DC Motors

[gsjhand]

Hello Guys

I have 600 Volts, 3200Amps swichgear supplying six DC motors with combined load of around 1000 HP . My power factor on switchgear remains around 0.45. Is this normal for DC motors?
Similarly, another switchgear of same rating which supplies VFD's of around same load remains at 0.75 P.F. All of our VFDs are 1336 AB vector drives. Dose the load and spead driven by AC and DC drives have any efect on power factor of incoming Ac supply of drives? If yes, is there any way I can calculate supply side power factor?

 

[Skogsgurra]

Yes. It has. And yes, you can.

Speed has a direct influence on power factor in a DC drive. Low speed -> low P.F.

The reason is that the current drawn by the motor is taken directly from the mains (no intermediate store like the VFDs have). So every ampere in the motor is directly reflected in the mains current.

The power to the motor is armature voltage times armature current. And varies as speed varies.

So, for a motor with constant torque over the speed range, the active power (P) is proportional to speed. Zero speed = zero active power. Maximum speed = maximum active power.

Since apparent power taken from mains is U*I and both U and I are constant (constant torque), the apparent power (S) is constant.

The result is that P.F. (defined as P/S) changes from a very low value at low speed to near one at high speed. Which may come as a surprise, since there are no inductive elements in the AC circuit.

The explanation is that firing delay of the thyristors makes current consumption move with regard to the voltage sinewave. There is, in fact, a close relation between delay (firing) angle and cos(phi).

There are also other factors like commutation and losses that complicate the picture somwhat. But the first approximation is as described above.

Gunnar Englund

http://www.gke.org

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