Re: [CAD_CAM_EDM_DRO] Determining servo motor specifications

whippo6 wrote:

>Hi,
>With the help of Steven Ciciora I have determined the following
>motor parameters for the Clifton Precision Model DH-2250-AU-1
>DC permanent magnet brush servo motors.
>
>Kv = Voltage Constant = 7.37 volt/K-rpm
>R = Terminal Resistance = 1.62 ohm
>
>I have also computed the other motor parameters/specifications
>at no-load, stall, peak power and peak efficiency loads, each
>at a terminal voltage of 24, 30, 36, 42 and 48 volts.
>
>My question is:
>
>How do I decide what current or torque load to use to compute
>the specifications at "Rated" load?
>
>I don't have any manufacturer's info for this motor.
>
>

Well, there are two limits. One is thermal. Whatever power you feed
into the motor that
is not delivered to the shaft (as power, which requires it to be moving)
ends up as heat.
Generally, in mid-sized mills and lathes for home-shop sort of use,
motor overheating is
not a real problem. The second is the peak current above which the
motor's magnets will
be demagnetized. That is pretty critical, and hard to determine, unless
you have more motors
than you need. I would think that 8 - 10 A is probably a safe value,
especially given the
low Kv, which indicates a low number of turns in the armature winding.
But, it is very
hard to determine this with non-destructive testing, especially if you
don't even know
what the magnets are made of!

>Secondly, how do I decide what terminal voltage to run the
>servos at for CNC use? I do notice that as the voltage is
>increased, the efficiency of the motor increases. So is it
>best to use a higher voltage? How high is too high?
>
>

You don't. The servo amp makes this determination! As long as the DC
input to the
amp is sufficient to supply the motor, then it works OK. If the supply
is not sufficient,
the motor will lag behind the commanded velocity at high speeds, and you
will get
a following error. but, these motors have such low Kv, that unless you
use a 20 TPI
leadscrew, or a very high belt reduction ratio, you don't need much
voltage. For 3000
RPM, you need 7.37 V x 3 = 22 V, plus a little for the drops in the
servo amp. Probably
a 36 V supply would be sufficient. So, you figure out how fast you want
the machine
to move, combine with the leadscrew TPI and the belt ratio (if any) and
you can then
compute the voltage needed to get to that motor RPM.

>The manufacturer's data sheet for the newer Clifton
>C23-L40-30 servos states that its allowable terminal
>voltage is 12 - 48 volts.
>
>The data sheets for the other motors do not specify the
>allowable voltage range, they only specify the rated voltage.
>
>

The RPM range needed determines the allowable voltage range. Really,
voltage is only
a worry if the motor will be connected to a stiff DC power source, like
a car battery.
A servo amp is always current limited, so it should protect the motor
from a sudden
application of full voltage.

>Finally, how do I determine the peak current limit so that
>I do not fry the magnets?
>
>

As I said, this is very hard to do. You will likely have to make
assumptions.

Jon