Electric Motor Configurations

The motor was probably supplied for Star duty - check the kW rating; connecting it in Delta would require a Delta rating. The reason why motors are Star/Delta connected, is to reduce the starting current (and torque) by starting in Star, these are both increased by a factor of 3 when switched to Delta. Connecting a motor designed for Star connection in Delta will burn out the motor windings - toot suite. Speed in RPM will be same in both modes. 60Hz signals a US design.

Jaymack     

No, this is getting very odd. A 3 phase induction motor can obviously run as star or delta, the windings get exactly the same everything, the only difference is that the voltage applied for operation will have to be changed so that the winding currents are equal for either connection. So a 400 V star-connected motor will operate correctly with 1/root 2 times the voltage, that is 230V in delta. There is no difference whatsoever in the motor design, the heat or winding currents, or mechanical power. Star-delta starting is a red herring, the motor must be rated for 400V delta operation, the star starting just reduces the start current substantially.

Running motors from VFDs can have some effects on the motor heat, but this can be controlled by setting the current limit to the normal full load current, then even running very slowly will not cause overheating, except that the cooling fan will be less efficient and so the temperature needs to be watched. When the VFD measures the motor parameters at setup it will (should!) make suitable allowances for the supplied waveforms. The peak voltage is not a problem, the changes are made to the VFD output waveforms. There should be no problem running a 60Hz motor with a VFD at 50Hz speed, but this is not advisable without the VFD, there is a risk of excessive current and significant loss of efficiency. A VFD always supplies a square waveform at high frequency, it is the average current and effective frequency supplied that controls the motor speed. Special motors are sold for VFD operation, these have better magnetic material with less hysteresis loss, giving higher overall efficiency, but most other motors work just fine.

"A 3 phase induction motor can obviously run as star or delta" ............ If the motor is wound as such. If however a motor is designed for running in star then the frame size will be smaller and cheaper for the reduced heat loss. The rating plate should state the limitation.

Jaymack

"A 3 phase induction motor can obviously run as star or delta" ............ If the motor is wound as such. If however a motor is designed for running in star then the frame size will be smaller and cheaper for the reduced heat loss. The rating plate should state the limitation.

Jaymack

This is becoming a very interesting discussion!

Jaymack - I'm not understanding - what is the difference in applying 380V to this motor with the windings in STAR or 220V to this same motor with the windings in DELTA?

Either way, the physical coils have the same voltage across them. Please enlighten - what design considerations would make this OK in STAR but not DELTA?

Just be clear - I'm not talking about STAR/DELTA starting.

Im talking about re-configuring into DELTA permanently and applying (sq root 3)* 380 = 220V to said winding.