Re: [CAD_CAM_EDM_DRO] Need a circuit breaker.......

doug98105 wrote:

> Group,
>
> It's recommended by the maker of the DC servos on my lathe that I
> install a 7 amp slow blow fuse or circuit breaker to protect the
> motors (2 motors burned up in last 4 years, $820/each).
>
> Motors are rated 4.6 amp continuous, 18.4 peak. Max volts 100.
>
> The motor manufacturer offers a breaker protected motor, but it won't
> fit in my space with the breaker protruding from the side. Their
> circuit breaker reset is sided mounted with protective cover, .63"
> square by .813" tall (fairly small considering that includes the
> cover). They buy the breaker from Newark, no part number is
> available to me, though.
>
> Anyone have suggestions, brands, etc. I can't seem to come up with
> much locally. What I find here are rated 32VDC, would that be okay?

Why are these motors burning up? Is the servo drive blasting them
with current, do they run very hot in normal use? What I'm asking is
if the burnout is a sudden malfunction that wrecks the motor, or 2 years
of steady abuse? A breaker protected motor might cost you the lathe!
Putting a fuse in series with the motor will probably work, although
fuses that are hit with a lot of surges may fail at the worst possible
time. Also, when the motor fuse or breaker opens, the motor loses
all braking ability from the control. In other words, an E-stop will
not stop that axis!

It seems a little odd to put a 7A fuse on an 18A motor. It may be that
the slow blow element will allow the system to perform up to the amount
needed without false trips.

If the circuit breaker is a plain breaker, not a temperature sensing one,
then it doesn't have to be mounted on the motor. In fact, it would
probably be better to put it in the control cabinet, away from all the
undesirable things on the machine (oil, coolant, vibration, etc.)

A 32 V breaker is not designed to interrupt current at 100 V DC.
Unfortunately, a 120 V AC rated breaker is ALSO not rated to interrupt
DC at 100 V. Both will probably perform the task a few times, which
may be enough.

But, this really doesn't solve the problem. Something is wrong here!
Servos should not BURN UP motors. Wear them out, yes, and maybe
this is all a symptom of motor wear out. But, if perfectly good motors are
being fried due to long hours of hot running, then you need to either
cool the motors (can be VERY effective) or replace them with larger
motors that can take the demands the machine places on it.
If perfectly good motors are being destroyed by faults in the servo
amp, then replacing $800 motors or having a lot of tripped breakers
ruining parts (and possibly the machine, too!) is not a solution.

Do you notice a slow degradation of the motors over time, and finally
they burn out? The current limit on the servo amps may be set
too high, and short pulses over 18 A may be slowly erasing the
magnets, making the motor work harder and harder until it fails.
The slow-blow fuse won't solve that problem if the pulses are short
enough.

Jon


Jon