PSU question

[iforeman]

Hi

I worked it out as 70% of 4x5.6=15.68A for 4 motors. It looks like forgot to add in the 2 phases Thanks for catching that! So it should be a requirement of 31.6 Amps correct?

[m_c]

Steppers use both phases, however only 1 is ever fully energised, so you only need to use the basic amp figure for PSU calculations.

Plus with modern drives, it's very rare for the drive to draw the full rated current anyway, and it's even less likely you'd be able to run at motors to meet the condition where the full power/current would be needed.

[iforeman]

Hi

Yes, I was thinking about this and went to check the detail in the Gekodrive STEP MOTOR BASICS GUIDE, This is a extract from Section 6: Power Supplies

The easiest factor in choosing a power supply is its current rating, which is based on your motor
ratings. A motor control will always draw less than 2/3 of the motor’s rated current when it is parallel
(or half-winding) connected and 1/3 of the motor’s rated current when it is series (or full-winding)
connected. That is to say, a 6 amp per phase motor will require a 4 amp power supply when wired in
parallel and a 2 amp power supply when wired in series. If multiple motors and drives are used, add the
current requirements of each to arrive at the total power supply current rating.

With that in mind it looks like my original estimate of Approx 16 amp would be correct.

[JAZZCNC]

Hi

Yes, I was thinking about this and went to check the detail in the Gekodrive STEP MOTOR BASICS GUIDE, This is a extract from Section 6: Power Supplies

The easiest factor in choosing a power supply is its current rating, which is based on your motor
ratings. A motor control will always draw less than 2/3 of the motor’s rated current when it is parallel
(or half-winding) connected and 1/3 of the motor’s rated current when it is series (or full-winding)
connected. That is to say, a 6 amp per phase motor will require a 4 amp power supply when wired in
parallel and a 2 amp power supply when wired in series. If multiple motors and drives are used, add the
current requirements of each to arrive at the total power supply current rating.

With that in mind it looks like my original estimate of Approx 16 amp would be correct.

Depends if the Motors are 4 wire or 8 wire.? If 4 wire which most large motors are then high chance they'll be series wound not parallel. In which case you won't need so much.

Regards PSU then I wouldn't mess around and just build Toroidal PSU to meet your exact requirements.

[Neale]

I suspect that the Gecko recommendations are based on linear power supplies. The trouble with switch-mode supplies is that they can hard-limit at max current which gives stepper drivers a problem when they demand short but high-current pulses. Linear supplies with their big smoothing capacitors will handle this with no problem - it doesn't matter in this situation if the output voltage drops a few volts when you demand higher than rated current for a few milliseconds. If you are looking at cascading a switch-mode inverter off the back of a switch-mode power supply, it might not handle this kind of load very well. Linear supplies needed to be rated for average demand, switch-mode for peak demand, as a general guide.