Re: LARGE Gantry table: Geckos on large servos.

Peter,

Jon is right; it does work that way and nicely too. Here's the math:

Motor: 5 mH, 2 Ohm, 80VDC rated (about a 500W output motor)
Drive: 25 kHz switching frequency

Ripple current (p-p) = V / 2*f*L = 80 / 2 * 25,000 * 0.005 = 0.32A
RMS current (triangle waveform) = I (p-p)/4 = 0.08A
I^2*R heating = 0.08^2 * 2 = 0.0128 Watts

The motor will be stone-cold when stopped and unloaded.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Peter Reilley" <micrio@m...>
wrote:

>
> ----- Original Message -----
> From: "Jon Elson" <elson@p...>
> 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
> >
> >
> >
> > Addresses:
> > FAQ: http://www.ktmarketing.com/faq.html
> > FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
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