Re: capacitor

Jon,

As speed increases, the motor "falls behind" or develops a phase lag.
This is identical to the phase lag current has relative to voltage in
an inductor.

During deceleration this phase lag decreases. As it does, the
drive "sees" an increasing voltage because current and voltage become
more in phase.

Once this "in phase" voltage exceeds the power supply voltage,
reverse current begins to flow if the rate of deceleration is high
enough.

The servomotor situation is considerably simpler. Since the motor
cannot develop a "back EMF" greater than the power supply voltage, it
cannot return any current to the power supply (apart from the
relatively insignificant L*I^2/2 component). To do so would require a
Kv generated voltage in excess of what is available from the supply.

Rather what happens, the motor is driven with a torque (current) that
opposes the current motion, decelerating the motor. This current has
to come from the power supply, so current flow is always into the
drive.

If you are in for an inch then you are in for a mile. The difference
between the 300+ lbs you mention and the motor's moment of inertia is
a quantitative one. If there were returned energy, it should exist in
either case, only the duration would be different.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> 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