Re: [CAD_CAM_EDM_DRO] How to determine Torque from these motor numbers?

Ok, two people beat me to this while I was typing and calculating so
this might be redundant.

Kv(rpm/volt)/1352=Kt (torque constant). This is valid for permanent
magnet DC motors. So your motors have a torque constant of about 54
oz-in/amp. Out of curiosity how big is your motor? These numbers
describe a large motor.

As far as how much current or power they can handle depends on a few things.

Heat that must be dissipated in the motor is found with
Rm x amps^2

1.7 watts at 1A
6.8 watts at 2A
15.3 W at 3A and so on.

If the motor is turning, it gets more complicated but for these motors
the heat due to current accounts for most of the heat generated. The
heat must be dissipated in some way. This will be mostly based on the
surface area of the motor and the airflow around the motor. At no load
it's just V x A. You can see that even at no load at 30V your motor is
generating 17.4W of heat but this isn't that important. To give you an
idea the motors I'll be using have a Kt of 14 oz-in/amp and spec 5.4A as
max continuous current. Rm(motor resistance) is 1.0 Ohm. This is 29W.
It is 3.5" in diameter and 4.8" long.

Is it OK to run the motor over 30V? Well the manufacturer put the
number there for a reason. It may be that the motor can not hold itself
together at over 720RPM. It may be that brush wear is not acceptable
above that RPM. Going to 45V would probably be OK but I would not go
much higher without fully inspecting the motor. Max RPM on my motor is
5000 RPM which equates to 50V for my motor at no load. Max voltage is
specified at 80V Bus voltage and 104V Terminal voltage, though I'm not
sure what the difference in terms means.

Like Jon said, I wouldn't be too concerned about the voltage. If I
wanted to test the motor you have, I would lock the shaft and put it on
my power supply at fixed current while watching the temperature. Start
at Mariss' suggested 3.7A and see what temperature the motor stabilizes
at. If it's pretty low you can run more current if it hits 250-260
Fahrenheit and keeps going up the continuous current will be lower than
that.

Greg



Earl wrote:

> Howdy all again.. hope everyone is having a great holiday.
>
> I have some Ametek DC motora here with a few more specs. How do i
> derive the Torque from these numbers, or is more data needed?
> So far I have found out:
>
> Resistance is about 1.7 Ohm, useing the following readings when
> applying the following to the motor with the shaft locked:
>
> 10.5V drew 6.5A = 1.6 Ohm
> 16.5V drew 9A = 1.8 Ohm
> (my meters wont go above 10A reading)
>
> "30VDC nominal" is printed on the motor, but seems to want to run at
> higher voltages.. it is OK to run it above 30VDC?
> No load resulted in these numbers:
>
> 30V .58A = 720 RPM
> 50V .67A = 1260 RPM
> 60V .73A = 1500 RPM
>
> Like I said, Id like to know the Torque of these motors, as well as
> if they are a good canidate for a small/medium CNC mill project
> useing Geckos. Ball screws are yet to be obtained.
>
> Thanks for any help (again). :)
> Colin
>