PM brushed DC motor without cogging torque?

[onemilimeter]

I'm aware that permanent magnet (PM) machine exhibits cogging torque. I'm looking for PM brushed DC motor without cogging torque. Is it possible? If you know any company that sells this type of PM brushed DC motor, please kindly let me know.

By the way, how smooth is the electromagnetic torque generated by a brushed DC shunt motor? Does brushed DC shunt motor produce torque ripples?

Thank you very much

 

[onemilimeter]

Hi DickDV,

Thanks.

Kindly refer to the specs of model VD6202D again. If you look at the title of the datasheet, i.e. "IEC fram, 180VDC Armature, 200/100 VDC field, shunt wound, TEFC", what does "200/100 VDC field" mean? Does it mean we can connect the field winding to either 200VDC or 100VDC? If YES, the field current shown in the datasheet, i.e. 0.5A, is the field current when the field winding is connected to 200VDC or 100VDC?

Please refer to the datasheet again. I try to work out the field resistance and armature resistance at rated conditions (i.e. P=1500W, n=1750rpm, Va=180VDC, Vf=200VDC, Ia=11A, If=0.5A) as follows:

w = n*2*pi/60 = 183.26 rad/s
Rated torque, Torq = P/w = 1500/183.26 = 8.1851 Nm
K = Torq/Ia = 8.185/11 = 0.7441
E = K*w = 0.7441*183.26 = 136.36 V
Armature resistance, Ra = (Va - E)/Ia = (180 - 136.36)/11 = 3.9669 ohm
Field resistance, Rf = Vf/If = 200/0.5 = 400 ohm

Please correct me if my calculation is wrong.


Now, I wish to calculate the Ia when the DC machine is driven as a generator at 400rpm with reduced Vf=150VDC. Do you is it possible to calculate? Please advise.

Thanks.

 

[Compositepro]

The field winding is two coils that can be connected in parallel for low voltage or series for high voltage.

I've never seen a D.C. motor where the field winding could overheat a stopped motor. The motor will simple stay warm.

 

[DickDV]

Compositepro, DC motors definitely do overheat with full field current at zero speed. That's why many DC drives have a feature called "field economy". When activated, the drive will reduce the field current to some nominal value after the motor has been stopped for a pre-determined length of time. Field economy also is used to avoid the waste of energy involved in keeping the field fully excited during long stopped periods.

1mm, Compositepro is correct about two field windings, each 100V .5Amp. When in series which is normal, the combination makes 200V .5A and when in parallel, it is 100V 1A.

Motors that are wound 200field/180armature are generally intended to be driven by DC drives powered from 240V single phase AC.

As to your calculations, you'll have to consult a textbook for that. I am not sure of your "K" calculation which would affect everything else. For me, I would hook up a 200V field supply and spin the motor at 1750rpm and measure the voltage open circuit on the armature leads. That would be the internal generator voltage.

As you drop the field voltage from 200 to 150V, it will be a bit difficult to predict what the generator output voltage will be due to the current possibly not being linear.

I'd test this in the lab as well rather than try to predict.

 

[onemilimeter]

What does "base speed = 1750rpm" mean? Does it mean the speed at FULL LOAD (i.e. at Ia = 11A) is 1750rpm?

I just received the torque-speed curve from Baldor, which shows that at FULL LOAD, the speed is around 1610rpm. Which one is correct?

 

[onemilimeter]

Hi DickDV, thanks again for your suggestion. Unfortunately, we have not purchased the DC machine yet. Due to limited budget, we wish to make sure that we will buy the right DC machine that will suit our application before we purchase.

The "K" in my calculation is EMF constant, which is equal to torque constant, when it's expressed in SI unit.

If the full load field current If=0.5, do you think it is OK to assume that, at If=0.25, the "K" constant will be half as well?

Thanks.

 

[DickDV]

DC motor base speed should be the speed of the shaft when the following all occur simultaneously:

1. The motor is producing full rated torque
2. The field is excited to its nameplate amps
3. The armature supply voltage is at nameplate voltage.

I don't know why the Baldor curve stops at 1610rpm. That doesn't seem right to me.

I am not familiar enough with the various constants in a DC motor to comment reliably to your question about K factor.

Finally, the field current doesn't vary dependent upon the motor load. So "full load field current" isn't a correct term. Correct would be "full excitation field current" or "nameplate field current".

Sometimes field current is deliberately reduced to shift the armature speed upward at all corresponding armature voltages. There is a matching reduction in torque when this is done.