Re: servo voltage question

Jon,

Even if it is a "minor war" I never received the invitation to
participate; I feel let down.

Switching type drives apply a voltage to a motor. This voltage has
little to do with the power supply voltage. All the motor sees is
some percentage of that.

Voltage by itself doesn't really matter and a user doesn't control it
or measure it anyway. What matters is a motor's maximum rated RPM and
torque.

It is perfectly OK to use a 40VDC rated motor with a 70VDC power
supply so long as you don't ask for any speed over what the motor is
rated for at 40VDC. The motor won't know any better because all it
will see is 40V. The PWM will go to a 60% duty cycle (0.6 times 70V =
40V more or less). Everything is cool then.

The drive is equipped with a current limit trimpot. Current is
limited on a pulse-by-pulse method. If you have a 15A rated peak
motor, set the limit on the trimpot to that then.

Overvoltage under your control by not asking for a speed above what
the motor is rated for. Over-torque can be prevented by setting the
torque limit trimpot correctly. That's what it's there for.

Otherwise go with your 72VDC supplies for 40VDC or even 24VDC motors.
They won't know the difference if you behave.

Mariss

--- 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