Re: Need a circuit breaker.......

Peter,

I know very well why the motors burned out. We're using the machine
to bar feed small parts. Once every year or so, we get a jam up of
cut-off parts which stops the Z axis from moving forward. Since it
runs unattended the servo burns up before anyone notices the jam up
problem.

Normally I assume the servo amplifier would error-out with an over
current condition before the motor burned up except the machine
builder chose to disable that function to get more performance out of
the motors (dumb??).

The motor builder suggested the fuse or circuit breaker solution.
They have become very familiar with this situation. I asked if other
users of this machine were having servo problems and had they sold
many replacement motors? The answer was, "yes, hundreds."

As I mentioned, the motor builder now offers this motor with a built-
in 7 amp circuit breaker. Unfortunately, the circuit-breaker type
motor won't fit my particular version of the machine because of
interference with the circuit breaker cover and the machine
structure.

Since the motor builder is using the 7 amp breaker I'm guessing that
must be a reasonable value for this motor. They tell me they have
had none of their breaker-ized motors burn up.


I have since gotten the info from the motor builder regarding the
circuit breaker they're using.

thanks,

Doug

--- In CAD_CAM_EDM_DRO@y..., "Peter Seddon" <peter@s...> wrote:
> Sizing servomotors for CNC applications can be a bit fraught as
they are
> often operated on very short accel/run/decel cycles with little run
time at
> any sort of load. The peak currents during accel and decel are
often twice
> or more times the rated current.
>
> I would like to know why they are burning out and to do this you
will need
> to measure the running currents and the duty cycle.
>
> Regards Peter
>
>
> ----- Original Message -----
> From: "Mr. sausage" <sirsausage@h...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Tuesday, March 26, 2002 3:02 PM
> Subject: Re: [CAD_CAM_EDM_DRO] Need a circuit breaker.......
>
>
> > The question I have is: Why are these motors "burning out"? Are
they not
> > properly sized for the job? Unless they are seeing fault
conditions
> > (hitting stops, brakes on etc) they shouldn't normally be
overloaded to
> the
> > point of destruction.
> >
> > Thermal overloads, circuit breakers etc. cures the symptom, not
the
> problem.
> >
> >
> > >From: "Peter Seddon" <peter@s...>
> > >Reply-To: CAD_CAM_EDM_DRO@y...
> > >To: <CAD_CAM_EDM_DRO@y...>
> > >Subject: Re: [CAD_CAM_EDM_DRO] Need a circuit breaker.......
> > >Date: Tue, 26 Mar 2002 09:07:49 -0000
> > >
> > >Is that the best way to protect motors from burning out? A 7Amp
slow blow
> > >fuse will carry 7Amps minimum for ever which is above your motor
rating.
> > >Fuses and circuit breakers are really designed for short circuit
> protection
> > >and not thermal protection.
> > >
> > >The best way to protect a motor is by embedded thermal detectors
which
> > >detect the actual motor winding temperature but are installed
when the
> > >motor
> > >is built and best suit ac motors which have stationary windings.
The
> > >alternative is a thermal overload relay in series with the motor
but this
> > >often trips too early as it does not have the thermal inertia of
the
> motor.
> > >
> > >I would try a thermal overload relay and leave fuses and circuit
breakers
> > >for short circuit protection.
> > >
> > >Regards Peter
> > >
> > >
> > >----- Original Message -----
> > >From: "doug98105" <dougrasmussen@c...>
> > >To: <CAD_CAM_EDM_DRO@y...>
> > >Sent: Monday, March 25, 2002 11:16 PM
> > >Subject: [CAD_CAM_EDM_DRO] Need a circuit breaker.......
> > >
> > >
> > > > 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?
> > > >
> > > > thanks,
> > > >
> > > > Doug
> > > >
> > > >
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