BLDC Dead Spots

[jimmythefool]

Belated Happy New Year!

I have built a sensored BLDC controller based around the Fairchild FCM802 and FAN7382 MGDs. The motor is an 8 pole 24v sensored type.
Running, the controller works fine, but I have been having some starting problems. There are 24 steps per revolution. 16 of the steps are ok, but there are 8 steps in 4 equally spaced pairs, where the motor will not start. Is this normal? I dont think the FCM8202 has a way of 'jogging' the motor into a good spot before starting.

This is my first attempt doing a bldc driver, and any experience would be gratefully received

 

[dr pepper]

I'm not totally certain but if its sensored it ought to pull away whereever it stops.

Have you checked for an open coil?

What is the feedback, is it hall effect devices or a magnetic disc or whatever

 

[jimmythefool]

Hi Dr P. Thanks for the reply.
It's a hall sensor type.
I'm pretty sure it's not a missing phase problem. I did try disconnecting each phase, and the result is obvious. The motor is running too smooth to have a missing phase I think.

 

[dr pepper]

I didnt mean a whole phase missing, depending on how many poles the motor is there will be multiple coils, if only one of them fails you wont neccesarily notice any difference in smoothness, but the motor will be hard to start if that particular coil is next up on startup.

 

[Mr RB]

It could also be one of the 3 sensors failed, will give a similar starting symptom and some running functionality.

 

[jimmythefool]

Thanks for the replies guys. I am going to look at it again this aftermoon.

Yesterday I stepped the motor round manually on a power supply by switching the phases by hand. It stepped round in 30' increments as expected, and needed 12 steps to complete 1 mechanical revolution. When I have it running on the driver board though, it seems to have 24 steps. This is where the problem occurs. There are 4 equi-spaced positions where the motor appears to stop between steps, and the Hall signals are '000, so when it comes to starting again, there is no output. Moving the motor one notch, and it works fine.

Any ideas welcomed. Please feel free to state the obvious, this is my first BLDC controller, so i wont be offended!

 

[shortbus=]

Are you sure that you operating in sine wave mode and not in square wave mode?

 

[Mr RB]

... There are 4 equi-spaced positions where the motor appears to stop between steps, and the Hall signals are '000, so when it comes to starting again, there is no output.
...

That does not sound right, the hall outputs should have about 50% duty cycle (N and S poles) and the 3 sensors should overlap (ie like quadrature, but "hexrature"? I made that word up);

001
011
010
110
100
101

or;

______~~~~~~
____~~~~~~__
__~~~~~~____
~~~~~~______
~~~~______~~
~~______~~~~

There should not be any position where all 3 sensors read the same.

 

[dr pepper]

Hexrature, I'm going to blag my boss with that next time I cant fix something.

 

[jimmythefool]

Many thanks for the replies, greatly appreciated.
Had a manic week, but I am going to sit down for a few hours on Sunday, and look everything over, and go through all your suggestions .