RE: [CAD_CAM_EDM_DRO] Re: can amps be mixed with motors?

That's sort of right Tom.

If the bigger motor is for the same voltage, it will behave OK until you ask
more than 500W from it, then it may pull more current than your drive can
handle.

The problem is this hi current condition could occur under light work, due
to initial acceleration.

Think of the motor as a variable resistor, the more power you want, the
lower the resistance will drop, down to a fixed level. On the bigger motor
this fixed level will be lower and pull more current. Also think of the
resistance as being bouncy, so it can bounce down to this low level quite
quickly during transient conditions, the more bouncy the better the torque
speed response of the motor. As it bounces down to the lower level, you may
not see much current spike on an ammeter, it will be for a very short time,
but a short time is all that's required to pop the fets in your drive, and
normal fuses aren't quick enough to offer protection.



The normal method to run a big motor from a small drive is to add a series
power resistor so that the combined resistance seen by the drive is the same
as that of the smaller motor.



The resistance values you are looking for are the stall value, you can
measure this with a good digital ohm meter across the motor terminals while
it is stationery. You can also calculate it knowing the stall current and
supply voltage. Don't worry about wasting power in the resistor, during
normal use the extra resistor will be very small compared to the operating
resistance of the motor which is much higher, it only becomes appreciable at
stall or heavy acceleration conditions when the motor resistance drops to a
low value to grab more current.



Without the resistor you may be OK. With it there is a much higher
probability you will be OK.



If you want to look at a simple write up for diy'ers. Rutex publish this
method on their site, look for a download document called mystique.



Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of caudlet
Sent: 30 January 2007 19:57
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: can amps be mixed with motors?



--- In CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com, "toolspindoctor"
<clarkfamily5@...> wrote:

>
> I have a used Yaskawa motor and amp. The motor turns out to be bad
> and needs to be replaced. It was a 500watt motor. Can I use a 750
> watt motor with an amp rated at 500 amps and just expect less than the
> rated power or will it blow the amp?
>
> thanks!
>

The draw of the motor is in voltage and amperage. You *have* to use
the right voltage, then the motor "demands" the amount of current
needed for the load. If you use the 750 Watt motor and place it in a
situation where it needs over the 500W the supply can furnish, the
best thing that can happen is that the amp has some form of protection
against overload and will just shutdown. If not you have a blown Amp.

If the 750 motor is rated at the same voltage as the old 500 and you
are confident the load will never exceed the 500 level (or the amp has
electronic overload protection) then it would work and actually not
run at less power than the old motor. Of course you can't get 750W
from a 500W (I assume you meant Watts instead of 500Amps!) supply. As
long as you don't ask it to then there should be no problem.

Watts is a measurement of power consisting of (roughly) the applied
voltage and the demanded power from the motor and it's load.

So the simple answer is that as long as you match the voltage of the
old motor and you don't let the load exceed what the old 500 Watt saw
it should work. This is a bit like putting a bigger fuse in when one
blows. It will work as long as things don't go into overload.

Tom Caudle
www.CandCNC.com





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