Producing constant torque in the simple electric motor.

1. May 10, 2012

tasnim rahman

In the simple electric motor, applying a constant current produces a pulsating torque. But if we apply a pulsating current will it produce a constant torque?

2. May 10, 2012

Staff: Mentor

Probably not constant (it depends on the current and the geometry), but at least in the same direction if the frequency is right. This is the concept of motors with alternating current.

3. May 10, 2012

DragonPetter

With a constant current, shouldn't you have a constant torque with only torque ripple that is dependent on how well the commutation timing is done (commutation at the right time, when the magnetic fields are perpendicular)?

4. May 10, 2012

tasnim rahman

What happens if we apply pulsating DC?

Sorry, I don't think I understood you. Applying constant current in a simple electric motor produces pulsating torque, from BANI cos$\theta$.

5. May 10, 2012

DragonPetter

In both AC and DC equations, torque is directly proportional to a DC current value, and so torque is constant at a constant current. I assumed you were talking about torque ripple, but maybe there's something that I'm unaware of though.

6. May 10, 2012

Bob S

Although the torque in a DC motor with a permanent magnet stator is proportional to B·A·N·I·cosθ, the "pulsating" effect of the cosθ factor is negligible due to the large number of commutator pads. The real problem in using a commutator-type dc motor as a torque motor is that under locked rotor situations, the commutator will get overheated and burned. Low frequency pulsing (like from a switching regulator) may help. In a locked rotor situation, a Hall-effect-sensor type BLDC (brushless DC) motor with a permanent magnet stator is best.

7. May 10, 2012

tasnim rahman

Play the DC motor animation on this page? Is this the kind of "torque ripple" you're talking about?

But in the simplest version of DC motors with two commutator pads the pulsating effect exists, right?

8. May 10, 2012

DragonPetter

By definition, that is a torque ripple, but not the kind I was thinking of. In that animation, I believe the current is not constant because the back EMF will be varying, and this is in series with the DC source voltage. I think the animation immediately after the one you refer to supports this. It shows the voltage generated at the terminals being non-constant (even though the rotor appears to be moving at a constant velocity), and this is the back EMF. Someone correct me if I'm wrong.

Last edited: May 10, 2012
9. May 10, 2012

tasnim rahman

The second animation is for DC generators, where constant torque produces pulsating DC.

I think here the back EMF remains constant. In the image attached, the right diagram represents electromagnetic phenomena, where applying a current in the conductor in the direction shown, in the magnetic field (where field lines are pointing into plane of paper), produces a force F and therefore a movement leftwards. Now if apply the same movement to a stationary conductor in the same magnetic field, it will produce a current in the opposite direction by electromagnetic induction. My reasoning is that, in a motor, if constant current in a particular direction, produces pulsating torque, then the pulsating torque will induce a current in the opposite direction, by electromagnetic induction.

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10. May 10, 2012

Bob S

Only the very simplest dc motors, like the ones found in high school physics labs,and usually are not self starting, have two poles. Even the simple toy electric motors in the 1896 Sears catalog or 1894 Montgomery Wards catalog (e.g., Little Hustler-see pic) had 3 poles and 3 pads. Modern dc motors have 20 or more pads, so the cosθ pulsation effect is minimal. Also, because of the inductance of the rotor windings, the current variation due to using a switching regulator as a current source is minimal. There is no back emf if the current pulsing is minimal or if the rotor is locked.

added There is back emf when the rotor is turning.

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11. May 10, 2012

DragonPetter

A DC motor with the shaft mechanically rotated will generate a voltage on its open terminals, which can be used to power other things. This is how a generator works in a simple comparison, and you can consider a DC motor as a DC generator if it is connected as the 2nd animation shows. I must ask you, how can the back EMF remain constant if the B fields of the rotor and stator are not always in the same direction with respect to eachother? I think the back EMF is varying in this motor, and so the current must be varying too if the source is a DC voltage supply.