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

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, March 11, 2004 12:45 AM
Subject: Re: [CAD_CAM_EDM_DRO] LARGE Gantry table: Geckos on large servos.


>
> Kim Lux wrote:
>
> >Comments below.
> >
> >On Wed, 2004-03-10 at 11:53, Jon Elson wrote:
> >
> >
> >>The Gecko doesn't have output filters, which causes 80 V (or whatever
the
> >>DC supply voltage is) square waves to be applied to the motor's
terminals
> >>at all times.
> >>
> >>
> >
> >I think you mean whenever the motor is commanded to move by the drive.
> >You are saying that the drive controls the motor velocity/power with
> >pulse width modulation and that the voltage applied is the supply
> >voltage, ie 80V in this case.
> >
> >
> >
> 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. In other words the motor
would
always disapate the maximum heat even when it is stopped. Not a very
efficient system.

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.

> >If the motor had 80V across it all the time, it would be moving all the
> >time.
> >
> >
> But, it reverses polarity 25,000 times a second. The net current
> averages out
> to zero.
>

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

> >
> >
> >>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.

> >
> >It may also cause high currents to flow just to
> >the windings if the capacitance to ground is low, or if there are EMI
> >filters in the motor. It might be necessary to put an inductor in series
> >with the motor.
> >
> >
> >
> >I'm not following this argument.
> >
> >
> If the motor's inductance is too small, the triangle wave current
> becomes large.
> The solution is to increase the inductance with a series inductor.
>
> >
> >
> >>Finally, if the Gecko failed when stopping, the problem is due to the
> >>back EMF of the spinning motor PLUS the extra voltage created by
> >>currents in the motor inductance produces a DC supply voltage GREATER
> >>than the DC supply normally produces. In other words, during
deceleration
> >>of large motors and heavy inertial loads, the bus voltage can rise to
> >>dangerous
> >>
> >>
> >
> >I've had this argument with other people. Here is my understanding:
> >
> >a) the terminal voltage of a DC <servo> motor is equal to the IR "drop"
> >of the armature, plus the KV product of the motor's speed.
> ><big snip>
> >
> >Therefore the terminal voltage during deceleration can never really be
> >greater than the supply voltage.
> >
> >
> This is true in a completely linear system. But, this is NOT a linear
> system,
> it is a switched-mode system, and inductance becomes VERY important.
>
> So, the motor can be thought of as the ideal DC motor, as you described
> above,
> PLUS an inductor in series with it! Now, if you are extracting energy
from
> the spinning motor, you are applying the reverse polarity from the power
> supply across the motor for short periods, and allowing the current to
> build to a large value. Then, you reverse the polarity for a while as
part
> of the PWM switching cycle, so that the motor back EMF and the power
> supply are of the same polarity. But, there's also that inductance with
the
> large current in it. As the polarity reverses, the like polarity of the
> two sources (DC supply through transistors and the back EMF) would
> act to reduce the large current in the inductor. AND, when you reduce
> the current flowing in an inductor, you get a large voltage which acts
> to raise the motor terminal voltage appreciably, as it forces a large
> current back into the power supply.
>
> >Think of the decelerating motor as a generator being turned at a
> >constant speed and the deceleration being a bigger and bigger load on
> >the motor. As the load increases, the terminal voltage drops.
> >
> >
> But, if you were to short the motor for an instant, and then when the
> current
> became very large, break the circuit, you'd get a big spark! This is
> exactly
> what happens in the simple PWM servo system every PWM cycle.
>
> These systems are always on, one polarity or the other. The PWM refers
> to the duty cycle of the waveform. There are other designs that provide
> for the source transistors to be off for part of the cycle, but the
> Gecko 320/340
> is not of that type.
>
> Jon
>
>
>
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