Electrical power frequency changes

[tc002]

I am working with a processing plant in India. All equipment is designed for 50 hertz but the frequency of incoming power ranges from 46 to 51. What are the effects on motors and capacities and long term effects on motor life?
please respond to tcinc002@aol.com

 

[boomsystems]

Should you have mentioned about voltage variation simultaneously, the subject would have been more clear. However, voltage remaining same, you will be slowly killing the rotating machines and transformer connected to this supply due to overfluxing of most of the equipment. However, in case of 51 Hz supply you will get better mechanical output from the same machines saving on electricity cost if your mech load is of that nature.

 

[Aakhrinaam]

I agree with Boomsystems on the overfluxing aspect but slightly sceptical about a system frequency of 46 Hz.

Usually systems are not allowed to run under 47.5 Hz and even that is very rare.
From my experience operation between 48.5 and 51.5 would be normal in India.
Are you connected to the grid or this is in-house generation ?
How are you measuring frequency ?

 

[allmend]

It is important to know how long the frequency deviates from 50 Hz.

According to European Norm for voltage characteristics (EN 50 160) the frequency of an in-house generation grid should be 50 Hz +/- 2% (49-51 Hz) 95% of time during a week and 50 Hz +/- 15% (42,5-57,5 Hz) 100% of time during a week.

It is important to measure the frequency a whole week and if there was frequency lower than 42,5 Hz, you have to take measures.

 

[xabproject]

dear tc002,
go by this fundamental approach and apply to motor/trf/etc without any fear(as i see you are in Chemical Engineering group):

V (Voltage) = fx (flux) X fr (frequency)

[forget about the constant for the moment]

So,
fx = v/fr

Hence, if fr is decreased with V constant you will see a rise in fx, which leads to
1)higer iron losses and heating,
2)waveform distortions (flat topping) accompanied by generation of harmonics, again leading to higher losses and heating.
3)The decrease in fr itself leads to lower speeds (specific to running equipment), increased slip, increased losses and increased heating;
4)add to it the lowered cooling by the rotor/shaft driven fan.
5)lower speed will result in lowered output in Q and H of pumps, outputs of mills, etc

Gloomy picture, isn't it?
But then if the decrease in fr is accompanied by a decrease in V, to hold fx constant, you have none of these problems, but then torques decrease because they are proportional to V X V.

However, intelligent plant operators, while operating systems in ISLANDED mode, resort to short periods of lower frequency operations since the load imposed on the islanded system generators decreases on lower frequency. You can run longer with limited supply of steam. A good technique, before fuel supplies are restored to normalcy! Some even use it for better coupling of rotor to stator on Syn Motor if rotor current rise is under some influence of a limiter!

Now the effects of increasing the frequency (with / without constant voltage) might be clear to you. But lowering the working flux level is accompanied by poor rotor to stator coupling, tendency to overspeed, inefficient power trf, etc

In order to ensure the system behaves as per the intent of the design, it is necessary that you run the equipment within tolerance limits such as +/- a or b % frequency variation unaccompanied by voltage variations, +/- c or d % Voltage variation unaccompanied by frequency variations, +/- e or f % combined voltage and frequency variation.
a...f can be found by consulting electrical engineering codes/standards.

hope this helps

best regards

 

[tc002]

I appreciate the input, very much. I'm a process technologist and only noticed it when nothing made sense on a mass balance and weigh scales were going nuts. We were operating at less than capacity and I wasn't paying attention to power useage. We have some medium voltage fans that I am most concerned with, 3000 HP.
Power is not consistent, neither the frequency nor the voltage. Typical of the condition. Dependent on the time of day, we are connected to the grid or to captive power. So you can see the effect on life.
Thanks