Controllers for bldc motors, efficiency

[ingesa]

Hi,
I posted this in the general discussions, but moved it here as it is more suitable:

I'm writing this to be sure that I understand this right:
Besides switching the circuit between the phases at the correct time, the mission of an ordinary bldc controller is to control the current to the motor by pulse width modulation (PWM), that is varying the on/off pulse duty to achieve a certain amount of amps. The available voltage to push current through the armature is the battery voltage minus the back EMF, and as a result the controller will increase the PWM duty when the motor speed increase until the motor approaches the max speed (the back EMF approaches the battery voltage), then I guess the PWM will be a full ON cycle (because the available voltage to push the current through the armature approaches zero). The mechanical power produced by the motor equals the back EMF times the current running through the armature.

Isn't this a very inefficient way of controlling the power? Let's say that at one motor speed the PWM duty must be 50% not to exceed the max amps. In this operating point the battery voltage could be much lower and the PWM duty could be 100%. Then the power drawn from the battery (volts * amps) would be much lower, and at the same time the power delivered by the motor would be the same.

Wouldn't it be better to regulate the voltage to the motor speed, just keep the voltage sufficiently above the back EMF to obtain the wanted amps? There are several voltage regulator circuits which can both step up and step down with acceptable efficiency. Then it could be possible also to increase the speed range in which the motor can run at high efficiency.

Or maybe such controllers exists, only it is too expensive to be used on e-bikes?

 

[Jeremy Harris]

PWM varies the motor voltage, not the motor current.

50% duty cycle would supply the motor with 50% of the battery voltage, unless the controller preset current limit was exceeded, when the controller would cut the duty cycle back to less than 50% to reduce the motor voltage and hence reduce the current it draws.

Normally (i.e. when current limiting isn't active), the throttle position determines the PWM duty cycle; 50% throttle = 50% PWM duty cycle = motor voltage that's 50% of battery voltage.

 

[amberwolf]

Wouldn't it be better to regulate the voltage to the motor speed, just keep the voltage sufficiently above the back EMF to obtain the wanted amps?

Jeremy explains it in detail, but that is exactly what the controller does, via the PWM, because the motor's inductance "smooths" the PWM to an average.

 

[ingesa]

Thanks for responding. Yes, I suspected it was something I got wrong with my reasoning. So that means that it actually runs more amps through the armature than through the battery at 50% PWM duty? I was just thinking that the PWM was chopping the current flow from the battery and distributing it to the phases so that the actual electron flow throgh the phases was equal to the electron flow out of the battery (fluid flow analogy).

 

[Jeremy Harris]

Thanks for responding. Yes, I suspected it was something I got wrong with my reasoning. So that means that it actually runs more amps through the armature than through the battery at 50% PWM duty? I was just thinking that the PWM was chopping the current flow from the battery and distributing it to the phases so that the actual electron flow throgh the phases was equal to the electron flow out of the battery (fluid flow analogy).

The two functions of a BLDC controller, power control and commutation, are separate.

The phase switching for commutation is either determined by Hall sensors, or by zero crossing detectors looking at the BEMF for sensor-less controllers.

The throttle sets the PWM ratio for all three phases (the PWM runs synchronously for all phases).

Phase current can be several times greater than battery current if the motor is loaded and the RPM is relatively low. In effect, the controller and motor combine to make a buck switched mode converter, converting a high battery voltage into a lower motor voltage but at a higher current.

 

[miuan]

I can't imagine you're better off running two DC converter devices in series when one set of mosfets can do the same job. Maybe you could propose a certain setup using known components (motor, controller, converter) so we can comment more specifically.

 

[ingesa]

Thanks Jeremy for a clear answer. So the sitation for my Magic Pie 3 e-bike is not as bad as I thought at low speed One thing that I'm not very impressed with this type of motor and controller is the speed range. It would be nice if the controller also could work as step-up converter to overcome the backEMF limitation of 48V in my case.

 

[Jeremy Harris]

Thanks Jeremy for a clear answer. So the sitation for my Magic Pie 3 e-bike is not as bad as I thought at low speed One thing that I'm not very impressed with this type of motor and controller is the speed range. It would be nice if the controller also could work as step-up converter to overcome the backEMF limitation of 48V in my case.

If you want the motor to run faster (and this is exactly the same for an ordinary permanent magnet DC motor as it is for a BLDC motor) then you either have to rewind the motor to get a higher velocity constant (kV, the rpm per volt constant for the motor) or you have to increase the supply voltage. The Magic Pie 3 is harder to mod as it has an internal controller within the wheel. It is possible to open up the wheel, remove the limited power built-in controller, feed the motor wires out directly and then use a separate controller that will allow a higher battery voltage to be used. This is a fair bit of work, though.

 

[ingesa]

Yes, I guess the easiest way will be to buy more batteries and a new controller handling the voltage. I know Lyen has controllers for a reasonable price. Actually it's not too much work to take out the internal controller from the hub, I have allready changed it twice. These controllers are apparently not of high quality (but since I changed it the last time it has actually lasted for about two months..) I have thought of buying more batteries for some time, but I thought that it maybe meant running the motor at low efficiency at a larger speed range. But as you explained, this is not the case (well, I guess it maybe is a little more losses if the controller must run on low PWM duty compared to high or 100% duty)

 

[rolf_w]

Hi,
...the mission of an ordinary bldc controller is to control the current to the motor by pulse width modulation (PWM), that is varying the on/off pulse duty to achieve a certain amount of amps.

agreed - the battery (voltage source) is parallel to the motor winding (inductance and resistance) thus pwm-ing it in a full H-bridge produces a 'saw toothed' current (which is almost dc in 'normal' setups but its spikyness gets really nasty for very low impedence windings).

The available voltage to push current through the armature is the battery voltage minus the back EMF, and as a result the controller will increase the PWM duty when the motor speed increase until the motor approaches the max speed (the back EMF approaches the battery voltage), then I guess the PWM will be a full ON cycle (because the available voltage to push the current through the armature approaches zero).

agreed - in steady state the pwm-ed voltage difference and the impedence produce just enough current and magentic torque to overcome the mechanical resistive torque.

The mechanical power produced by the motor equals the back EMF times the current running through the armature.

agreed - the mechanical torque produced is fundamently the product of two magnetic field components in the machine's airgap, which (properly shaped and timed) are proportional to the bEMF and the armature current.

Isn't this a very inefficient way of controlling the power? Let's say that at one motor speed the PWM duty must be 50% not to exceed the max amps. In this operating point the battery voltage could be much lower and the PWM duty could be 100%. Then the power drawn from the battery (volts * amps) would be much lower, and at the same time the power delivered by the motor would be the same.

hmm, to deliver a steady state mechanical power a matching average electrical power has to be supplied by the battery and, after the inverter, by the 3 phases to a usual bldc motor, thus V_rms*I_rms is aprox. constant too. If inductance is high the current is almost dc (or a ramp or sine, whatever has to be shaped) at moderate pwm frequencies, it is thus the pwm duty cycle which controls V_rms and the el. power. The pwm switching is lossy (e.g. FET heat in transients) therefor it is efficient to keep the FETs On (100% duty) which is the case when the bemf is close the battery voltage. Thus match motor speed with vehicle speed.

Wouldn't it be better to regulate the voltage to the motor speed, just keep the voltage sufficiently above the back EMF to obtain the wanted amps? There are several voltage regulator circuits which can both step up and step down with acceptable efficiency. Then it could be possible also to increase the speed range in which the motor can run at high efficiency. Or maybe such controllers exists, only it is too expensive to be used on e-bikes?

as said above: if inductance is high pwm is pruducing smooth currents and adjustable V_rms. There are developers here on ES who try to handle very low impedance motors (see 'colossus'). None of the current bldc controllers handle well the peak currents involved. One solution is to add an external inductor in series which creates a voltage divider and a current limiter. However, this is not adjustable. One has to controll extremely fast the phase currents (in some ns). I doubt this can be achieved via an external adjustable voltage source.
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