Frequency variations and how loads are affected?

[ulrichj]

I would like to know what affect variations on utility frequency would have on loads.This could be for a variety of loads whether they be quadratic in nature, resistive or inductive motors? Could anybody help or at least point me in a direction to obtain information relating to this?

 

[electricpete]

Resistive load would have no reaction to change in frequency.

Motor load - I have to think about that one. Torque for constant freq is proportional to (V/f)^2 which shifts torque speed curve down. The torque speed curve will also shift to the right corresponding to increase in sync frequency. Speed will be determined by intersection of load Torque-speed curve with the new (higher freq) torque speed curve. Clear as mud right? Maybe someone else can analyse it a little further?

 

[james007]

when frequecny increase induction motor loads give more out put like blower, pumps, compressors etc and you get improved PF in the system . load current also increases . For process industry you get more output when you operate at higher frequency .
regards
James

 

[electricpete]

I looked up how composite power system loads operate and got the same result as james. As frequency increases the motor will usually speed up drawing more real power, but reactive power drawn by motor will decrease.

Note this is not an absolute for every induction motor. The two effecrt described in my first message work in opposite directions (torque speed curve shifts towards higher speeds but lower torques). But for overall modeling of large number of motors it is correct.

 

[ulrichj]

So in effect if a utility decided to relax its frequency limits slightly e.g. from 49.8Hz to 49.5Hz, industrial loads such as induction motors will only be affected in that there will be variations in torque, power and current.
An individual motor is not likely to be affected in a manner detrimental to the motor or its operation?

 

[electricpete]

NEMA standard (US) is that motors should be able to deliver rated horsepower under conditions up to +/- 5% of rated frequency (provided voltage is within 10% of nameplate).

You're only talking about a 1% variation in frequency. Should not be a problem for induction motors.

Certain types of clocks might run slow though.