At 07:35 12/12/00 -0500, Kirk S. Hegwood wrote:
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>>If this is a small 12VDC conventional brush motor ...
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>[Kirk S. Hegwood] It's not a brush motor.

It's not? This makes a big difference then and you may need to disregard some of the things I said previously. If it is not a brush motor, then it must be a brushless or stepper or switched reluctance (or some other such thing) motor (since I rather doubt you have a homopolar motor). If you are supplying the drive electronics with 12VDC rather than just driving a
simple brush motor then some of my previous comments need revision.

First, I suggested that a DC power supply needs to be current limiting. This will work very well for a conventional brush motor. However,
if you are supplying drive electronics rather than just a motor, then when the voltage drops (due to current limiting) the electronics may not operate properly. That is, the voltage may temporarily drop below the minimum operating voltage for the electronics and the motor will never start.

The other thing is I suggested an unregulated supply using a transformer and rectifier. Again, if you are supplying drive electronics there is a good chance that the voltage ripple from this will be too high for the electronics to tolerate. There should be a rating for this in the motor drive system specifications.
If you try to overcome this by adding various filtering, then you are just basically making your own power supply and may as well just buy one.

I'm not really sure I understand the motor system you have. If you have some sort of brushless (or other) motor, and the motor and drive electronics are both supplied from 12 volts then there could be several problems which are affected by the design of the drive. I suggest you ask the company which manufactured the motor-drive system what size and type of power supply they recommend. I could guess at several different solutions, but I don't have any data to base it on, and I would just be guessing.
For example, when you power up a small brushless DC motor/drive, the drive electronics may draw a brief peak of current to charge its capacitors. The actual motor starting current though may be relatively low as the drive
electronics acts to limit the current in order to protect *itself* from excessive current. This is just one example of what may happen. I can't make
any more specific suggestions without knowing more about the motor.

I would be very interested in more information on this motor. The only small 12VDC brushless motor/drives I am familiar with are used in a few automotive applications and it doesn't sound like you have one of those. What is this thing? Do you have a manufacturer's name I can search for?
The small industrial brushless and stepper drives I am familiar with are all powered from 120VAC and therefore have their own motor power supply built in. I think I saw some literature a few years ago on a 24VDC brushless
motor/drive, but I don't recall who made it.


**********************
Michael Griffin
London, Ont. Canada
[email protected]
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Do away with the power supply. You can try with a bridge rectier connected to a step down transformer and a filter circuit to supply 12V DC. For your motor protection use an external thermal overload.

Pradeep Chatterjee
M-30/7,
TELCO Colony,
Jamshedpur-831004 (India)
Email: [email protected]
Ph:91-0657-487549 (R)

 

Hi
I can suggest a simple light weight cost effective way to solve your problem if you do not need to start the motor at full speed. You can use SGS L298 with a pwm circuit to control the speed. This way you can start the motor at low speed and than change duty ratio to increase the speed of the motor. When you reach full speed disconnect L298 from the motor by relay contacts. You need the relay as with L298 you can never reach full speed because of the voltage drop on the bridge.You can do away with the relay if you can provide a higher voltage than 12v to compansate the voltage drop. Another way would be to build another step up regulator say from 12 to 15 volt with a current limit.