Re: Question for Mariss - Servo Motor Current

Peter,

I assume 20A is the stall or locked rotor current. Keep in mind your
motor is delivering no power at that point (lots of torque but zero
speed). Peak power occurs at 1/2 of stall torque, 10A in your
instance. At peak power, the motor efficiency cannot exceed 50%,
meaning for every Watt delivered as mechanical power, another Watt
goes up as heat. The motor would not have long to live if it is run
at this load for other than short intervals. More than likely, the
maximum continuous torque of your motor will be 20% or less of your
stall torque (and current as well).

That being said, your continuous current per motor would be more like
4 amperes and your motor would be about 80% efficient. The maximum
continuous mechanical output power would be around 210W. 210W can
lift (or push with) 550lbs at 200 IPM, (250kg @ 5,000 mm/min). At
peak power, the motor would output 325W.

If you want, you can get by with a 4A per motor power supply. You can
increase it to 10A per motor if you want to reach the peak power
current. The power supply would have to 2.5 times bigger to gain only
50% more motor power, which you can only use for a very short duty
cycle at that.


That is where reduction gearing comes into play. Find your motor's no-
load RPM and divide it by the maximum RPM you sanely want your
leadscrew to turn at. That result will be your reduction ratio. More
than likely your current will be much less because the torque load on
the motor will be less.

Think of it as sitting in a fine sports car (5-speed manual
transmission) at a red light. Which gear do you want to be in when
the light turns green? Probably not 5th gear, I'll bet.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Peter <prenolds@s...> wrote:
> Hi Mariss,
>
> I've just converted my BP clone from steppers (G210's) to servos
> (G320's) and have confirmed as expected, that I need a bigger power
> supply (PSU). Your white paper and most of the group's discussions
have
> been related to stepper motor current requirements - so I thought
I'd
> ask about PSU guidelines for servos. My motors are rated at 20
amps max
> but I'm hoping I won't need a 60 amp power supply!
>
> Setting the G320 current limit pots to max, I connected a 0.1 ohm
25W 1%
> resistor (thanks, Digikey) in series with my existing PSU (65VDC 12
> Amps, 15,000uF cap) and with my scope, measured the idle currect
pulsing
> to about 5 amps (for all three motors) during servo ticks. When I
> started the mill running some 3 axis exercises, as I increased
> accelerations and speeds to near desireable rates, I observed the
load
> current peaking to around 35 amps during a motor stop/reversal.
The PSU
> output voltage would drop way out of regulation (more than 10V)
during
> these peak loads and I observed the G320s would momentarily
illuminate
> their fault LED's. Short term I can run lower accelerations while
I
> beef up the PSU.
>
> Clearly I can increase the capacitance by adding another 15,000uF
cap,
> but is the 2/3 current design rule (i.e. 40 amps) applicable for a
servo
> power supply design?
>
> Cheers, Peter