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
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?
RE: PM brushed DC motor without cogging torque?
Bill
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"Why not the best?"
Jimmy Carter
RE: PM brushed DC motor without cogging torque?
RE: PM brushed DC motor without cogging torque?
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?
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RE: PM brushed DC motor without cogging torque?
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?
I read "Brushless" instead of "Brush." Maxon also makes brushed DC Servo Motors with "Ironless Rotors."
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RE: PM brushed DC motor without cogging torque?
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?
RE: PM brushed DC motor without cogging torque?
RE: PM brushed DC motor without cogging torque?
Thank you very much
RE: PM brushed DC motor without cogging torque?
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?
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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?
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?
RE: PM brushed DC motor without cogging torque?
RE: PM brushed DC motor without cogging torque?
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?
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?
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?
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?
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?
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?
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?
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?
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.