Magnetising current and efficiency of electric motors

[maypot]

Hi,

Is it correct to say that an electric motor with a 50 % magnetising current ( no-load current ) is less efficient than one with only 30 % magnetising current , bearing in mind that the loading current margin is higher in the second case .

Bob

 

[Skogsgurra]

That is almost a trick question.

The low magnetizing current points to a lower flux and that would point to a higher slip - which means low efficiency.

On the other hand, if there are enough turns to make up for the reduced flux, then the slip is reduced and that part of the losses back to normal.

But, more turns will almost certainly mean more resistive losses and hence a lower efficiency.

So, with same frame, I would guess that the one with the lower magnetizing current has the lower efficiency. At least when the numbers are 30 and 50 percent.

There are surely people doing motor design out there. What do they say?

Gunnar Englund

http://www.gke.org

 

[aolalde]

The efficiency (EFF) of an electric motor is proportional to the total produced losses.
EFF = Power out-put / (Power out-put + Losses)
The sources of loss are: Stator winding i^2R, Rotor winding i^2R, Magnetic core losses, Frictions and windage and Stray losses.

The magnetizing current is only one of the factors generating losses. Besides consider that this current is inductive and then 90° out of phase. When the active current (in phase with the voltage) is high (full load) the influence of this magnetizing current is minimal, when the active current is low (reduced motor load) the effect of this magnetizing current is important.

Look for the final full load current in the nameplate, if that is higher most probably the motor is less efficient.