Out out voltage of 3 phase inverter

davezawadi (David Stone):

I have a VFD at home on my lathe (Colchester Master) which takes in single-phase nominal 230V and outputs 3 phases to the motor, rated at 230V in delta. The actual peak voltage to the coils is about 350V, not too far from that during a mains cycle. The motor is 4kW, and I use it between zero and twice its normal speed, ie.100Hz. The VFD can reverse direction too, and can run between 0 RPM and 3000 RPM. The switching is at a few kHz, a bit screechy at times of big load but not bad. The advantage is that I can go at any speed with a knob, change speed whilst turning, and get very low speeds when needed for tapping, and it will plug into a single phase supply (it will blow a 13A fuse at full load fairly easily). The diagram is not really right the sine wave is current, not the pulse voltage, or though of course the two are related.

Must be an old one! Current model has a 7.5 kW motor.


I cannot recall having to set the voltage or frequency on my puny little Myford. I did, however, need to set the min and max speeds. At very low speeds, there is sod all torque - I can grasp the chuck and stop it. Not too long ago, the machine stalled when parting off and then the VSD died. It may have been coincidence, but it may also have been the result of insufficient torque, which led to overload after stalling.


Another advantage, which David has not mentioned, is that a VSD allows frequent stopping and starting (and reversing if necessary). When turning a metric thread on my imperial lathe, between passes, I have to back off the tool and run the lathe backwards before I stop and go again. If I did not have a VSD, it is possible that the duty cycle would be exceeded and risk damage to the motor.

davezawadi (David Stone):

I have a VFD at home on my lathe (Colchester Master) which takes in single-phase nominal 230V and outputs 3 phases to the motor, rated at 230V in delta. The actual peak voltage to the coils is about 350V, not too far from that during a mains cycle. The motor is 4kW, and I use it between zero and twice its normal speed, ie.100Hz. The VFD can reverse direction too, and can run between 0 RPM and 3000 RPM. The switching is at a few kHz, a bit screechy at times of big load but not bad. The advantage is that I can go at any speed with a knob, change speed whilst turning, and get very low speeds when needed for tapping, and it will plug into a single phase supply (it will blow a 13A fuse at full load fairly easily). The diagram is not really right the sine wave is current, not the pulse voltage, or though of course the two are related.

Must be an old one! Current model has a 7.5 kW motor.


I cannot recall having to set the voltage or frequency on my puny little Myford. I did, however, need to set the min and max speeds. At very low speeds, there is sod all torque - I can grasp the chuck and stop it. Not too long ago, the machine stalled when parting off and then the VSD died. It may have been coincidence, but it may also have been the result of insufficient torque, which led to overload after stalling.


Another advantage, which David has not mentioned, is that a VSD allows frequent stopping and starting (and reversing if necessary). When turning a metric thread on my imperial lathe, between passes, I have to back off the tool and run the lathe backwards before I stop and go again. If I did not have a VSD, it is possible that the duty cycle would be exceeded and risk damage to the motor.