power supply for stepper motors

[jpoge66]

I'm in the process of looking for a power supply to power 5 stepper
motors for a robotics type application I am developing. The five
motors are size 17 bipolar hybrid stepping motors and I am powering
them with 5 microstepping drives. My specific questions involve
looking at the power requirements for my drives and motors and using
those figures to determine what I'm going to need in a power supply.

The drives are rated at 12-24 VDC, and have an adjustable max current
of .25 to 1.4 Amp RMS/phase or .35 to 2.0 Amp/phase peak. The motors
were tested at 1.4 Amp RMS at 12 volts and have plenty of torque at
this range to satisfy my calculated requirements. I intend to start
at 12 volts and hopefully this will be enough, but it would be nice to
have a power supply that went up as high as 24 volts if I need to get
more torque out of the motors than I expect.

The dives also require 5 volts at 7 mA for the logic circuits.
According to the manufacturer if a separate 5 volt supply isn't
provided it's acceptable to use a resister to drop the voltage. A
separate computer power supply might also be a possibility to power
the logic circuits of the drives.

My ideal solution would be to have one power supply to power all five
drives. Something with an adjustable voltage or separate channels
between 12 and 24 volts, or possibly even a separate 5 volt source for
the logic circuits. I think my amperage requirement would then be 2
amp/phase x 2 phases/motor x 5 motors = 20 Amps. This is assuming I
need to use the peak amp figure and not the RMS figure. I think this
would then give me a power requirement of 12 volts x 20 Amps = 240
Watts (using 12 volts) up 480 Watts (using 24 volts). But maybe it is
not the most economical, practical, or even possible to use the same
power source for all five motors.

So, my questions are:

Did I calculate the correct specifications for power requirements for
a power source?

Does anyone know good places to look for the power source described?
The more economical the better.

Would I be better off with 5 smaller power supplies rather than one
big one, and if so where can I find those?

Also, am I correct in this assumption: A 20 Amp power supply,
supplies UP TO 20 Amps, but only when 20 Amps are being drawn. I need
a 20 Amp supply to hook up 5 – 2 Amp/phase motors with 2 phases each,
but in situations where I just want to move one motor at a time then
only a maximum of 4 amps will be used for that time and the power
supply will only put out 4 amps. Is that correct?

This isn't exactly my area of expertise and any help would be greatly
appreciated.

Thanks,
Jason

 

[steve vorres]

It sounds like you can use a PC desktop power supply for this
application. Sized smaller than half a shoe box.
You can buy a PC supplies from your local computer store for less than 50
bucks.
Or A used one is perfectly fine and you could get one from an older PC for
almost free.

You are correct in your understanding that a Motor only draws up to that
specified current amount.

2 amps DC x 2 coils (phases )= 4 amps per motor
4 amps x 5 motors would consume a max of 20 amps

A power supply rated at 20 amps will provide any where from 0 to 20 amps
depending on the load.
The power supply will get warmer as you approach its maximum rated
current.

Most PC supplies will provide atleast 10 amps on the 12 volt connector.
they also provide 5 volts.

You can probably connect two separate supplies in series for more voltage
, but you should not ground them together.

Stepper motors draw current when they are moving AND they draw current
when the are commanded to be stationary.

Another potential solution is to use some medium sized 12 volt lead acid
batteries or gel cells.

A 12V 7 AH battery will provide 20 amps for 12-15 minutes, a larger
AH battery will provide proportionately more run time.

Two batteries can be connected in series for more current.

Have fun

 

[James Newton]

Stepper motors draw current when they are moving AND they draw current
when the are commanded to be stationary.

This is true the vast majority of the time, but smart controllers can
reduce the current used to hold or even turn it all the way off in
applications where friction will provide enough stability.

The linistepper controller (an open source system using a PIC
Microcontroller) has a low power hold mode and can be modified to shut
off completely when the motor has not been active.

http://www.piclist.com/linistepper

The advantage of this is not only that the power supply needs supply
less current, but also that the motor will heat less and will heat
evenly.

Another (minor) point has to do with the linearity of microstepping:
Believe it or not, if a stepper motor holds for a long time on one set
of coils, the difference in temperature in the coils will cause e.g.
half steps to be not exactly half way from one step to the next.

James Newton.