Re: [CAD_CAM_EDM_DRO] Question for Mariss - Servo Motor Current

Peter 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!

You will need apower supply capable of 60 amps PEAK, if you give maximum
acceleration to all 3 motors simultaneously, and if the current limit is set
to 20 Amps for each motor.

>
> 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.

Watch out for incorrect readings. Long-term trends, in the millisecond
range, will be accurate. but, small pulses, in the tens of microseconds,
may be incorrectly exaggerated, as this may not be a non-inductive
resistor.

A suitable capacitor bank should be able to deliver the peak current burst
you need. But, it may take several soda-can sized capacitors to do the job.
Wathc out again - those big cap banks can be dangerous, as the current
capacity is enormous during faults.

> 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.

Yes, 35 A is not a surprise!

>
> 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?

No, the rules are totally different for servos. Stepper motors draw large motor
currents when idling, but the voltage per phase is low, hence power input
is low.

Servos, draw very little current when idling, much less than steppers, but the
current can go to 100 % of the limit continuously when accelerating. As the motor
speed increases, the applied voltage increases, and so power drawn from
the supply increases roughly linearly.

Jon