Re: Servo motor question

Tim Goldstein wrote:

> 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.

What model tape drive?

> 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.

Hmmm, the .5 Amp rating sounds REAL fishy. If that really is the limit, at
24V, that is a 12 Watt motor! It shouldn't be able to even move a Bridgeport
table. If 0.5A is the continuous duty rating, there is some hope. First
thing I'd do is use an ohmmeter on the armature terminals. If you get
readings below 1 Ohm (you'll have to wiggle the motor to get a good
commutator contact, then let the meter reading settle) then you are in
good shape. We have found some motors that have about 5 Ohm
armature resistance, and similar voltage/RPM ratio. The resistance
is the killer (I think). If you have a couple of amps running through
a 5 Ohm motor much of the time, it will run VERY hot. If the
average current is 1 amp or less, and the peak is just a couple
of amps, then it might do fine.

Still, it's hart to answer the question without enough data to calculate
Kt and Ke. These are the Torque and voltage/RPM constants for
the motor. Using my own servo amps as a guide, a Kt of no less than
25 in/Lb per Amp (assuming a modest belt reduction ratio), or
maybe 50 in/Lb per amp if you want direct drive. Doubling or even
quadrupling these numbers gives a good safety factor. As for
Ke, you must have an armature voltage no higher than the servo
DC power supply when the motor is driving the machine at the
desired rapid feed rate. But, you want the Ke not much less
than half that figure, so you can deliver decent power without
excessive currents.

If you can find direct drive motors from a 75 IPS, start-stop
tape drive, you are onto something. A 125 IPS start-stop
drive would be even better, although some of these motors
start to get big and heavy.

Jon