Re: [CAD_CAM_EDM_DRO] LARGE Gantry table: Geckos on large servos.

Peter Reilley wrote:

>>>On Wed, 2004-03-10 at 11:53, Jon Elson wrote:
>>>
>>>
>>>
>>>
>>>
>>Yes, at idle there is a square wave where 50% of the time there is 80 V
>>across
>>the motor one way, the other 50% of the time the polarity is reversed.
>>
>>
>The
>
>
>>motor experiences a triangle current wave of some amplitude determined
>>but the applied voltage and the motor's inductance (and the switching freq
>>of 25 KHz.)
>>
>>
>>
>
>I think that you are wrong on that one. While it could work that way, i.e.
>that motor
>would not move, the IR loss would be maximized.
>

Believe me, I have looked at this on an oscilloscope, it DOES work this way.
With suitable motor inductance, the current deviation on each cycle is quite
small, so the iron losses are not great, and the IR loss is controlled
entirely
by the RMS current, and not affected by frequency, peak to peak deviation
or anything like that. The RMS current is usually an amp or less except
with
very low inductance motors, like printed or pancake motors.

> In other words the motor
>would
>always disapate the maximum heat even when it is stopped. Not a very
>efficient system.
>

The maximum heat would only occur when a high current flows through the
motor. At 25 KHz, the half cycle is 20 uS. How much current rise will you
get applying 80 V for 20 uS? Assuming a 1 mH motor, it comes to 1.6 A.
For a motor with a peak rating of 15 A, that is not going to be a big
problem.

>
>There are three states in the drive; full on plus, off, and full on minus.
>When there motor
>is stopped there is no pulsing of the power. When the motor is moving the
>power is pulsed
>with one polarity only. The duty cycle of that pulsing determines the
>power output of the motor.
>

I have designed a servo drive which DOES work this way. But, I assure you
that the Gecko 320/340, and a HOST of other servo amps do NOT work like
this.
I'll mention such amps as Servo Dynamics, Copley Controls and Westamp for
examples.

>>
>>
>
>The current would average to zero but the power loss would not.
>

Because of motor inductance, this does not happen. If the motor was purely
resistive, then you would be completely right. But, I know of no electric
motor that has no inductance.

>
>
>
>>>
>>>
>>>>This may cause high circulating currents at 25 KHz to flow
>>>>through the transistors.
>>>>
>>>>
>>>>
>>>>
>>>I think the only time current should be flowing through the <output>
>>>transistors is when power is being supplied to the motors.
>>>
>>>
>>>
>>Yes, seems reasonable, but MOST PWM full-bridge driver schemes use
>>the 50% duty cycle scheme.
>>
>>
>>
>
>This would inply that the power supplies must supply full current all the
>time.
>They do not.
>

No, because the inductace stores energy and then returns it when the current
reverses, there is VERY little loss, other than magnetic hysteresis in the
armature iron.

>
>

If I'm wrong on this design detail of the Gecko 320, I'm sure Mariss
will correct
me.

Jon