What's the meaning of current rating of BLDC motor and it's relation to a phase peak and RMS current? [closed]

Considering battery powered BLDC motor having rated voltage and current specs, what is exact meaning of the rated "current"? Since BLDC motor is NOT powered by a DC current (enjoy the twisted logic of terminology) rather it is another device - a motor controller - which generated controlling AC phases in a form of PWM sequences, then, if I understand the "rated current" spec as a maximal current allowed from the rated maximal voltage, how could it be translated to the true motor spec? In other words if my understanding is correct stating that rated current and voltage are maximal specs drawn from the DC source, then these specs cannot even be applied to the motor directly! Hence my question.

For simplicity let's assume that efficiency of the motor controller is 100%, then how can we translate these manufacturer' provided (virtual) specs into actual specs of the motor itself? As we know the electromagnetic field and MMF which drives the rotor is caused by a current flowing through the coils of the stator, so we need to "translate" the rated current from manufacturer into the peak or RMS current of the motor phase(s). If it is so then how to do this translation? Assuming the peak of PWM driving phase is equal the rated voltage, can we get a peak and max RMS current of a single motor phase?

I am guessing that total RMS current of the 2 phases (out of 3) which are in ON state at any time should be about equal to the max "current rating" of the motor (minus whatever losses "on the way" through the motor controller), but I am not sure if this is correct - need your confirmation or denial :)

Added 4.29.18 : consider this example REX 30 motor from http://www.rotexelectric.eu/products/bldc-motors/rex-series/ Their units might be confusing, this motor is almost the same but cleanly defined parameters ( I am running this motor): http://www.nt-power.eu/doc/nt-power-datasheet-motor-15kw.pdf

closed as unclear what you're asking by Charles Cowie, laptop2d, Dmitry Grigoryev, Sparky256, Andy akaMay 13 '18 at 13:32

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• Usually the motor has its Kv listed. This will allow you to calculate Kt. What else do you want to know? Winding resistance? If you know the winding resistance and Kv, you can figure out just about everything you need to know. – mkeith Apr 28 '18 at 20:40
• – mkeith Apr 28 '18 at 20:45
• I believe that manufacturers of small BLDC motors specify them as if the electronic speed controller (ESC) is part of the motor. The DC current is actually the DC input to the ESC. The Kv and other motor constants refer to the performance as a machine emulating a commutator motor with a permanent magnet field. Brushless motors used in electric vehicles and industrial applications are more often treated as permanent-magnet synchronous motors. I don't think you can get sufficient information to translate the virtual motor information to the real physical information. – Charles Cowie Apr 28 '18 at 21:03
• In all these comments does everybody agree that rated current is a max current from a DC source through whatever controller before max temperature is reached? Or Charles Cowie hint that in case of powerful EV motors it is that but what? And Tony Stewart did not mention WHAT IS DEFINITION of rated current ? Is there no standard for it's meaning? – VladBlanshey Apr 28 '18 at 21:10
• At this point I am going to recommend your question is closed as too broad. I will remove my vote if you make it more specific. – mkeith Apr 28 '18 at 21:14

Who knows what the REX30 specs mean. Basically if you buy this motor, you get what you get. Take a look at the max current rating for the top motor (the 2700 rpm version). The units given are kW. I don't think it actually is intended to be current at all. I think it is power in kW at max current or something. But the range is from 8 to 20.

The seller is not doing a good job of providing the needed engineering data for this motor. It doesn't even say how many pole pairs there are. I guess you have to count them yourself.

But it does give the Kv (64 RPM/V). So we can calculate Kt.

Kt = 60 / (Kv * 2 * pi)

So the Kt is 0.15 N-m/A. So if you know how much torque you need, you can figure out how much armature current will be required to achieve it. The Kv will give you a rough estimate of how much voltage is required to achieve a specific speed, but you would also need to know the winding resistance and output torque to get a real estimate.

As far as the maximum current goes, you can make some guesses. The output power seems to be somewhere between 8 and 20 kW at 2700 RPM. Note, I am making an assumption that the power ratings are for a motor speed of 2700 rpm. If not, then everything that follows will be wrong.

At 2700 RPM, 8kW requires 28.3 N-m. And 20kW requires 70.7 N-m. This is based on output power = torque * speed, where power is Watts, torque is N-m and speed is rad/sec.

So at 8kW, the armature current would be 28.3 N-m / 0.15 A/N-m = 190 A. And at 20kW, it would be 70.7/0.15 = 474 A.

I will run through the calculations for the NT Power motor. But I am not sure they match with the graphs. From the data table, the Kv is 38.5 rpm/Volt, so Kt is 0.248 N-m/A.

Max torque is listed as 61 N-m. So armature current at max torque would be 61 / 0.0248 = 246A.

If we assume max power of 15kW occurs at max torque of 61 N-m, then speed would be 15000/61=246 rad/sec.

246 rad/sec = 2350 rpm.

So according to these calculations, unless I made a mistake, the motor should be able to put out 15 kW at 2350 rpm with 246 A of armature current. The back EMF would be 61 V. The voltage required to supply 246 A would be substantially higher than just the back EMF, because it would have to overcome the winding resistance.

What exactly is the max current specification in general? I don't think there is a single accepted definition. You have to be familiar with motor theory and examine the data from the manufacturer, and if there is any doubt, ask the manufacturer. But I will tell you what max current SHOULD be. Max current should be the maximum armature current that the motor can handle without over-heating. If Kt is known, the max current can be used to calculate the max torque also. Obviously there is an assumption that torque = Kt * Ia, which is only true when the motor is being controlled properly. (Kt is the torque constant and Ia is armature current).

In some cases, max current may be max DC current (from battery or DC bus) and this may be essentially the same as armature current if the DC voltage is well matched to the motor operating speed.

Hope that helps.

• Thank you for your input. You are right that motor specs are quite confusing. Given what is provided there your answer seems to be the only way to make sense out of it. I'd like to get answer more related to my question though and looks like that example does not allow that. Sorry. Could you please add your analysis to the 2nd link? Now I see it is better for this topic. Though I never tested it's max but my tests show specs match under 90 amps from battery. Using max V, I and Power can you arrive at motor phase' RMS current? (max power does correspond to max RPM) - this would match the topic. – VladBlanshey May 1 '18 at 15:30
• @VladBlanshey, updated the answer. – mkeith May 2 '18 at 17:13
• Thank you - this is useful input. Interestingly that your arrived RPM at max power is close to what my no-load tests show even though according to BLDC theory max power is not at max torque because definition of "max torque" is the point where motor stalls (hence no rotation and no power:) Looks like manufacturer' given "max torque" is whatever torque is at max power point... I voted +1 for the useful input but I regret I didn't get any input on the meaning and use of the max current spec. – VladBlanshey May 3 '18 at 10:34
• In the context of a BLDC with controller, max torque is not defined as stall torque. The stall torque may be of interest, but max torque SHOULD be based on max armature current. For a BLDC, torque is Ia * Kt, where Ia is the armature current and Kt is the torque constant. Max torque is the torque achieved when Ia is at its max. The max Ia is determined by power dissipation considerations. The resistance of the wire will give rise to heating in the stator. To a first approximation, torque is constant from 0 RPM to max speed. – mkeith May 3 '18 at 16:01
• About definitions and relationships between max torque, speed and power in the context of DC motors: lancet.mit.edu/motors/motors3.html#tscurve – VladBlanshey May 4 '18 at 17:43