Counter EMF

[ptonsparky]

You guys go first.

I have always wanted to see a Grizzly in the wild, but as in he is over theeeerrrrrre with me just a few steps more this direction.

 

[ATSman]

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.

I agree with the first paragraph. To add to the 2nd paragraph, and the way it was explained to me way back which helps me to understand motor theory: during initial start, electrically, a motor is a transformer with a shorted secondary winding with rated voltage applied to the primary. As the armature starts to turn and accelerate the back EMF of the armature field interacting with the stator field increases then levels off. As the EMF increases, the motor current decreases till full RPM is achieved and the motor draws it's normal "running current" based on the mechanical load it is driving. If the mechanical load on the armature is such that the RPM goes to zero (locked-rotor)
the current increases burning up the insulation and windings just as in the shorted transformer.

 

[LISHAJI]

This appears to happen with a centrifugal device like pump or blower driven by an induction motor. Run a blower with out connecting to any duct or any head(for a layman meaning no load) the motor would trip. Well one would think there is no load on the blower but its just the opposite. I have corrected many pumping application , that was tripping, by throttling the valve. To many throttling appears to be increasing the load but its actually doing just the opposite.

 

[Besoeker]

This appears to happen with a centrifugal device like pump or blower driven by an induction motor.

I have dealt with a lot of projects that used three-phase cage induction motors on centrifugal loads.
It is usual to check them uncoupled (for direction of rotation at least)
Uncoupled, they run at close to synchronous speed. Coupled to the driven load at rated load, it will be a little lower.
But it won't run away under either conditions.

 

[LISHAJI]

But it won't run away under either conditions.

Thats correct. It reminds of what one of our Prof. at my Engineering school use to relate- a run away DC motor which tore itself from its footing and damaged the concrete ceiling above!!

 

[Besoeker]

I have corrected many pumping application , that was tripping, by throttling the valve.

If you have to do that, it is a good candidate for a VSD.
It's one of the prime energy saving applications.

 

[ActionDave]

I have corrected many pumping application , that was tripping, by throttling the valve. To many throttling appears to be increasing the load but its actually doing just the opposite.

I have this discussion concerning irrigation pumps from time to time. It is counterintuitive,at first,
that less pressure (too many sprinkler heads or too big of a pipe) equals more work for the pump.

 

[T.M.Haja Sahib]

Running motor at no-load

Running motor at no-load

It is well known that some induction motors take no-load current of even around 50%.During no-load running, its efficiency is very poor.Under these circumstances,I suspect that its temperature rating may be exceeded due to increased internal losses as a result of poor efficiency,if run at no-load for a long time.

 

[iwire]

During no-load running, its efficiency is very poor.Under these circumstances,I suspect that its temperature rating may be exceeded due to increased internal losses as a result of poor efficiency,if run at no-load for a long time.

A motor running with no load is not efficient but it will not overheat unless there are other issues.

 

[T.M.Haja Sahib]

it will not overheat unless there are other issues.

Like?

 

[iwire]

Like?

Bad bearings

Insulation failure

Over voltage

Under voltage

 

[iwire]

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?

In the specific case of a diesel powered generator it is bad to run them unloaded or underloaded.

This is not bad for the generator it is bad for the engine, an unloaded diesel does not run hot enough to completely burn the fuel and you get a condition called 'wet stacking' which is unburned fuel collecting in the exhaust system.

Besides being a total waste of money, a properly operating genset or motor will most likely run perfectly well without load. Why would you even want to?

Most generators are run once a week to exercise them if you do this weekly testing without transferring the load to the generator the generator runs unloaded. In many cases load banks (think big electric heaters) are installed to load the generator during the weekly exercise so that the diesel engine gets up to ful operating temperature and will not wet stack.


http://www.dieselgeneratorset.us/wetstackingmeansinadieselgeneratorset.htm

 

[Besoeker]

During no-load running, its efficiency is very poor.Under these circumstances,I suspect that its temperature rating may be exceeded due to increased internal losses as a result of poor efficiency,if run at no-load for a long time.

With all the respect due, that's utter nonsense.

Take a cage induction motor.
If you define efficiency as output power/input power then, at no load, it is a bit meaningless.
Losses are reduced running off load.
Think about it.
Windage and friction will be about the same. Maybe very marginally higher because of the very slightly higher speed but the cooling from a shaft mounted fan, the common arrangement, the cooling will more than compensate for those losses.
R_m losses will stay about the same.
The stator resistance (R₁) losses, even working with your 50%, something I don't recall ever seeing, will reduce by a factor of four.
And rotor current will be close to zero. So vanishingly small losses.

A knowledge of motor performance is helpful for this.
But just application of a little common sense and i²R would have got you there quite easily.