Calculating transformer current for steppers

[dfro]

I am trying to calculate transformer current necessary to power some steppers, and there is a gap in my understanding.

Are there two ways of expressing a motor's amperage rating - amps per phase, and total amps?

The reason I ask is that in 1/2 step mode, 2 phases will be powered, thus drawing twice the current. Correct?

So, if I see a stepper motor rated at 2.4 amps, what do I do? Do I multiply the current by two or do I just use the 2.4 number, when calculating the total current that the transformer must be able to handle?

[jeffs555]

A lot depends on the drives and also on the power supply voltage relative to the motor voltage. If you use a resistor type drive, you will need 2x the phase current like you said. If you use a chopper drive, the current will only be on for a fraction of the time, so the average current from the supply will only be a fraction of the motor current. The higher the voltage of the supply relative to the motor voltage, the less current required from the supply.

[LOKI]

Hi jeff555,

related to this topic, If I plan to use 4 stepper motors powered at 35V and they have a rated current of 1.5A (each), while using chopper drives (L297-298 type) the total powersupply's current can be...how much? (sorry, but English isn´t my mother tongue)
thanks in advance,

[NC Cams]

The current of the L297-298 style stepper drivers is controlled/monitored by the sense resistors that are placed between the 298 and ground.

The power supply can be arbitrarily capable of 1000 amps output.

The current that will be drawn is purely a function of the output voltage and the motor impedance (crudely E=IR) in concert with whatever limiting the sense feedback resistors provide to the 297 and the 297's processing of same feedback data.

The following links may be of help:

Stepper sizing:
http://www.cnczone.com/forums/showthread.php?t=17707

Small stepper P/S
http://www.campbelldesigns.com/files...ply-part-1.pdf

[fyffe555]

Originally Posted by dfro The reason I ask is that in 1/2 step mode, 2 phases will be powered, thus drawing twice the current. Correct?

Almost. 2 phases will be powered in microstep by the driver, but the total draw of both coils is the same as one phase full step. microstepping and the power provided to each coils is entirely (usually) a driver function.

To get 1/2 step, or any microstep, two coils either side of the rotors pole are powered so allowing the driver to hold the rotors pole between steps. Half step means the rotor is held midway between 'steps' or pole. 1/4 means the pole is held 1/4 / 3/4 between steps. For 1/2 step each coil either side of the motors pole is held at the same current/voltage to keep the rotor hovering there, each coil drawing 1/2 the total rated power under control of the driver. 1/4 step means that one coil is held at 1/4 the total current/voltage and the other coil is held at 3/4 the total current/voltage so holding the rotor pole 1/4 the distance between steps.

The total current and voltage across two coils in microstepping is the same as the motors rating for single coil full step operation. You do loose some torque in microstep.

Note microstepping doesn't reliably increase the resolution of the motor, it's intended to reduce resonance.

[dfro]

CampbellDesign.com has a very good essay on how to design a simple power supply to power an interface/driver system at:

http://www.campbelldesigns.com/how-t...wer-supply.php

It has the equations necessary for calculating the transformer and smoothing capacitor sizes, for the size and number of stepper motors you are using. I imagine you can always put components in that have even more current capacity that what is calculated - for example if you know that later you will be adding an A axis later, or you just want headroom.

My questions came up while reading that paper. They use, as an example, a stepper with a voltage of 1.26 and a current of 6.6 amps. When people talk about a motor having a current rating of 6.6 amps, leaving out the 'per phase' part, should I assume that is what they mean? My guess is, yes, after reading what fyffe555 wrote on this thread.

I am planning on using the picstep, which uses the LMD18245T chopping driver chip. So, if the motor, which it is driving, is in a 1/2 step, it will chop 3.3 amps worth of current to one phase and 3.3 amps worth of current to the other phase. Correct?

[NC Cams]

FYI: the Campbell link posted in post #4 is the DIRECT link to the PDF.

The Campbell link posted in #6 will eventually get you to the same PDF.

[LOKI]

Hi Nc Cams
sorry for this late reply.
Thank you for your response.

Alex