Re: servo voltage question

I run the 38vdc Ameteks in my BP retrofit at 75vdc for 3 years now. It
quadruples the horsepower which translates to more accuracy through
better holding power. Rarely do they even get warm.

Mike

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> 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