Re: [CAD_CAM_EDM_DRO] servo voltage question

cnc_4_me wrote:

>What power supply voltage do you use for servos with a pwm
>drive...Example, if your motor is rated for 40VDC do you use a 40VDC
>power supply or do you use something higher like 70VDC...
>
>

I've been having a minor war with Mariss Freimanis at Gecko over this.
You have to know how the voltage rating of the motor was written.
If it is an absolute voltage limit, never to be exceeded, then you don't
want to go over it. This will be true for "real" servo motors made by
such makers as Electro-Craft (now Reliance Electric), Yaskawa, Fanuc,
etc. Those motors also have a peak current rating, which is also never to
be exceeded.

But many motors sold in the servo market have a very different voltage
rating.
Makers such as Ametek use this. It is a "bus voltage" rating. You can
connect
the motor to a DC bus of that voltage with the motor stationary, and it will
not be harmed. To illustrate, the typical Ametek motor, sold by
Camtronics and
many others, has about a 4 Ohm armature resistance, a 12 A peak current
rating, and a stated 40 V maximum voltage. Simple Ohm's law calculation
states you can NEVER even obtain 12 A through this motor with a 40 V
power supply. I = E/R or 40/4 = 10 A! Now, with the motor running at
1/2 its rated speed, the back EMF will be about 20 V. That subtracts
from the
applied voltage, so you'd only have about 20 V left to drive current
through the motor.
With a 40 V power source, you can only get (40-20)/4 = 5 Amps! If you
want to
spin the motor faster, there's even less current possible. So, by
limiting the supply
to the 40 V rating, you lose at least HALF the possible power output of
this motor.
But, you can't "erase" the magnets in it with only 40 V. If you turn
the voltage up
a bit, the only thing standing between the the motor and destruction of
the magnets
is the servo amp's current limit. If you trust that current limit to do
its job, then
you should be able to safely turn up the supply voltage to get rated
performance
from the motor.

Now, that gets you into the safe operating area discussion. The motor
CAN be
made to safely deliver maximum rated speed and maximum rated torque
simultaneously
for only some time interval. If you need both at the same time, you'd
better not
demand it for more than a fraction of a second a couple of times a minute.
Really massive industrial servos are designed to survive more severe
service than
lighter commercial-grade motors, and those often don't come with safe
operating
area curves. So, you have to use some common sense in how far you are
going to
push things. And, of course, a Gecko drive will happily deliver 10 A to
a locked
Ametek motor until the smoke comes out. I've seen it happen!

Anyway, Mariss warns to strictly adhere to the manufacturer's ratings.
But, I hope
I have shown that at least with some motors, you absolutely cannot even
achieve rated
performance if you do that.

Jon