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PM brushed DC motor without cogging torque?

PM brushed DC motor without cogging torque?

PM brushed DC motor without cogging torque?

(OP)
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

RE: PM brushed DC motor without cogging torque?

(OP)
I mean, for the best brushed DC shunt motor available in market, does it produce torque ripples?

RE: PM brushed DC motor without cogging torque?

before we play twenty questions about torque ripples, what is your application?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PM brushed DC motor without cogging torque?

Yes, brushed dc motor has torque ripples and for most applications it is not a problem because of rotor inertia.  

RE: PM brushed DC motor without cogging torque?

A DC motor, whether shunt or PM field, that has a skewed pole armature is specifically designed to minimize torque ripple.

Exactly how much and whether it is suitable for your application will require a discussion with the motor supplier.

RE: PM brushed DC motor without cogging torque?

The Parvex AXEM series DC motors would meet your requirements. Typical construction consists of 8 permanent magnet poles and the rotor is a 6 layer printed circuit card that implements a wave winding with 193 commutator bars. This makes for very smooth operation especially at very slow speeds.

However, the thermostat that comes with the motor does NOT act as an effective motor overload relay. SquareD makes single pole holders for melting alloy relays - the class 10 would be the FB series. As overloading one of these motors causes disastrous damage to the rotor and the servo amplifier the overload realy should be sized slightly smaller than the nameplate rating of the motor.

Also remember that at standstill only 80% or the motor RMS current produces torque - the nameplate torque only applies to a rippless power supply such as a battery and variable resistor. The torque producing component can be measured with a repulsion moving iron meter even though such a meter theoretically needs to be read using the average of reversed reading for direct current. ( A true RMS meter with its bandwidth set at 135 Hertz will also do this. ) Peak commanded current for the current loop in the drive can be 1.6 times full load for a load that does not hit and stay at a positive stop. For a load that hits and stays at a positive stop the commanded current that produces torque needs to be set at 80% of nameplate and the motion controller needs to be programmed to ignore not being able to exactly reach final position.

Total continuous motor current would need to be measured with a true RMS meter with its bandwidth set at about 16 times the carrier frequency. Harmonics beyond the 15th are less than the measurement accuracy of the meter and do not add up to anything consequential.

Therefore, if lets says that the load requires X amount of motor torque, the nameplate rating of the motor should be 1.5X torque so that maximum total continuous RMS current will be less than the overload relay setting.

RE: PM brushed DC motor without cogging torque?

(OP)
Thanks for your replies.

I wish to use the DC machine as a load driven by a PM brushless motor. This system will be used to test the vibration behaviour of the PM brushless motor. Therefore, I hope that the DC machine will not produce any cogging torque, which might affect the vibration measurement of the PM brushless motor. I'm looking for a DC machine with output of 1~2kW.

Thank you very much

RE: PM brushed DC motor without cogging torque?

Why would a DC machine when driven by another motor produce any torque let alone cogging torque ? That DC machine will be working as a generator unless its speed was above that of the PM motor driving it. Your post is a bit confusing.

RE: PM brushed DC motor without cogging torque?

an across the line control motor...

RE: PM brushed DC motor without cogging torque?

(OP)
Thanks edison123. I agree with you that I should not call it "torque" when the DC machine is driven by another motor. I believe DC generator does exhibit vibrations. Let's say the DC generator is driven by another motor, which gives very smooth torque. When load, which is connected to DC generator, draws power (e.g. current), do you think the DC generator, besides cogging torque if it has, will produce any "electrically" or "electromagnetically" induced vibrations?

Thank you very much

RE: PM brushed DC motor without cogging torque?

If you need to check your PM motor vibrations, run it decoupled and measure the vibrations. When it is coupled to another load, there are other sources of vibration introduced, like misalignment,  that cannot be blamed on your PM motor.

A DC generator does not produce cogging torque but it has  its own souces of vibrations like brush chatter etc.

If you can explain your requirements a little better, many people will offer good suggestions.

RE: PM brushed DC motor without cogging torque?

(OP)
Thanks edison123. In fact, I have opened another thread earlier, which explains the experimental setup and requirements of my tests as follows:
http://www.eng-tips.com/viewthread.cfm?qid=252223&page=1
I really hope that you will join the discussion in that thread and provide your advices and suggestions. Thanks in advanced.

By the way, I find a DC machine as follows:
Product: DC Shunt Wound Motor
Manufacturer: Baldor
Armature: 180VDC
Field: 200VDC
Base Speed: 1750rpm
Additional: 20:1 speed range with constant torque

Would someone please explain what does "20:1 speed range" mean in this case?

Thank you very much
 

RE: PM brushed DC motor without cogging torque?

That means that the motor can be used to produce full nameplate torque down to 1/20th of the base speed without overheating.

Actually, the motor will produce full torque all the way down to locked rotor but, if continued for more than a minute or so, will overheat the motor while below the 1/20th speed point.

RE: PM brushed DC motor without cogging torque?

(OP)
Thanks DickDV. Let's say the full load armature and field currents are 11A and 0.5A respectively. If we limit the armature and field currents to 11A and 0.5A respectively, can we operate the motor at FULL torque below the 1/20th speed point without overheating?

 

RE: PM brushed DC motor without cogging torque?

The lower speeds are achieved by lowering the armature voltage and the HP goes down proportionately thus maintaining the constant torque. Such motors have externally driving blowers to avoid overheating.

RE: PM brushed DC motor without cogging torque?

1mm, you didn't tell us what the full load amps is on the motor but I assume that the 11amps in the armature is less than full load.

You are correct in saying that reduced amps (torque) will permit slower running than the nameplate limits.  Baldor should be able to send you a torque-speed curve for that motor that shows the low speed derate requirements.

You must leave the field current right at specification.

RE: PM brushed DC motor without cogging torque?

(OP)
Hi DickDV,

I'm really interested to find out why below the 1/20th speed point will overheat the motor.

Kindly refer to model VD6202D as shown in Fig. 1 below. Let's say we maintain the armature current and field current at their rated values, i.e. 11A and 0.5A respectively, below the 1/20th based speed, do you think the motor will overheat?

Thanks

Figure 1

RE: PM brushed DC motor without cogging torque?

1mm, looking at the data you sent, it is clear from the picture that this motor is cooled with a fan driven by its own armature shaft.

Clearly, when the armature shaft slows down for low speed operation, the cooling fan also slows down.  At some point, the fan is turning too slow to move enough air to cool the motor.  In the motor you have been using, at full load, that point is at 1/20th of base speed.

You can run slower than that but you cannot do it at full load.  You would have to reduce the shaft loading which would then be reflected in reduced armature amps.

In the case of a shunt wound field motor, the field itself generates some heat so, even completely unloaded, you will need some cooling capacity.  Since 1/20th of 1750 is only 35rpm, I would think that that should be the minimum speed at any load including unloaded.  Below that, shut the motor off including the field current.

Certainly, do not leave the field energized with the motor standing still.

Reviewing all this makes me appreciate AC induction motors again!!

RE: PM brushed DC motor without cogging torque?

(OP)
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.

 

RE: PM brushed DC motor without cogging torque?

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.

RE: PM brushed DC motor without cogging torque?

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.

RE: PM brushed DC motor without cogging torque?

(OP)
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?

RE: PM brushed DC motor without cogging torque?

(OP)
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.

RE: PM brushed DC motor without cogging torque?

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.

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