Re: [CAD_CAM_EDM_DRO] Need help and motors, and quickly...

Weyland wrote:

>On Tuesday 14 October 2003 02:01 am, Jon Elson spake:
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>>I don't understand the problem. What is the voltage/current rating on
>>these motors? (If you don't have that, what is the winding resistance?)
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>>From the data sheet -
>Toque @ 50 sps = 540oz/in
>Holding torque = 635oz/in
>Detent torque = 22.6oz/in
>Phase current = Unipolar = 3.5;Bipolar = 5.0
>Phase resistance = 1.29 Ohms
>Phase inductance = 3.3 Ohms
>Rotor inertia = 17.4oz/in
>Power disapation = 30 Watts
>These are eight wire motors.
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>>What current do you have the Gecko drives set to?
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>I've tried both 3.5 and 5.0 Amps.
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OK, at 3.5 A and the 1.29 Ohms, that is 4.5 V Thats not as bad as
I feared, but it is pretty high. Do you have these motors set up in
series or parallel? You should get better performance in parallel
connection. You should even do better in single-winding mode, that
simulates the unipolar drive. Some of your specs don't make much
sense. Inductance is measured in Henries, not Ohms. Rotational
Inertia is measured in torque/sec^2, so i guess that is 17.4 Oz-In/sec^2.

>...hence Mariss's explanation that these are possibly
>the worst motors I could have bought for my situation...
>

Well, I don't know. A number of people are using very similar motors,
and although they don't get wonderful performance, they are getting a
lot better results than you are. The oldest Bridgeport Boss-3 machines
used real Superior Electric Slo-Syn motors that were designed and sold
as low-speed synchronous motors. On some of them you could still find
the label that said 72 RPM, 120 V, 60 Hz. Bridgeport opened the motors
up to rewire them as stepper motors. They get feedrates up to 60 IPM
or so on a full-size Series I Bridgeport. (Admittedly, these machines
had ballscrews.) So, I am concerned that you may have another problem,
and spending a lot of money on new motors may not fix it. The possible
causes I can think of are a power supply with too little capacitance to
deliver a steady voltage, or step pulses with a lot of timing jitter.
One other possibility is feedback from the motor wiring to the step
inputs of the drivers.

I should mention that I am not using software-generated step pulses,
but my own stepper controller board. It puts out a smoother train of
step pulses than software can do. It shouldn't make any difference at
the speeds you are experiencing trouble at. But, then, it seems to me
that these motors you have should be able to do much better than you are
getting.

Jon