Re: [CAD_CAM_EDM_DRO] sizing transformer for servo system ?

Anders Wallin wrote:

>Hi All,
>
>I am setting up a servo control system for a 3-axis mill using 90VDC
>servos which are specified as 7.8A maximum continuous current and 38A
>peak current.
>
>On the transformers I am looking at, 2x38VAC seems to be a common
>secondary configuration. Wired in series this is 76VAC and rectified
>about 107 VDC minus two diode drops.
>
>But what about the power rating ? I'm certainly not going to calculate
>it for max peak current on all three motors (38*3=114A or 11kW!), but
>should I size it for max continuous current on all three motors
>simultaneously (3*8A= 24A or 2.4kW) ?
>
>
>

It seems unlikely that you could ever draw this much power on a mill.
Note that
because the servo amps are switching power supplies, the only way the input
to the servo amps would be 8 A * 90 V = 720 W would be when the motors were
spinning near rated RPM. If the motors were delivering rated stall
torque at zero
RPM, the armature voltage would be a couple of volts due to windinbg
resistance.
In that condition, the power draw would be only 8 A * 2 V = 16 W.

>We do run 3D parts where all three axes would be moving simultaneously,
>but the 3D toolpaths are finishing passes with low chip load. The heavy
>cutting is done using the X/Y motors, and Z needs power when drilling...
>so I'm thinking 2*8A or an 1.6kW transformer would be enough ??
>
>
>

Plenty, and even still too much. I would guess that a 1 KW power
supply, or 10 A
continuous rated current would suffice. Certainly don't go above 1.6
KVA, it
will just be overkill.

Jon