Motor heat , sinewave vs square

[Jestronix]

How much heat energy is lost when using a square wave controller vs sinewave . I know on inverters square wave cause motors to run hotter, but is this true on controllers?

 

[nutnspecial]

Wouldn't that parallel directly to the efficiency of each? Aren't sinewave more efficient?
Heat would be of negligible difference unless you were really pushing the controller though I think.

 

[Lebowski]

the motor current should be sine wave (so only fundamental) for best efficiency...

 

[Punx0r]

It would appear that it's a variable factor depending on the ratio of PWM frequency to motor inductance:

PWM and motor Heating

A popular 'old wife's tale' is that PWM causes the motor to heat more than pure d.c. Like most old wives' tales, this springs from a partial truth nurtured by misunderstanding. The 'myth' comes about because, if the frequency is too low, the current is discontinuous (or at least variable over the pwm waveform) because the motor's inductance cannot maintain the current properly during the off period of the waveform. So the motor current will be pulsed - not continuous. The average current will determine the torque but heating will be an integral of the current squared (heating is proportional to I²R)- the 'form factor' of the current will be greater than unity. The lower the frequency, the higher the ripple current and the greater the heating.
So consider an oversimplified case where the current is either on or off. If the current flows for, say, 1/3 of the time and you require a torque from the motor equivalent to that given by 1 amp continuous, them you clearly need an average current of 1 amp. To do this with a 33% duty cycle you must have 3 amps (the current flows for for 1/3 of the time).

Now a current of 3 amps will give 9 times (I squared) the heating effect of 1 amp continuous.

But if 3 amps is flowing for only 1/3 the total time - so the heating in the motor is 9 times for 1/3 the time - or a factor of 3 greater than the steady 1 amp! This waveform is said to have a 'form-factor' of 3 (or is it 33% - no doubt someone will put me right!)

However - if the pulse repetition frequency is high enough, the motor's inductance will cause a flywheel effect and the current will become stable. For instance the Lynch motor has an inductance of only 39 microhenrys (being one of the lowest inductance motors I know of) and a resistance of 0.016 Ohms. The 'Time constant' for an L-R circuit is L/R which (for the Lynch motor) gives 2.4 mSec. For an SEM DPM40P4 (1kW) the inductance is 200 microhenries and the resistance 40 milliohms - giving a time constant of 5mSec.

As a rule of thumb and to avoid too much maths, the pulse repetition period must be significantly shorter than the motor's time constant.

Other factors that affects PRF are:
If is it is in the audio band the motor can emit a whine (caused by a phenomenon known as 'magnetostriction', so keep above the audio band.
A MOSFET circuit dissipates most while switching from one state to the other so the frequency should not be too high - MOSFETs can be used upto 100kHz with care, but this is getting a bit high.
RF emissions: these increase with increasing frequency, so keep the frequency as low as possible!

It is clearly difficult to chose a 'best' compromise between these but an optimum frequency would seem to be around 20kHz.

http://www.4qdtec.com/pwm-01.html

I suspect that if the PWM current is discontinuous this would also manifest in increased torque ripple?

 

[liveforphysics]

How much heat energy is lost when using a square wave controller vs sinewave . I know on inverters square wave cause motors to run hotter, but is this true on controllers?

The added heating would be precisely the efficiency difference between the two operating strategies.

So, if you're running say 1kW, and trap drive is say ~85% efficient, and you switch to sine drive and it's ~90% efficient, then trap is heating the motor with ~150W of heat the shed, and sine is heating it with ~100W of heat to shed.

Those values of improvement are actually realistic for many motors, but if the motor happened to be created to have trap-wave shaped BEMF, then switching from trap drive to sine drive could even hurt efficiency rather than help it. That said, a motor designed to have trap BEMF is going to suck at everything anyways.

 

[Jestronix]

How much heat energy is lost when using a square wave controller vs sinewave . I know on inverters square wave cause motors to run hotter, but is this true on controllers?

The added heating would be precisely the efficiency difference between the two operating strategies.

So, if you're running say 1kW, and trap drive is say ~85% efficient, and you switch to sine drive and it's ~90% efficient, then trap is heating the motor with ~150W of heat the shed, and sine is heating it with ~100W of heat to shed.

Those values of improvement are actually realistic for many motors, but if the motor happened to be created to have trap-wave shaped BEMF, then switching from trap drive to sine drive could even hurt efficiency rather than help it. That said, a motor designed to have trap BEMF is going to suck at everything anyways.

That's a bit to take in . , so a controller that is sinewave and say 5% more efficient, that being energy in vs out. But what are the efficiency losses at motor based on square vs sine ? Which is different for different motor types I guess, there could be another 5% or more in this area ?

 

[Punx0r]

Luke was talking about the motor efficiency (85 Vs. 90%)