Is VFD system better than slip ring system in sq cage motor?

For high torque requirement, we need to over size both sq cage motor and variable frequency drive. Should we invest this solution? or operate the same system? sq cage Which reason sq cage motor and VFD system better than slip ring system?

Some slip-ring motors can generate a very high starting torque, much higher than torque available at the output of squirrel-cage motors inverter driven. First of all you have to state this maximum value of starting torque. Please check also if - in case of problems at start-up for excessive load - there is the possibility to rotate the crusher using a low-power gearbox with a free-wheel, or if you can reverse shortly the direction. Another possibility is the addition of an hydro-coupling between motor and load. That will really make the motor starting easy. Most hydraulic torque converter can multiply the torque. Also in a dusty environment like a cement plant, a totally enclosed induction motor will certainly last a long time.

I recommend using high efficiency induction motor with variable speed torque control. The prices of permanent magnets have increased ca 300% during last 12 months. Also, the price of special permanent magnet high torque motor will be very high too. The use of squirrel cage induction motor with frequency converter will consume much less energy compared to phase rotor induction motor with resistive slip control, which is quite probably used. The energy saving can be essential.

You have few options here:
1. Slip energy recovery with the current motor but with dual inverter on rotor side, which may not be too expensive as it is lower power on rotor side and no change in motor.
2. Slip ring motor with shorted slip ring and stator side inverter control, which may be expensive due to higher cost of electronics for the higher power rating of stator side.
3. Sq cage motor with stator side inverter control, which may be expensive because of motor replacement and higher cost of electronics for the higher power rating of stator side.