supply voltage effects and motor design

[noel0589]

Hi all,

It is common motor knowledge that motor torque declines by the square of the change in supply voltage.
However, there are motors designed for 230 or 460 volts, and torque is the same at all rpm for 230 or 460Volts, with only a change in drawn motor current.
so in this case supply voltage can drop by half, but torque does not change at all. Is this a function of design?
can someone explain?
Thank you.

 

[aolalde]

To make the story short, keep constant the torque by keeping the volts/turn/hertz constant.
For your example; 230/460 volts motors; a change of connection is required. Series connection at 460 Volts, parallel connection at 230 Volts.
If all coils per phase are series connected you have double of the turns connected to 460 volts, parallel connection has half of those turns connected to 230 volts but with two paths for current which takes care of the increased current at 230 volts to handle the same power. For this case the frequency is constant so the motor spins at the same speed in both connections.
When VFD’s are used; the frequency is changed too but the voltage must drop proportional to frequency, so the volts/turn/hertz ratio is kept constant..