Re: Servo motor question

> From: "Tim Goldstein" <timg@...>
>
> I was just a local salvage yard and they had a few DEC Magtape units that
> had what appeared to me to be servo motors to drive the tape reels.
>
> My question to you servo guru's (Matt, Jon, & ??) are these motors of any
> use for a servo setup? The marking were DC Motor, 24 v. 0.5 amp 600 rpm
> assembled by Indiana General.

Jon is the real expert on this, but some of the issues are:

1. Does the motor "cog" when you turn it? These probably won't given their
application but if you run into any others this is one thing to test.
Another, better way to test this parameter is to rig up an adjustable DC
supply and run the motor at the minimum voltage to make it turn and note
whether it rotates smoothly, or in lurching steps. Smooth low speed operation
is a must as we often run at fractions of an RPM for slow feed rates.

2. Another issue is armature resistance, should be low to minimize internal
power dissipation.

3. Yet another thing is what is the max current you can push through the
motor without demagnetizing the field magnets. This probably won't be listed,
but you need to know if you can achieve enough torque without wrecking the
motor. One way to gauge some of this would be the weight of the motor. The
motors we want weigh more than 5 lbs. and more than 8 would be better. This
is obviously a crude indicator of value, but it's one that can be easily
measured.

4. Is it a low speed motor? Yours obviously is at 600 RPM. I'm assuming that
the figures you quote are the no load speed and current draw at 24 volts.

5. Do they cost more than $20? If not, buy one and send it to Jon to evaluate
;). If they pass his inspection, BUY THE REST! I've spent a good bit of time
and money trying to locate a source of servo motors worth using for retrofits
and I'll take a batch myself if Jon blesses them with his approval. I'd
prefer something face mount, 3"-4" in diameter with a 5/8" shaft (1/2" would
be OK), double shafted would be great.

Matt

P.S. The blue books arrived today in the mail, so I'll be sending yours to
you on Monday.