PWM switching in BLDC motors: ratio between switching losses versus conductive

Question

There were some good posts regarding choosing the PWM switching frequencies considering the needs of the motor control. Here I am focusing rather on understanding of how to decrease losses in the power bridges of the controller. Mainly analyzing two main components of these losses: the conductive losses (power MOSFETS + inductor + resistive load) versus switching losses in power MOSFETs (Miller effect between full open and fully closed state).

Consider example of 3000RPM motor , 50Hz outer loop frequency, having 4 pairs of poles whence 200Hz outer loop motor frequency. According to the accepted "rule of thumb" the required PWM switch frequency starts from 10 x 200Hz = 2kHz This number is thought of been able recreating the "natural sinusoidal" EMF of the motor coils needed for voltage control of the coils current.

Let's assume that I have other means to change the voltage applied to the half bridges MOSFETs in order to control the speed of the motor (in this case to maintain steady 3000RPM). Is it correct to assume that I can reduce switching losses in the power inverter if I just apply the same switching frequency 2kHz thus not recreating sinusoidal waveform at all but only switching polarity of motor phases to make motor run with constant speed ? (Again, assume for a moment that issues of the close loop, position sensing and fine tuning are passed to another block - the voltage controller , adjusted for a permanent speed.) Or are the conductive losses increase so dramatically that saving on switching losses will not help at all? What kind of losses prevail within say 100Hz to 10kHz switching range ? Would conductive losses increase dramatically in the lower end of this range so that eliminating switching losses don't matter ? Or is it more beneficial to use upper end of the range to have minimal total losses?

Yet one more way to put this question: Is PWM switching frequency chosen only to recreate sinusoidal voltage control of the motor or is it also helping to decrease overall energy losses in the inverter compared to the long period of the ON state conductive losses in the MOSFETs?

Anybody who had heavy practical experience with motor control please share your thoughts. JonRB please? Edit: To clarify the expression "to recreate sinusoidal voltage control" is only one purpose of using PWM, another one - more frequently used for - is to control voltage level below of the provided fixed voltage source. In any case, this post is not about voltage control but about power losses in the inverter : the balance between conductive losses vs switching losses.

Answer 1

According to the accepted "rule of thumb" the required PWM switch frequency starts from 10 x 200Hz = 2kHz This number is thought of been able recreating the "natural sinusoidal" EMF of the motor coils needed for voltage control of the coils current.

The PWM frequency must be much higher than the commutation frequency to get a reasonable representation of a sine wave, but also to reduce current ripple. High current ripple causes greater loss due to the increased rms current (which heats the controller and motor) relative to average current (which produces torque).

Current is smoothed out by the action of winding inductance, so low inductance motors need higher PWM frequency. Slotted iron-cored BLDC motors are typically run at 8kHz, while slotless and ironless motors may need 32kHz or higher. Another reason for using >20kHz is to reduce audible noise.

Is it correct to assume that I can reduce switching losses in the power inverter if I just apply the same switching frequency 2kHz thus not recreating sinusoidal waveform at all but only switching polarity of motor phases to make motor run with constant speed ?

Controllers that use '6 step' commutation rather than 3 phase sine waves can have reduced switching losses because PWM is only applied to each MOSFET for 2 of the 6 steps. At full power there is no PWM so switching losses are further reduced.

Or are the conductive losses increase so dramatically that saving on switching losses will not help at all? What kind of losses prevail within say 100Hz to 10kHz switching range ?

Most BLDC controllers use FETs which have low switching loss below 10KHz, so I doubt that it is significant. However current ripple increases as frequency is reduced, so the controller should suffer higher conduction loss at lower frequency. All the controllers I have tested ran at 8kHz or higher, and even at that frequency there was high current ripple. Recirculation currents are another factor. Most controllers rely on the FET body diodes to carry the flyback current when the FETs are turned off. This results in higher conduction loss below 100% PWM.

Answer 2

You are absolutely correct, motor speed can be controlled without the use of PWM by changing the voltage. There are a couple of considerations to this approach.

Brushless motors are not all wound for sinusoidal drive. You would want a motor with a more trapezoidal back EMF for this approach. You mention two functions of PWM in your post. One is obviously speed control, and the second is to match the back EMF of the motor to the drive waveform. When you go without PWM, the drive mismatch from the EMF means that the instantaneous current requirement will change ("current ripple") and so will the instantaneous torque. If you have a large inertial load, you will have a lot of current ripple as the motor goes from a position where there is high torque (the back EMF voltage is low with respect to the drive) to a a position where there is less torque, but your inertia will keep the speed relatively constant. If you don't have a lot of inertia you may actually have some "cogginess" in your output speed.

Your real question is then concerning your switching losses. If you have a driver with a lot of stray inductance and are switching at high PWM rate, your losses can be reduced by your approach. We run downhole motors in this manner. You will still need PWM to keep your current from spiking during start-up. Get the best motor wind you can, but you will have to put up with current ripple of 15-20%.

Answer 3

The main reason for operating the FET's in a PWM mode at all is to alter the effective applied voltage. The effective voltage applied to the motor is simply VDC * duty cycle. VDC is the battery or DC bus voltage. Very often, this effective voltage is not modulated to approximate a sine wave. Instead, it follows a simple six-step commutation scheme. If you are not familiar with six-step commutation, please look it up using a search engine. Please don't be offended if you already know all about this. (I have no way of knowing what you know or don't know).

If you are able to control VDC rapidly using feedback, there is no real benefit to using PWM control at all. Losses will be lower if you simply turn the FET's on and off fully once per commutation state. There are many, many control systems out there that do not try to approximate a sinusoidal waveform. They are only using PWM to match the effective applied voltage to the back emf.

You mentioned conductive vs switching losses. The problem is that during PWM, the conductive loss (due to Rds(on)) is always present, because the motor current always flows through a FET. Each time the FET's switch, additional losses are incurred due to charging and discharging FET gates, and also there are resistive losses in the FET channel due to transition from low resistance to high or vice-verse. So to a certain extent, there is no trade-off here. Lower frequency is more efficient in this special case where VBUS is controlled.

Please note that when you apply a voltage to the motor which is very different from the back EMF voltage, it may result in very high current and torque. Essentially, setting the voltage sets the speed of the motor. Acceleration is the derivative of speed. If you change speed rapidly, it can lead to violent action. So if you rely on DC bus voltage to set speed, you need to vary it from very low (near zero) up the maximum, and if the motor speed changes due to sudden load change, you need to respond very quickly by changing the DC bus voltage to match. It may be worthwhile to add some form of current sensing so that you can treat sudden over-current as a fault condition and possibly respond in some fashion.

Finally, note that there may be conditions where you end up with regeneration current being delivered from the motor to the DC bus. The DC bus needs to have some way of accommodating this without blowing up. Regeneration can occur if you need to stop the motor fast, or if an external force (like another motor, or a falling mass) is driving the motor and you attempt to reduce speed. In short, there are a lot of system-level issues here that need to be considered, but are probably outside the scope of what you asked.