Re: [CAD_CAM_EDM_DRO] Re: capacitor

mariss92705 wrote:

> Jon,
>
> OK. "When in doubt, run an experiment" is my motto, so I did. Let's
> start with the stepper first.
>
> The stepper experiment:
>
> (1) Take two identical step motors and couple one to the other via a
> flexible coupler. In this case the motors are double-stack 34s, 4A
> per phase, 4-wire.
>
> (2) Connect a drive (Gecko of course) to one motor and use a 24VDC
> power supply.
>
> (3) Connect a 'scope to a winding on the undriven motor.
>
> (4) I run the combo up to a speed just short of 5,000 full steps per
> second at 24 VDC. The reason I stop is I don't want to bust my fine
> Tektronix scope; the voltage across the undriven motor reads 400
> volts peak-to-peak at 1.25 kHz!!!!
>
> (5) I switch over to a sturdier (600VAC max) multimeter and get 800
> volts peak-to-peak, 400V peak, or 283 VRMS at 10,000 full steps per
> second where the whole mess stalls. All this from a 24VDC supply.
>
> How is this possible? Hint: it's the phase relationship between the
> motor's lagging shaft position and the driving voltage at high speeds.

Ah, then this would be an unloaded situation, only. If the motor was
being decelerated by a drive, the voltage would be much lower, I think.

>
> The DC servomotor experiment:
>
> (1) Take a large size 42 servomotor and hook it up to a drive (G320).
>
> (2) Toggle the direction input at a 1Hz rate.
>
> (3) Connect a DC power supply (60VDC) thru a rectifier to the drive
> so no reverse current can flow.
>
> (4) Put a 'scope across the supply pins at the drive to monitor the
> voltage. Remember, no current can flow back into the supply, so this
> should a cause tremendous voltage spike during deceleration.
>
> (5) Increase the speed on the motor until it is pulling 20A during
> decel/accel on each direction reversal.
>
> Rather than a voltage spike, the scope shows a 5VDC dip in the 60VDC
> line on each reversal! This is due to the voltage drop as 20A passes
> thru the 18 guage wire I am using for this lash-up.
>
> Where did the energy go that is returned during deceleration? It was
> dissipated in the motor's armature resistance and the drive's
> MOSFET "on" resistance. Exactly the same way as it was delivered to
> the motor during acceleration in the first place. It is symmetrical.

First, you only have armature inertia there. It is a completely different
story when you have 300+ Lbs of machine table moving. I never
exercised my servo amps much on the bench. Just a few quick checks
and I put it on the machine. (I'd been using a modified stereo amp
previously, which didn't work well at all, but it moved the table.)

I can tell you that energy does flow back to the power supply on my
system. I won't put the diode in, because I KNOW it will blow up
my amps. I'll see if I can get a reading on it sometime soon.

Jon