Re: Drive mfrs recommendations for MOTOR cables ?

Ballendo,

Excellent question! I just did some measurements so here's my
recommedations:

Ours, like most switching type drives, generates substantial
switching noise on the motor cables. Good practice is to have some
sheilding so as not to interfere with radios, TVs and other equipment
sensitive to RF interference.

STEP MOTORS:

Use a 5 conductor sheilded cable. Connect 4 of the leads as required
to the motor. Use the 5th lead to connect to the motor case at one
end and to your "single point" ground at the other end. Connect one
end of the sheild to this same "single point" ground. Do not connect
the other end of the sheild. See (1) for reasons.

Braid or foil, it makes no difference. If twisted pairs are
available, use one pair per motor winding.

Wire guage (within reason) makes very little difference. Using the
wire guage as the motor leads is usually sufficient. See (2) for
reasons.

Very long motor cables (>20ft to 200ft) present a special problem.
Series chokes of about 100uH should be used on each motor lead at the
drive end. See (3) for reasons.

SERVO MOTORS:

Use the same techniques for the motor wiring as for step motors. The
encoder wiring should be a seperate sheilded cable. Use a twisted
pair cable like CAT-5. For single-ended encoders, have only one
signal per pair, the other wire should carry ground or +5VDC. Again,
ground the sheild at one end to your "single point" ground. See (4)
for reasons.

(1)The windings on a step motor are on the stator. They form 1 plate
of a capacitor. The wire insulation and the bobbin they are wound on
forms the di-electric (insulator) of a cap and the motor case forms
the other plate of a capacitor. This capacitor can be as large as
3,000pF (measured) on a size 42 DC servomotor and stepper.

MOSFETs switch very rapidly and have a dv/dt of 2V/ns on the motor
leads. The capacitive current then is 6A (I=C*dv/dt). This pulse
current is substantial and must be returned to ground to insure it
does not interfere with encoders in particular.

(2) Wire gauge contributes to the total motor resistance and
inductance. Inductance can be figured at about 2nH per inch. On a
20ft cable this would amount to only 0.5uH per wire and 1uH for the
round trip. This is negligable compared to the 1mH to 50mH inductance
the motor has.

Resistance produces a voltage drop when current passes thru it. The
effect of resistance is voltage drop subtracts from the voltage
available to the motor.

I measured the resistance of some 22 guage, 5 conductor sheilded
cable (from Home Depot) I have around. A 10ft length measured out at
0.35 ohms for a round trip (10' out, 10' back). At 7A this resistance
would drop 2.45V and dissipate 17W.

At high speeds step motor phase currents for a 7A motor drops to
2.33A maximum. The voltage drop then becomes only 0.82V. This amounts
to 3.5% performance hit whe using a 24VDC power supply and about a 1%
performance drop at 80VDC. It can be neglected.

Heating in the cable is 1.7W per foot at 7A. This would make the
cable slightly warm to the touch but should not be a problem
otherwise.

(3) The same test cable as in (2) measures 600pF wire to wire
capacitance for a 10ft length, or 60pF per foot. The wire to sheild
capacitance was 100pF per foot.

This works out to 3,200pF of capacitance load on the MOSFETs for a
20ft cable. The resulting charge and discharge currents are large
(see 1) and can begin to fool some switching drive's current sense
circuitry. The result would be squealing and hissing noises from the
motor and possibly reduced phase current due to the drive prematurely
terminating the switching cycle.

Series chokes in the motor leads (100uH) at the drive serve to de-
couple this capacitance, allowing the drive to function normally.

(4) A length of CAT-5 cable I have measures out at 30pF per foot.
This would form an RC time constant of 3uS for a 10' length and 10K
pull-up resistors in a typical encoder circuit. This 3uS would be the
width of the cross-talk pulses if both conductors in the pair were to
carry both encoder channels. This may degrade the quadrature decoder
and cause false feedback signals. Use ground or +5VDC (encoder supply
power) on the other wire in the pair to prevent this cross-talk.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:
> Hello Dan, Dean, Mariss, Hans, Tim,?,?,
>
> I'm wondering what our list member step drive mfrs recommend using
> for cabling FROM their drives TO the motors?
>
> Shielded?
> Individually? Overall? Braided? Foil?
>
> Twisted pairs?
>
> RE: Gauge... Any modifications to the "standard" amps ratings as
> supplied by the wire mfrs when the wire is used for PWM drives?
>
> Effect of wire that is "too big", if there is any?
>
> Now how about a few specific apps (I've tried to pick
some "typical"
> values here):
>
> 12VDC PS, 2A drive, 1A motor rating, 3 foot cable length?
> Same with a 12 foot cable?
>
> 36VDC PS, 2A drive, 2A motor rating, 10 foot cable length?
> Same with 5 foot cable? 20 foot cable? 4A motor?
>
> 60VDC PS, 5A drive, 4A motor rating, 18 foot cable length
> Same with 5 foot cable? 6A motor?
>
> Does drive type (unipolar, bipolar, bilevel) affect these answers?
>
> Any other thoughts or considerations? (besides the need for oil
proof
> outer covering if used in a machining environment)
>
> Thank you in advance,
>
> Ballendo
>
> P.S. Any special consids for servos, versus steppers? (Just the
> motor. We'll assume everyone knows to keep the encoder signals away
> from the drive current) Can I just use SJO to the servos?