AC Induction Motor - Voltage and Frequency Variance

[Sparky2003]

I am looking to perform some qualification testing on 15HP AC Induction Motors (3 phase). I need to vary the voltage from 200V up to 460V and the frequency between 50Hz and 60Hz. At the moment I am looking towards a Variac for the voltage and Generator for the frequency. Can anoyone offer some advice and related info on varying ac voltage and frequency.

 

[moturcu]

It is not clear what you meant by qualification testing. However, as a rule of thumb, while you are playing with voltage and frequency simultaneously, you have to keep V/f ratio constant.

If you let this ratio increases, then the motor will be saturated and generate less torque but more heat. As long as you operate below rated speed, you have to keep V/f constant.

For operation above rated speed, you can keep it constant as long as you do not exceed the voltage rating.

Bottom line, more V/F leads more flux and saturation which causes drops at the torque output of the machine. However less V/F leads less flux and less torque which causes poorly utilization of the machine.

 

[Marke]

Hello Sparky2003

With many generators, you can change the frequency and the voltage, so you may not need the variac.
You can also achieve this with some inverter drive systems, but the voltage is not sinusoidal, so this could be an issue for your measurments.
As moturcu suggests, you need to be sure that you do not over flux the motor by running the voltage too high at a given frequency.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[buzzp]

Given the stated frequency and voltage, V/Hz will not be constant. Are you trying to prove operation over the stated voltage and frequency ranges? What are you trying to do exactly? What is the nameplate data on the motor?

 

[jbartos]

Suggestion: Search for a suitable
AC-DC-AC converter. Visit

http://www.thomasregister.com

and type Converters: AC to AC under Product or Service, which will return 42 companies to approach to

 

[GusD]

Hi Sparky

If you don't maintain your volts/hz ratio throughout the
changes you could cause some motor problems.
As you say ,this is only a test,so it might not matter much.If you plan on running it on a more permanenent bases you might have to reconsider your plan.
The old days of VFDs did much the same by changing the Hz without changing the Voltage.Repair shops loved the opportunity .

GusD