Lower current at higher speed?

[abfer]

In a vector controlled drive with speed regulation mode, is it possible that drive draws lower current in high speed than in low speed at steady state and with same load conditions?

 

[alexit]

Yes, motor may draw less current at high speed than low under same (torque) load. Efficiency is not linear across voltage/speed range, there are points where 0.25% change can be detected. If was to be 0.25x large difference I would suspect measurement equipment might be reading in error.

 

[cswilson]

At a fixed field strength (direct current magnitude), torque is proportional to quadrature current, regardless of speed. Above base speed, the direct current magnitude is reduced to weaken the field in order to keep the back EMF down, but now more quadrature current is needed to produce the same amount of torque. (If the direct current is cut in half, twice the quadrature current is required to produce the same amount of torque.)

The total current magnitude is the vector sum of the orthogonal direct and quadrature currents. I suppose that for very low torque levels, the reduction in direct current can more than compensate for the required increase in quadrature current. But given that the direct current below base speed is only about 10-15% of the maximum quadrature current, the torque levels required for this to happen would have to be very low indeed.

Curt Wilson
Delta Tau Data Systems

 

[cswilson]

Alexit -- Remember that we are talking about vector-controlled motors here. The dynamics are very different from running directly off the AC lines.

Curt Wilson
Delta Tau Data Systems

 

[DickDV]

Notice that cwilson says no change at the same torque and alexit says .25% change at the same torque.

I think in the field for all practical purposes, both have said the same thing! No change!

Or am I missing something here?