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3 phase PWM

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Avi

Mon Apr 09 2007, 06:43AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Does anyone know how the timing diagrams for 3 phase pwm should be both graphically and mathematically, to opeate a 3 phase motor?
I found a graph on google and edited+added to it but I dont know if it is correct specifically the first phase if it ever requires an offset.
Will what's in the graphic at any point become out of phase with the motor?

Electroholic

Mon Apr 09 2007, 07:05AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

if you want to do pwm, you will need 3 halls to sense the position of the rotor.(assuming its a brushless 3 phase in star configuration) then you will know which phase is suppose to be on, and then you pwm that.

ragnar

Mon Apr 09 2007, 07:06AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Welcome Avi,
For three-phase motors, the PWM has to be applied to all the phases sequentially, right?

I'm not sure I'd understand why you'd ever require an offset -- normal turn-on and variable turn-off times / duty, each cycle, seems like the only way to me..? Jeez, this actually feels quite complex -- I'm having trouble getting my head around H-bridges for more than one phase.

Electroholic -- Avi already knows how to, and has driven, a three phase motor, I think he wants to know if there are exceptions involved in PWMing it.

Avi

Mon Apr 09 2007, 07:41AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Please note, i am not asking about motor or circuit design.
But i should point out that it has 3 hall sensors each with inverting and non inverting outputs.
the motor coils are a star configuration (with common not connected) but i would assume correct phase driving is importaint with all types of multiphase coil configurations.

I am questioning the timing diagram of the 3 PWM channels and how they relate to eachother and the hall sensor timings.

the PWM has to be applied to all the phases sequentially

all PWM channels are updated at the same time, the on and off points can be changed reletive to the cycle period length.

I'm not sure I'd understand why you'd ever require an offset

changing the offset would cause a potential difference between any 2 channels to either be present or not present. Additionally it also may relate to an amount of physical angle error with the motor, but im not sure.

Electroholic

Mon Apr 09 2007, 10:53AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

i still don't quite understand your question.

if you want to generate a 3 phase signal using pwm, there are two ways to do it. Edge aligned, and centre aligned.

if you want to vary the speed of a 3phase motor, its just the fundamental frequency of your signals.

if you want to vary the torque, then you will have to play with the PWM within the phases, to moldulate the pk-pk voltage of the phase signal.

and most of the time, the halls are designed/positioned to give a signal with 50% dutycycle and 120degree out of phase.

Bjørn

Mon Apr 09 2007, 11:25AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

My feeling is that the reference point should not be the rising edge of the first pulse, but the middle of the first pulse and use dual edge PWM. If not you will get phase differences that are not optimal.

Imagine the pulses in the middle of sinewawes, if you stretch the first pulse it will not longer be in the middle of the sinewave unless you stretch both edges.

Steve Conner

Mon Apr 09 2007, 12:49PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The textbook way of doing this (AFAIK) is to synthesize three sine waves 120 degrees out of phase by using PWM with a carrier frequency much higher than the actual motor drive frequency (aka modulating frequency) you're synthesizing. Then the phase relationship between the PWM carrier and the modulator doesn't matter, since they're not even the same frequency.

In your diagram, you have the carrier and modulating frequencies the same, so the question of phase becomes important. My solution would be to bump up the carrier frequency so you have, say, 5 pulses per half cycle of the modulator.

If you are driving a "brushless" synchronous motor with Hall sensor feedback, then the modulating waves need to be kept in a certain phase relationship with the outputs from the Hall sensors to make it work efficiently. IIRC, the motor works best when there are nearly 90 electrical degrees between the rotor and stator fields, since then the motor is giving the most torque it possibly can without slipping out of sync. An ordinary sync motor needs a wide safety margin to prevent this, but of course one with Hall sensor feedback can't slip out of sync.

I believe this is done by the motor designer physically putting the Hall sensors in the right place, such that their outputs can drive a 3-phase bridge directly. This gives square wave drive, not sine wave.

In this case, the best way to apply PWM is to keep the original drive circuit, and chop the DC bus voltage to the bridge at a frequency considerably higher than the highest speed you expect the motor to go. That's how a lot of inverter drives used to work before DSPs (and programmers!) got good enough to do the required math in real time, such that the 6 devices in the 3-phase bridge would do the PWM too. The inverter synthesized three square waves 120' apart with constant duty cycle, and a buck converter ahead of it adjusted the DC link voltage to change the motor speed. If it was an induction motor drive, the inverter frequency was changed along with the DC link voltage.

Bjørn

Mon Apr 09 2007, 01:09PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The controller in this case is a 60 MHz ARM7 with dual edge PWM in hardware. The main problem is to find the formulas for calculating the values.

Steve Conner

Mon Apr 09 2007, 01:19PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, if I understood right, and you're going to have carrier and modulator frequencies the same like that picture suggests, and you're determined to use a DSP in the loop (though it's huge overkill since you have the Hall sensors: you should literally only need a TL494)

The centre of each pulse should correspond with the centre of the "High" portion of the square wave from the appropriate Hall sensor.

The width of each pulse you set depending on how much torque/speed you want. The pulsewidth does not relate directly to torque or speed, though. Instead, the motor will have roughly the same torque/speed characteristic as a brushed PMDC motor driven by a PWM'd DC voltage.

Starting the motor smoothly from a standstill, when none of the Hall sensors is giving output and you don't know the rotor position, is an exercise for the reader

Avi

Mon Apr 09 2007, 01:56PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

The hall sensors are analogue, they output a sine wave. Iâ€™m not sure what voltage they are designed for but it seems to output nicely in the middle from a 3.3v supply.
The graphic is not very realistic however because they all show the same duty.

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