3 phase BLDC motor drive waveforms dont look right?

[treez]

Hello,

The following file "3ph BLDC" is an LTspice simulation of a 3 phase BLDC. the three other text files are the waveform description files of the six transistor gates, which need to be in the same folder as "3ph BLDC" when the simulation is run.

The waveforms simply do not look right, do you know why?

 

[FvM]

The waveforms simply do not look right.

Why? Which waveforms do you expect?

 

[treez]

The voltage on one of the phase coils is like this....
https://i42.tinypic.com/2dhcg78.jpg

its not symetrical , and should be like the 2nd page of this....

https://ww1.microchip.com/downloads/en/appnotes/00857a.pdf


..is it because I need to insert a voltage source for back emf?

 

[FvM]

Just wrong gate control sequence. The high side must be delayed by 1/6 period in each phase.

 

[treez]

Thanks FvM, tho' the individual coil voltages still look lopsided...it does look better though....(have I interptreted yourself correctly?....attached is the updated simulation and the three extra phase shifted gate drive files).

https://i43.tinypic.com/154fuxv.jpg

 

[FvM]

Timing is still inexact. All switching actions should take place at multiples of 5 ms (1/6 period), but they don't.

 

[treez]

Should each fet "on time" be T/6, and each fet duty cycle be 1/6?

 

[FvM]

No, the on time in the MHCP application note is 2/6, as in your simulation. And the phase shift between LS and HS should be 3/6, but it isn't.

 

[treez]

Thanks, I think its like page 4 of this...as you say...

http://documentation.renesas.com/doc/products/mpumcu/apn/reu05b0074_r8cap.pdf

..These are the sim and gate drive files now......the motor winding inductance rings like mad....what stops this happening in the real motor?

Also, here each fet has a duty of 33%.....is there a running method whereby each on pulse has the same centre point, but is only on for say a 20% duty , so as to reduce torque or speed?

 

[FvM]

The duty cycle might be varied between 0 and 50 %, or you can use PWM as in the Renessas AN.

The ringing is promoted by the far-off snubber values. Try 10n+4k to get an idea of achievable snubber effect.

 

[treez]

do you know how to calculate how high the current can get in a fet in a 3 phase drive....I mean in a full bridge smps its calculable, i'm not sure how to do it with a BLDC motor.

 

[FvM]

You can't calculate it without a motor model including the "external" emf.

 

[treez]

by external emf you mean "supply voltage"?

 

[FvM]

"External" emf means the generated motor voltage, the part of the motor equivalent circuit that can't be modelled by passive elements.