Re: [CAD_CAM_EDM_DRO] Re: Hot servo motor

k8zre wrote:

>That is a good thought. Will check. However, the servo gets hot
>just idling (no movement).
>

The problem is that the servo drivers have finite gain. If the axis and
motor
can move freely, it will move the motor to within one encoder count or
so of the commanded position, and the motor current will drop to nearly
zero. If the axis is stiff for some reason, the Rutex will deliver less
current
as the motor gets closer and closer to the commanded position. At some
point
the friction will prevent any further motion, and it will stall there.
If the
friction is substantial, then the encoder will continue to indicate some
distance from commanded, and the motor will be sent a steady current,
but not enough to break the static friction. If this current is a few amps,
then the motor will get hot.

> The Y axis seems to run smoothly without
>apparent binding by the hand wheel (servos off). I will be swapping
>drivers to the board to see what happens...maybe scope the motor
>outputs.
>
>

Scoping the motor outputs is difficult. Definitely don't connect the
scope ground
to either terminal. You will see a 100 V square wave with a varying
duty cycle.
You can connect a 10 A ammeter in series with the motor to see the average
current. It should be very close to zero at idle. You can apply a
little torque
to the shaft and see the current rise as the drive resists your attempt
to move
it off the null position. The current should return to zero when you stop
deflecting the motor. I'm guessing under some conditions that the
current does
not drop to zero. If the DC current does appear to drop to zero at
idle, then
the drive must be dithering at a very high rate. This usually is audible on
low-freq PWM drives. Most Rutex drives run at 19.5 KHz, so the highest
dither frequency would be 9.75 KHz, which I would certainly hear, and find
maddening.

These PWM drives that use the 50% duty cycle scheme can send a fair amount
of PWM carrier ripple current through the motor, and it can cause heating on
low inductance motors. but, if your motors are identical, then the ripple
current should be the same in all of them.

Jon