Re: [CAD_CAM_EDM_DRO] Re: CNC Stepper Power Supply Q's???

Mark wrote:

> No that was excellent. So from what your paper stated, and near as I
> can tell what I want is about a 50 VAC output from the transformer
> and roughly a 750 VA Transformer since the motors are 6.2 Amp motors.
>
> So we get 6.2 * 3 = 18.6 Amps * .667 = 12.4 Amps
> 12.4 Amps * 50 Volts = 620 VA Or should I multiply by 70?

Yes, use the DC voltage at the capacitor, as amps x voltage = power.
The transformer will be forced to deliver the RMS power in short pulses of high
current, which causes transformer dissipation to be much higher. So, a
capacitor input filter generally requires adding a safety factor to the transformer
rating, unless the transformer already is rated for "capacitor input". This can be
25% or more, depending on how conservative the maker is.

But, there is the other factor, that this is not likely to be a steady-state full
load. When idling, the current draw of the drivers will be much less. If your
motors are 6.2 A per winding, they can draw more than 6.2 A input when
delivering power at medium speed. But, a bulk dc supply should have no
problem handling short-term overloads.

>
> As for the Capacitor It appears that I need
>
> (80000 * 12.4)/70 = 14,171 uF of capacitance at 70 Volts. Apparantly
> there is no penalty for having more capacitance than required?
>
> Finally should the Bridge Rectifier be rated on the Input Voltage or
> Output Voltage, I assume output voltage?

No, it should be rated WAY over the output voltage. Every time the current stops
flowing in the winding, an inductive voltage surge is generated. When you flip
the main switch off, there will be a big surge due to the collapsing field from the
primary winding. For a 70 V power supply, a 250 V rectifier would be my minimum.
I might even go to 400 V unless I put snubbing capacitors or networks on the
rectifier to protect it. But, the higher voltage rectifier only costs a few CENTS
extra. VERY cheap insurance.

Jon