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Hasibul Jamil
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Join Date: Jan 2015
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Shaded Pole Motor Speed

01/22/2015 7:23 AM

I have a shaded pole motor rated as 240 V/50 Hz,50 mA . Its full load speed is 2750 rpm.

I need to increase its rotational speed, so i plugged it to 240 V/ 60 Hz . As the v/hz ratio is decreasing, i thought the operation is safe and actually it was. But problem is at 60 Hz its speed become less than 50 Hz (2400 RPM). I don't understand the reason for this type of motor behavior. Can anybody explains this to me?? Thanks in advance.
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Rixter
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#1

Re: Shaded pole motor speed.

01/22/2015 7:37 AM

Everything else being equal, it should run faster (unloaded) on 60 Hz. The torque would be less because the current draw is less. If there is a load on the motor, it might be that the lower torque is causing it to run slower.

Any mechanical load has a torque vs speed curve. The motor also has a torque vs speed curve. The speed of the motor with a load is where motor torque equals load torque.
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redfred
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#2
In reply to #1

Re: Shaded pole motor speed.

01/22/2015 8:17 AM

Sounds good to me. I expect everything else will not be equal. I expect the current will not be the same. I suspect an oscilloscope with either a shunt or clamp on ammeter will show that the real power produced will be close to the same because the mechanical load is too big for this motor. I suspect the mechanical load was already creating too much slip at 50 HZ. Increasing only the frequency just created more slip and a very slight decrease in current.

Just to be clear, I'm reinforcing your answer not refuting it.

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Hasibul Jamil
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#11
In reply to #1

Re: Shaded pole motor speed.

01/23/2015 1:40 AM

Thanks a lot buddy... but why should the current will deceases at 60 Hz?? can you please explain ???
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Crabtree
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#12
In reply to #11

Re: Shaded pole motor speed.

01/23/2015 3:33 AM

Because, being based upon coils of wire, the motor is an inductor. The impedance of inductors increases with frequency, so the current reduces, and available torque is proportional to current. If the torque that your load presents to the motor increases with speed, then the motor will slow down to a place where the load torque and the available motor torque are matched. And that's where it will stay.

Basically, you've now got a mismatch between the motor and the load. A gearbox won't help, so you need to change the motor, which is where you should have started.

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Hasibul Jamil
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#13
In reply to #12

Re: Shaded pole motor speed.

01/23/2015 3:42 AM

Thanks again.. ok , i will do what you suggested....
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Fredski
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#3

Re: Shaded pole motor speed.

01/22/2015 8:19 AM

carefully worded homework
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redfred
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#4
In reply to #3

Re: Shaded pole motor speed.

01/22/2015 9:19 AM

Very possibly true. However, a carefully reworded homework problem shows that the student does think. Just not clearly enough, yet. I'll help to lift a veil or two for somebody that is showing effort.

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PWSlack
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#5

Re: Shaded pole motor speed.

01/22/2015 9:48 AM

If it is in overload, as suggested by #1, then its behaviour would suggest that its existing overload protection arrangements are inadequate. If it is run in this way for any period of time, expect it to burn out and need changing for a correctly-rated replacement. In any case, review the protection arrangements and ensure they are appropriate for the motor.

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redfred
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#6
In reply to #5

Re: Shaded pole motor speed.

01/22/2015 11:13 AM

This is a shaded pole motor not a water pump motor. Frequently a shaded pole motor (think fan motor) will not need any protection because the heat produced by a locked rotor condition will not overheat the motor with no forced air motion.

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SolarEagle
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#7

Re: Shaded Pole Motor Speed.

01/22/2015 6:01 PM

It would seem the motor is undersized...try a PSC motor....

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JRaef
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#8
In reply to #7

Re: Shaded Pole Motor Speed.

01/22/2015 11:44 PM

His motor is 50mA, about 8.4W or 0.01HP. I doubt there is a PSC motor made that small, in fact I doubt there is any other type of AC motor made that small...

I'd venture to say that this is a muffin fan on the back of his PC, and he has over clocked his CPU, which is making it over heat. So rather than buy a larger fan, he is trying to push more air with it.

When you increase the speed of a fan like that, the load it expresses on the motor increases at the CUBE of the speed change. So just increasing the speed by 20%, the load that the fan impresses on the motor becomes 1.2 cubed, so 172% of what it was. Given that the wires are likely marginally sized, I'd say that the increased speed is causing the fan motor to overload, draw too much current, which drops the voltage due to the overheated wire, then drops the torque of the motor even further until is almost (or will eventually) stall.

There ain't no free lunch. Need to move more air? Buy a bigger muffin fan.

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SolarEagle
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#9
In reply to #8

Re: Shaded Pole Motor Speed.

01/23/2015 1:10 AM

Yes you are quite right, I didn't read the question properly....maybe something like this...

http://www.newegg.com/Product/Product.aspx?Item=N82E16835553002&cm_sp=Cat-Fans-PC-Cooling_3-_-Pillars-_-COUGAR-CF-V12HP_3

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Hasibul Jamil
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#10
In reply to #8

Re: Shaded Pole Motor Speed.

01/23/2015 1:15 AM

Hell Yeah.... thank you very much...
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Users who posted comments:

Crabtree (1); Fredski (1); Hasibul Jamil (3); JRaef (1); PWSlack (1); redfred (3); Rixter (1); SolarEagle (2)

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