460V Motor Overvoltage

[Pitt123]

The question came up what would happen if you ran a 460V motor at btwn 490-500V. I know Nema allows for +/- 10% but what would happen physically.

I know you would increase the V/Hz ratio and therefore have a larger flux density. Would the motor increase torque with higher flux density or keep torque the same with just less current to balance out. Also wont increased flux density cause heating damage to rotor?

 

[dkarst]

The short answer is it depends on the motor's design but this graph shows some typical relationships. As the V/Hz increases, eventually saturation occurs and the current increases.

 

[winnie]

The increased voltage would mean increased flux density, increased magnetizing current and increased magnetization losses.

The increased flux density would mean increased _avaliable_ torque at any given slip, but with a fixed load would mean reduced slip and reduced real current.

Depending upon where you were on the load curve, over-all current might increase or decrease, and temperature might go up or down slightly...but if the motor is not overloaded then operating a 460V motor at 500V is probably well within the design capabilities of the machine.

-Jon

 

[Pitt123]

A second part to my question is what happens when a motor is re-wound for a higher hp. I we kept the speed that same but wanted more torque then how would we have to rewind the motor? If flux density = V/(Hz*N) with N being number of turns then would there be less windings or windings removed from the motor to increase flux density and torque? Or could windings just be insulated more?

What if motor is re-wound for lower speed?

Thanks for the responses.

 

[winnie]

If you are talking about _continuous_ output, then there is not much you can do to increase motor horsepower if you keep the speed the same.

Producing more torque means producing more heat in the motor. Since heat is what limits the motor output, rewinding won't let you get around this.

You might be able to substitute higher temperature rated magnet wire, and operate the machine at higher temperature, but this would not give a large increase in continuous torque, and would lead to problems with the balance of system (eg. bearing lubrication).

You might be able to tweak the number of turns to get higher efficiency at an overload operating point, at the cost of lower efficiency elsewhere on the operating curve.

You might be able to tweak the number of turns in order to get increased available torque for transient conditions (eg starting), but at the cost of reduced efficiency, reduced continuous torque, and increased starting current.

In a nut shell, rewinding won't do much to change the continuous torque of the machine, so the only way to get real changes in output power would be to increase speed. Note that if you rewind the machine for fewer poles, then you will increase backiron saturation, and will likely need to reduce flux density and torque.

-Jon