Re: [CAD_CAM_EDM_DRO] max voltage for Electrocraft servo

grantfair2001 wrote:

>I have some used Electrocraft E543 brush servos. I did track some
>specs on them, but there was nothing on max. motor voltage.
>
>Does anyone know what this is? Or is there a way to determine this
>from the other specs (KT, KE, max continuous current, motor terminal
>resistance at 4 amps, etc.)?
>
>I know someone who is running these at 67 volts without any problems.
>But I am cautious (as all this servo stuff is new to me) and the
>Geckodrive 320 (which I haven't tried yet becuase of this question)
>information states not to exceed the motor's rated voltage by 5 volts.
>
>

Maximum motor voltage is important only with motors powered directly
from a DC supply bus. If the voltage is exceeded, the starting current
could damage the magnets.

In a servo system, it doesn't apply, as the servo amp is supposed to
limit the current to a safe value. You can determine a reasonable minimum
value for the DC supply, so that full motor performance can be obtained.

First, figure the max RPM you will need. Convert to KRPM by dividing by
1000.
So, if you need 3100 RPM, that's 3.1 KRPM. Multiply that by Kv and you get
the max back EMF the motor will develop. Now, take the armature resistance
(this is exceedingly hard to measure through the brushes, the best way is to
pull the brushes out and poke the commutator with meter probes. (for 4-pole
motors, use 2 adjacent brushes and divide by 4.)) Take the max peak current
and multiply by the resistance, and add to the above calculation. Then, add
10 % for modulation and drop in the servo amp. If your power supply
delivers
this voltage, then you can produce full rated torque at full RPM. If
you use
the "max voltage" from the motor manufacturer, it is DESIGNED so you can't
get any torque at rated RPM, totally defeating the purpose of using a servo
motor!

Jon