AC-motor behaviour 3

[FerrariF]

Dear colleagues,

I'm looking for info on the AC-motor behaviour in abnormal conditions. More specifically what happens if the frequency and/or voltage rise or fall with a constant load?

As I am no electrical engineer and I have to explain this it would be use to find some information on this subject.

Who can help me?

thanks a lot!!

Frits

 

[jraef]

To start with, every AC motor is designed around a specific voltage and frequency, so they all have an inherent "Volts per Hertz Ration" (V/Hz). As long as the V/Hz ratio is maintained, the torque output of the motor remains constant. Motors are designed to provide a specific amount of torque at the design speed, the product of which is expressed as HP or kW.

If frequency AND voltage rise and fall together, maintaining that same V/Hz ratio, the motor speed just changes with it. Because HP or kW is based on speed and torque, the HP output rises and falls along with it. But there are other limits as to how fast or slow any particular motor design can go, i.e. cooling issues and bearing ratings etc.

Frequency rises above rated without a change in voltage, the motor enters into "constant HP" operation where the torque will decrease as speed increases. The relationship is non-linear.

Frequency drops without a change in voltage, the motor saturates and overheats. Conversely, the same happens if voltage rises without a change in frequency.

Frequency drops without a change in voltage, the motor is under excited. Slip increases, motor overloads. The same happens if voltage drops without a change in frequency. Torque is lost at a rate of the voltage drop percentage squared, so if voltage drops to 80% of design, the torque drops to 64% of nominal.

And finally if you LOWER the voltage and INCREASE the frequency, the magic smoke comes out very quickly!

 

[FerrariF]

Thnx for the info. Do you also have some graphs? Because I need to explain this to some students and a graph sais more then 1000 words according to a chinese telling ;-)

thnx
Frits

 

[trist]

I performed a quick search, and you could probably use Table 2 on this page to see the consequences of a modest over- or under-voltage:

http://www.eere.energy.gov/industry/bestpractices/energymatters/articles.cfm/article_id=7

(You'll problably be able to perform better searches than I did)

Best regards

 

[CJCPE]

Here is the Cowern Papers explanation including a widely published graph.

http://www.motorsanddrives.com/cowern/motorterms12.html