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]

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

 

[waross]

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

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

 

[edison123]

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

 

[DickDV]

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.

 

[sreid]

There are motors with no cogging torque and the word to add to the search is "Slotless." Maxon Motors is one of many who make these.

http://www.maxonmotorusa.com/product_overview_details_ec_motor.html

 

[mc5w]

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.

 

[sreid]

Onemillimeter,

I read "Brushless" instead of "Brush." Maxon also makes brushed DC Servo Motors with "Ironless Rotors."

http://www.maxonmotorusa.com/product_overview_details_maxon_dc_motor.html

 

[onemilimeter]

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

 

[edison123]

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.

 

[edtech]

an across the line control motor...

 

[onemilimeter]

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

 

[edison123]

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.

 

[onemilimeter]

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

 

[DickDV]

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.

 

[onemilimeter]

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?

 

[edison123]

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.

 

[DickDV]

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.

 

[onemilimeter]

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

 

[DickDV]

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!!