start up.

Driving the motor with a vfd (by doing the correct sizing of the vfd), you will have higher starting torque with lower currents (same as operating the motor as a standard slipring motor). With the lower starting current you will not have any additional stresses on the motor winding.

The only problem which may arrise by supplying the motor with a vfd is the insulation system of the motor. Depending on the age of the motor, the winding insulation of the motor might not withstand the spikes generated by the vfd. This will lead to a premature winding insulation failure on the motor. I suggest you contact the manufacturer of the motor, to establish if the insulation system of the motor will be able to withstand the supply generated by the vfd.

If it is a new generation motor chances are that the insulation system will be ok, because most manufacturers uses insulation suitable for vfd applicatio as a standard.

I agree with your comments re control via VFD - essentially good control and no problems in my experience. The soft start may have the effect of prolonging stall currents at the varying voltages when starting slowly causing local heating which may damage the windings, the VFD offers 'at speed' control, efficiency and current draw with good torque.

If the motor is relatively close to the VFD then the voltage will appear as normal, so if within 10m of cable I don't think you need worry unduly re insulation breakdown under PWM. If any doubt with a 20-50m run, fit a standard motor choke and low PWM carrier frequency (4kHz or less) should be more than sufficient. The 'spikes' only cause problems when 'reflected' by the motor coinciding with incoming pulses causing volt doubling which can 'puncture' the insulation.