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BLDC Motor Torque at Start

10/28/2017 4:33 AM

Hi,

I am using a BLDC motor for driving a battery operated vehicle.

The specs of the motor are as follows-

10KW, 48V, 2000 to 6000 RPM range, Make - Golden motors

I did a lot of search on internet to know the torque applied by the BLDC motor at start.

As per the chart of the motor it gives max torque at around 3500 RPM. But the chart also shows that as the RPM goes down the output torque increases. So, I should get max torque at start and not at 3500 RPM. The chart ends at 3500 RPM!!!

Also, the motor RPM range is 2000 to 6000 RPM, even in that case I should get max torque at 2000 RPM.

Can anybody answer that?

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#1

Re: BLDC motor torque at start

10/28/2017 8:33 AM

Sure it's not the power that is max at 3500 RPM? Could you attach a picture of the torque-speed plot or at least a link?

BLDC motors, AFAIK have the same torque-speed of brushed DC motors, maximum torque at stall and decreasing torque as the motor increases speed and generates back emf.

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#2

Re: BLDC Motor Torque at Start

10/28/2017 11:16 AM

I think you're confusing torque availability at that speed with overall torque produced...

https://www.goldenmotor.com/eCar/HPM48-10000Curve.pdf

https://www.goldenmotor.com/

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#3

Re: BLDC Motor Torque at Start

10/28/2017 12:15 PM

Here is a better plot.

https://hackaday.com/2016/10/17/the-little-things-i-didnt-know-about-small-dc-motors/

Analysis of a dc motor is fairly straightforward. Input power is volts times amps. Output power is RPM times torque. Torque is proportional to amps and RPM is proportional to volts.

The motor acts as a generator, producing a voltage proportional to the RPM that opposes the applied voltage. The maximum RPM of the motor is reached when back emf equals the applied voltage. There is no back emf at zero RPM, and therefore the current draw and torque are maximum, limited only by the internal resistance of the motor. Increasing the applied voltage generates a new graph (above), increasing the torque (current draw) and maximum RPM (back emf).

When the motor is connected to a mechanical load, that load will have a torque-RPM curve associated with it. Generally, the faster the load is turned, the more torque required. If you plot both the motor torque-RPM curve and the load torque-RPM curve, where these curves cross will be the operating point. To move to a higher RPM (and torque) requires increasing motor voltage (new torque-RPM curve for motor).

https://nathotron.wordpress.com/courses/engineering-design-process-530-381/motor-selection/

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#4
In reply to #3

Re: BLDC Motor Torque at Start

10/28/2017 12:43 PM

Yes excellent explanation....

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#5

Re: BLDC Motor Torque at Start

10/30/2017 7:59 AM

Torque is something presented to the motor by the load.

  • If the load torque is too much for the motor, it won't rotate the load.
  • If the motor gets the load to turn, then the torque the load presents is less than that which the motor can produce.

Simplesξ.

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#6

Re: BLDC Motor Torque at Start

10/30/2017 6:38 PM

Does your motor have some sort of position feedback (encoder, resolver, etc.) or is it a sensor-less design?

Sensor-less designs are unable to make any significant torque at low RPM's. In a sensor-less design, the motor controller has to be able to spin the motor first to derive the relative rotor position by sensing the back electro-motive force (BEMF) when each of the inverter bridge switches are in the off state. Sensor-less motor controllers will apply arbitrary sequence of pulses to the windings to get the motor started by bumping it along. Once it gets the motor going, then it can "put the boots to it" at higher RPM's.

If a motor has position feedback, the controller 'knows' exactly where the rotor is relative to the stator and can apply the maximum current to the appropriate stator windings and generate the maximum torque. Even in a stalled condition.

Yes, in theory, you should be able to develop higher torque at the lower RPM's because you will have less BEMF to push current against. As others have said, the peak power for the motor may be 3,500 RPM and they just provide a maximum torque at that operating point given the voltage applied. If you need more torque, you need more current and to get more current at a given RPM you will need more voltage.

Then we can get into a discussion of direct currents, Id and quadrature currents Iq, but JRaef is THE guy on this stuff. I'm a neophyte on this stuff.

Good luck with your quest.

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#7

Re: BLDC Motor Torque at Start

10/31/2017 3:43 PM

A BLDC dc motor behaviors as a brushed independent excitation that means the pole magnetic flux [Fx] is constant [approximate if we shall neglect armature reaction ].

Back EMF[E]=kE*rpm*Fx≈kV*rpm

T=k*Fx*I T=torque,Fx=magnetic flux I=armature current

T=kT*I

I=(V-E)/R=(V-kV*rpm)/R

T=kT*(V-kV*rpm)/R

A=kT*V/R B=kT*kV/R

T=A-B*rpm

If rpm=rpmrated then T=Trated

If rpm=rpmmax then T=0 A-Brpmmax=0 A=B.rpmmax

If rpm=0 then T=A stall torque

Trated=B*rpmmax-B*rpmrated

B=Trated/(rpmmax-rpmrated)

A=B*rpmmax= Trated.rpmmax/(rpmmax-rpmrated).

P=kp*T*rpm kp=2*pi()/60

P=kp*(A*rpm-B*rpm^2)

In order to find Pmax=Prated dP/drpm=0

A-2.B.rpmrated=0 rpmrated=A/2/B=rpmmax/2

Tstall= Trated.rpmmax/(rpmmax-rpmrated)=2*Trated

Trated=Prated/2/pi*60/rpmrated

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