Counter EMF

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JdoubleU

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I have been told that if you do not have a load on a motor that this can damage. Is this because of the counter EMF needed to reduce the current. Does a load on a motor increase counter EMF.
 
I have run hundreds of motors with no external load connected, can't recall ever damaging them for that reason alone.

If you are thinking theory only I suppose it could be possible to have a problem with an unloaded motor, but reality is that the rotor itself is a load and the motor does consume some energy even when no external load is connected. A 1 HP motor will have a rated output of 1HP. The rated input will be 1HP plus what it takes to run the motor itself.
 
What about the case of a genset. I have been told that a genset with no load on it is bad for it. Why is it better on the genset to run it with a load?
 
Jakewhis said:
What about the case of a genset. I have been told that a genset with no load on it is bad for it. Why is it better on the genset to run it with a load?
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For the most part you are hearing old-wives tales.

Gensets are regularly run without loads: no-load testing, cool down periods, and running in standby mode. I am involved in a project where 6MW will be brought on-line to serve loads as small as 45kVA.
 
The problem with gensets running unloaded for long periods of time has to do with building up deposits in the engine, nothing to do with the generator portion other than the only way to put a load on a genset engine is to have a load on the generator. That's why some large standby gensets have "exercisers" that connect it to a load bank so they can simulate a load and prevent that deposit formation. On smaller ones it's usually easier to just run your load from the generator a couple times a year.

The myth on the unloaded motor vs counter emf issue is something I heard a long time ago too. I think it's a misinterpretation of basic motor theory. When a motor is first connected to a power source and it is not turning, the only thing restricting current flow is the wire resistance, which is so low that the initial inrush can be 10-20x the FLC and it that went on for more than a few cycles it would damage the motor. But the instant the motor starts turning, the counter emf begins to impede the flow. So I think someone misinterpreted "not moving" with "unloaded" and fostered that myth.
 
ActionDave said:
Not universal motors that I am aware of, at least not when they are hooked up to AC; but a DC motor can.
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You may be right. I can't remember off the top of my head. I have an application guide somewhere that has something about it but I don't have it handy.
 
ActionDave said:
Not universal motors that I am aware of, at least not when they are hooked up to AC; but a DC motor can.
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A series wound DC motor can.
As can a compound wound one that has lost it's shunt field.
I haven't witness it happen but I have seen the result of one that did.
Bit of a mess.
 
Besoeker said:
A series wound DC motor can.
As can a compound wound one that has lost it's shunt field.
I haven't witness it happen but I have seen the result of one that did.
Bit of a mess.
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I have not either, but I have heard tales of pieces of shrapnel flying and men diving for cover relayed to me by the motor guru in our shop.
 
mivey said:
Here is a neat little presentation on the DC motor runaway:
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=IAU12508
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Nice demonstration!

The one I saw was on a winch drive with a compound wound motor. The supply to the shunt field failed. Slightly more complicated.....the shunt field had two different voltages. One for high torque low speed and the other for low torque, high speed. It was in this latter mode that the shunt field supply failed. Not only was the motor in a bit of a state, there was a tangled mess of wire rope.

It was quite an interesting application, as it happens. It was a tin mining dredger in South East Asia. The dredger floats in a man-made pond* and a bucket elevator digs up the stuff at the bottom of the pond. The dredger moves from side to side on the pond and that's where the winches come into action. There are two forward and two aft on opposite sides of the vessel. They are anchored to the left and right hand banks of the pond. One set, forward and aft, say on the left had side, winches in pulling the vessel towards the left bank. Those on the opposite side which out (render). The bucket elevator thus works across the face of the excavation rather like the operation of a milling machine - just on a little bit bigger scale.

*Calling it a pond doesn't really convey the scale of it or anything else.
 
Besoeker said:
*Calling it a pond doesn't really convey the scale of it or anything else.
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Understood. There are many people here that would say that you live on the other side of the pond, so the word pond doesn't really carry much size implication sometimes.
 
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