Why won't my motor spin?

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Robert Reed
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Why won't my motor spin?

Post by Robert Reed » Sat May 17, 2014 9:34 pm

Have a blower motor in a piece of equipment that refuses to start at power up about 20% of the time. Did some investigation and here is what I found: It is a four pole brushless motor with two imbedded hall devices placed 90 degrees apart in close proximity to the axis of the shaft in which has a PM attached to it. These hall devices each drive a pair of transistors to supply current to the coils at the proper time. One hall device appears to be defective which is probably the cause of inconsistent starting due to a dead spot for position sensing (about 20-30 degrees) in that area. When the motor does run it looks like the correct speed which is 3000 RPM. A scope check showed 5 millisecond pulses to each coil in turn and a 20 millisecond time for one complete turn of the motor. However out of the four 5 millisecond pulses that should have been present, two looked quite weak (again the defective hall device?). There is no way to get at the imbedded hall devices, so an exterior drive method was needed. I have no experience with brushless motors or hall devices, so my assumption was to design a quadrature pulse generator with 5 millisecond time duration. By directly injecting a DC current into one coil at a time, I was able to determine direction and sequence of the drive circuit. Seemed simple enough. I built the generator with the correct rotation and pulse width and current requirement, connected it and was set to go. Then applied power and to my dismay the motor just sat there and hummed but went nowhere. Lacking a good knowledge of these devices, I don't know why it does not work as set up.

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jwax
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Re: Why won't my motor spin?

Post by jwax » Sun May 18, 2014 5:19 am

I'm guessing the motor shaft spins freely, with no rough spots?
When you power it up with your supply and it just hums, can you manually spin it to get it started?
John

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Re: Why won't my motor spin?

Post by Robert Reed » Sun May 18, 2014 9:54 am

Yes John the motor runs very free. Manually rotated it in small increments before power up and still nothing. Out of desperation, used a power drill with a rubber snubber attached and made gentle contact with the motor shaft to get it up to speed-still nothing! Would a simplified schematic help?

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jwax
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Re: Why won't my motor spin?

Post by jwax » Mon May 19, 2014 7:24 am

I'm not intimate with the brushless motor either, but sure- maybe a schematic would help Robert.

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Lenp
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Re: Why won't my motor spin?

Post by Lenp » Mon May 19, 2014 9:26 am

With a multi channel scope do you see any overlap of the pulses? Maybe 1,2,3,4 is wrong
Maybe it should be 1,1&2, 2, 2&3, 3, 3&4, 4, 4&1... or some other pattern.

Just a thought!
len
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Re: Why won't my motor spin?

Post by Robert Reed » Mon May 19, 2014 9:42 pm

Lemp
Qx base drive pulse from Hall device is sequential with Coil x current. does not appear to be any overlap from coil to coil current. Will try to upload current and revised circuit. Revised circuit is a monostable ring style generator with tickle pulse start. All connections and timing match up with original operation.Image

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Lenp
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Re: Why won't my motor spin?

Post by Lenp » Mon May 19, 2014 10:28 pm

Hi
Looks like your design would work for a stepper motor but maybe the brushless is different.
At http://www.allaboutcircuits.com under AC chapter 17 there is some info that may help. I think the devil is in the details.

Question... Why not just replace the motor?

Len
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jwax
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Re: Why won't my motor spin?

Post by jwax » Tue May 20, 2014 4:01 am

My question is, does Hall #1 fire both opposing coils via Q1 and Q2 simultaneously, and Hall #2 firing opposing coils via Q3 and Q4 simultaneously?
The revised circuit looks as if it fires each coil sequentially, rather than a pair of coils each pulse.

Lenp- where's the fun in that? :)

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Re: Why won't my motor spin?

Post by Robert Reed » Tue May 20, 2014 11:48 am

John
The coils appear to fire one at a time. And to reinforce that action (and again this is just my take on it) If you look at either Hall device you will note either transistor base connected to opposite poles of the hall device. Since each hall will see a positive and negative field from the rotor for each revolution, it will shift its output 180 degrees for each revolution. This would indicate transistor base drive on one side of it for a north pole field and transistor base drive to the other side for a south field and operate each transistor only at those appropriate times. Of course as mentioned previously, one hall device is bad ( drive channel measures 60 ohms across it and the bias channel measures 4-5K ohms). So it seems like it has to fire sequentially because two poles "lit" at the same time and 180 degrees apart would make it bind up.

Len
To me it looks like the brushless motor is a stepper motor of sorts. If I manually ground each pole in turn the motor will step in 90 degree steps to complete each revolution. unfortunately, I cannot do this fast enough by hand to get any speed out of it for a more conclusive test. I had seen your web site and there is not much info on the particular four pole motor. But one feature I did see and this has been haunting me throughout the whole design is in looking at that web design, it appears that each pole is triggered not from its own sensor but rather from the sensor preceding it. In other words as momentum swings the rotor up to one pole, the following pole is being energized just before the rotor arrives there. And that would make sense as a motor is basically a revolving magnetic field with another field always trying to catch up with it but never quite making it due to slippage always being present to a small degree. This blower motor is buried in a special cavity with its output port mated up to a aluminum casting. That casting is loaded with high speed TTL & ECL chips and they are power hungry! So the air circulation is vital at all times. I cannot locate the original HP part or any thing similar that would fit the cavity much less meet the power requirements (about 2-3 watts).

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Re: Why won't my motor spin?

Post by Robert Reed » Tue May 20, 2014 7:52 pm

Hey John and Len
Looked at this problem from a different angle and need some backup opinions for my theory. As stated I scoped a consequent drive pulse for each coils energizing and all this time was assuming that the Hall detector was located on that pole piece. but in actuality, the detector could be located on any pole. Now I am thinking that the detectors that drive any given pole coil may actually be located on a pole 90 degrees preceding it. So when the magnet is attracted to a pole, by the time it reaches it , the Hall device on that pole is now putting out a drive pulse for the next succeeding pole and so on and so on. Inertia keeps the magnet sweeping past that pole as attraction is also pulling it toward the following now energized pole. In this manner we have the magnet always chasing the magnetic field yet never quite catching up to it which I believe is what we want it to do. There is no way I can prove this theory as I have no way of telling their physical location in relation to the pole pieces. What do you think?
If this is the case (and unfortunately) there is no external circuit I can use to accomplish this action. This could only be sensed internally with -guess what- a hall detector and that brings me back to the root of the problem- a defective imbedded Hall device. Me thinks I am screwed.

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Lenp
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Re: Why won't my motor spin?

Post by Lenp » Tue May 20, 2014 9:28 pm

We'll maybe it's time for some pictures and maybe a junk box raid will save the day!
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

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Re: Why won't my motor spin?

Post by dacflyer » Wed May 21, 2014 5:43 am

my experience with brushless motors has been limited, but the ones i seen were like 3-phase
design, 3 wire types with a controller circuit boars. these were R/C airplane motors,,
extremely high speed and very powerful. these were able to swing a 8-10" propeller easily,
and they did consume a lot of amps. the motor had heavy wire coming out of such a small package.

good luck on your quest.

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jwax
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Re: Why won't my motor spin?

Post by jwax » Wed May 21, 2014 10:31 am

One more reference-
http://www.pcbheaven.com/wikipages/How_ ... tors_Work/
Only one hall effect sensor is typically required. A second is added to tell the controller the position of the rotor from a stopped position.
Your pulse generator is not "smart" enough to tell the controller when to accurately pulse each coil?

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Lenp
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Re: Why won't my motor spin?

Post by Lenp » Wed May 21, 2014 11:17 am

Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

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Re: Why won't my motor spin?

Post by Dean Huster » Wed May 21, 2014 6:17 pm

Tektronix used similar circuit for the fans in the "new" 400-series portables. The transistors were in a Motorola array that just happened to be in a symmetrical 14-pin DIP. If the motor was hung, you just unplugged the array and put it in the socket backwards. If the motor then started on power-up, you could blame the array. If it didn't, you could usually blame the motor. Good, quick troubleshooting trick.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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