Two direct current motors, same voltage...

One has stator and rotor windings, delivers X torque at Y rpm.
Another with permanent magnets instead of stator windings, delivering same X torque at same Y rpm.

Will the one with permanent magnets stator consume less current than the wound stator one ?

Same torque equation --> T=Kt1*I1*phi1=Kt2*I2*Phi2
Same speed equation --> E1/(Kt1*phi1)=E2/(Kt2*phi2)

This results in I1=I2*E2/E1, so as you can see, generally not, because the back EMF might not be the same. It depends on the construction of the machine (nº of poles etc..).
Even at same speed and same flux, 2 machines will have different EMF unless they have same Ke (or Kt constant).

Depends on the motors efficiency, that is quality of bearings, windings, internal characteristics of iron in rotor, motor power rating - and probably other factors. All those will make those two different motors.

And also: assuming motors are pwm controlled - if one motor indeed get delivered greater voltage (but identical current input for that specific motor load) to maintain rpm/torque - would that still be considering yes to your question?

Thanks gentlemen.
Perhaps I should rephrase to simplify the question :

A DC motor with separately powered windings for stator and rotor. The stator consumes 'W' electrical power to produce the magnetic field.
Replacing the stator winding with equivalent permanent magnetic field, will the altered motor consume 'W' less electrical power to yield the same mechanical power as before ?

Please forgive my poor wording to express the question.

Externet said:

A DC motor with separately powered windings for stator and rotor. The stator consumes 'W' electrical power to produce the magnetic field.
Replacing the stator winding with equivalent permanent magnetic field, will the altered motor consume 'W' less electrical power to yield the same mechanical power as before ?

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I1 and I2 in post #2 are currents through the rotor, so those expressions are valid for any connection of DC motors (regardless of the kind of excitation to the stator).

The answer to your question is yes, if and only if the "Kt" constant is the same.

Having used brushed DC motors extensively, I can tell you that the difference in a permanent magnet motor vs. a separately excited shunt field is between 3 to 5%, all other variables remaining the same. Because as you correctly guessed, the field winding does consume power.

But you have some concepts wrong. For brushed DC Motors, the field is always the stator. The rotor is always the armature.

If the permanent magnet is in the stator, this is no longer a plain brushed DC motor. Rather it is a brush-less DC motor or BLDC. This motor does not run on its own, it requires to be driven by an electronic circuit.
The losses of the electronic circuit have to be factored into the overall efficiency calculations.

Thank you CataM.

"The answer to your question is yes, if and only if the "Kt" constant is the same. " OK.

schmitt trigger said:

...Because as you correctly guessed, the field winding does consume power.

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Was not a guess, it is a fact; the stator field does consume electrical power. A magnet does not.

schmitt trigger said:

But you have some concepts wrong. For brushed DC Motors, the field is always the stator.

Click to expand...

I did not state otherwise.

schmitt trigger;1597433 If the permanent magnet is in the stator said:

This motor does not run on its own[/B], it requires to be driven by an electronic circuit.
The losses of the electronic circuit have to be factored into the overall efficiency calculations.

Click to expand...

An older outboard starter motor has a stator winding; a newer model replacement motor has permanent magnets. Both are still plain brushed motors, none a BLDC, and run on its own. There is no electronic circuit to drive any other than a battery.

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Now, another thing difficult for me to express in proper words : A permanent magnet instead of a field winding saves the power consumed by the winding.
The lesser electrical power consumption raises the efficiency of the motor; efficiency as a ratio of mechanical power exerted to electrical power inputted.

Is it wrong to say that a magnet contributes with some 'pseudopower' to the motor operation ? (The power now not consumed by the stator field )

If the permanent magnet was weak; would the motor yield a lesser mechanical power ?
If the magnet was stronger, would the mechanical output power become greater for the same electrical power supplied ?

Externet;
I agree that you did not state that the field is the not stator. My apologies, I was distracted while typing the reply..

"An older outboard starter motor has a stator winding; a newer model replacement motor has permanent magnets. Both are still plain brushed motors, none a BLDC, and run on its own. There is no electronic circuit to drive any other than a battery."


Again my apologies, I had meant to write "rotor", same reason as above.

The book: Electric Machinery 5th edition by Fitzgerald, A. E., Kingsley, Charles, Umans, Stephen D., provides an explanation to what happens to the "borrowed" energy in a permanent magnet motor.

Externet said:

If the permanent magnet was weak; would the motor yield a lesser mechanical power ?
If the magnet was stronger, would the mechanical output power become greater for the same electrical power supplied ?

Click to expand...

In post#2, "T" means torque. As you can see, it is proportional to the magnetic flux and to the current through the rotor winding. Take now your own conclusions.