Re: [CAD_CAM_EDM_DRO] DC motor - Controlled over-current - how much??

Hi Joe

Well, you could try to hunt up the motor specs on the web.
Is your motor a permanent magnet type? If so it is probably
similar to the servo motors on the bottom link below
(shameless plug). They are about 4.4 amp continuous
and 26 amps peak. The peak is usually a limit before
demagnetization can occur.

How long? The motor specs should say.
But let's try an educated guess.

Let's say a motor suddenly has high current and is stalled or nearly so.

Let's further guess that the winding temperature as it was running
previously at 4 amps is about 50 C. That is room temp plus your
25C temp rise spec.

Also we will keep the winding temperature to no more than 100C.

Finally let us assume the total copper mass in the armature is 1 Kg.
(wild guess here) and the winding resistance is 2 ohms.

Ok it has been running a while at 4 amps and we suddenly put in
14.

The windings will begin heating up right away because they are also the
heat generators. The heat does not have to flow anywhere. We
call that adiabatic.

The heat capacity of copper is about 400 J/Kg-K.
1 J (Joule) = 1 Watt second.

So I^2R*time = heat capacity * mass* delta temperature

solve for time
t= heat capacity * mass* delta temperature/ I^2R

= 400 J/Kg-K * 1Kg * 50K/ 196*2

= 51 seconds.

Seems like longer than it should be... but we used many assumptions
to keep the math simple. One is brush resistance. The brushes are
small and light, but the same current is flowing through (if two brushes).
They could get hot much quicker than the armature if resistance was
significant. So I would check that before I believed the 51 seconds.
The brushes might be the limiting factor by far.

I am a little bored on a thanksgiving morning... can you tell?
Better go out and rake leaves or something.

Les

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html
Surplus cnc for sale:
http://www.alltel.net/~leswatts/forsale.html

----- Original Message -----
From: "onecooltoolfool" <joevicar3@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, November 28, 2002 9:17 AM
Subject: [CAD_CAM_EDM_DRO] DC motor - Controlled over-current - how much??


> I have a 1 hp 180V DC spindle motor rated at 4.5 amps.
> In the drive manual they show over-driving the motor for up to 160%
> rated current, but they don't say for how long.
> The drive will over-current for up to 300% (14amps)
> Higher current is only needed for brief (less than 2 minutes) and
> only when drilling with large diameter drills which is rare.
>
> How much can I go over the rated 4.5 amps and for how long?
> There is a "Temp rise" rating on the motor plate of 25C.
> What does this mean and can I use it to estimate my overload capacity?