[poptarted]

Hi,

I've got a set up I've been playing around with running for a mobile work shop application.

2X 24 V, 6 A motors
1X 12V car battery
2X Gecko 203V's.
1 Step-Up 12/24V DC converter rated for 300 W, 12.5 Amps output continuous.

Having some issues with the drives quiting at high speeds steady state when both motors are running. I've seen improvement by adding 470 uF caps to the drive inputs, but no improvement by adding a 10000 uF to the Step-up converter output. I guess following the
C = 80000*I/V thing this should be more like 40000uF, but I'm wondering if it's more I'm pushing the limits of my step up converter and that's what's causing the drives to shutdown, but I think it might be something else.

I've also tried subbing in a 12V 20 A regulated power supply for the battery and seen similar problems.

Thoughts?

-Nick

[ger21]

The Geckos need 15V minimum, and your motors probably need much more to get higher speeds. What voltage are the motors rated at?

[poptarted]

The motors are 6 A,0.6 V, 0.1 Ohm resistance (0.4 mH) motors designed to be run at 24 V like I'm doing.

Your right, the Geckos need more than 15 V, so I'm running them with a 12 to 24 V DC-Dc step converter powered by the battery.

-Nick

[joeybagadonuts]

Poptarts,

Why not use two batterys in series, that might be better than the dc to dc inverter?

JoeyB

[poptarted]

Agreed on the batteries in series, far fewer headaches there, but I'm trying to keep the weight down to make it easier to move. Perhaps I should run some tests with two batteries to see performance differences.

-Nick

[joeybagadonuts]

Check it out

http://tinyurl.com/56kqna

[joeybagadonuts]

Nick,

http://tinyurl.com/5n5p2u

Get one of these and rework a old GM alternator.

JoeyB

[joeybagadonuts]

Or just mount one of these!

http://tinyurl.com/5zglty

JoeyB

[poptarted]

That battery is cool, and it's getting me thinking I might try something like that.

but it still doesn't solve my problems with the motor cutting out. Like you said, two batteries in series ought to prove if it's the step converter or something else, like the two motors BEMF messing with each other, or the controller which in the past hasn't been a problem.

Just one motor gets up to speed (~1000 rpm) and then poops out in a couple of seconds, so I guess it's either the Gecko over heating (I have it heatsinked and fan cooled set to 6 Amps) or I don't have enough a big enough Cap on the system in the system to run it at the right speed, but like I said earlier, adding a 10k uF the cap didn't seem to do anything, but I may need to add a 20k uF for each motor.

The alternator might work out well in a car application. But I bet I'd still need to do some power conditioning stuff.

-Nick

[joeybagadonuts]

How may amp output is your converter?

JoeyB

[poptarted]

The DC-DC converter is 12.5 Amps output

-Nick

[joeybagadonuts]

Guessing the low voltage may be causing the motors to draw more current than the drivers may be able to handle.

Or the inverter is outputing really dirty D.C. running at its maximum capacity.

You will have a much better answer once you try the two battery combo.

Might suggest using a 36 volt inverter however it still only has a 270 watt continuous rating.

JoeyB

[cjmerlincnc]

Hi, Getting high revs from steppers is a function of voltage so raising the voltage will enable you to get more speed. Having said that, you will find larger steppers will not rev as high as smaller ones. Sometimes it is better to use a slightly smaller stepper with the right gearing to attain a higher speed.

If you are using batteries as a supply, adding larger capacitors will not help attain more speed.

Capacitors are only used to smooth the ripple on AC driven power supplies as more current is drawn from the supply. They also soak up the back-EMF produced as a motor decelerates. If you do not have enough capacitance, B-EMF will raise the voltage of the supply or as you draw more current from an AC driven supply you will get more ripple, lowering the available voltage that the motors can use.

Unless you use some trial and error method by changing components, you really need a scope to see whats going on.

Using batteries as a supply is a good idea for testing as you wont have a ripple problem.

Try making the wires from the drive to the steppers as short as possible and with as large a diameter as you can fit in the drive terminal block.
Same with the supply wires.

You mentioned that adding a capacitor near the drive helped, maybe your wires are a bit too long or too small.

I would be inclined to borrow a few batteries and raise the voltage to 36v.



Regards

[poptarted]

A single motor runs just fine from a 24 V AC-DC power supply with a 10k uF cap on the supply terminals.

2 batteries in series to make 24 V work fine too.

Adding in the step up converter means switching is occurring, but that in at 54 kHz. So it is an AC source in that sense, but I'm not sure. I assume that the 80000*I/V was derived from 60 Hz full wave, so I guess if I have a 54 kHz driver I'd use C = 10e6*I/(2*f*V) = 10e6*6/(2*54e3*24) = 23 uF, which may or may not be significant at all. So I guess if I'm testing from an AC source through the Step up converter, I'd need ~20k uF per motor and it should be something small from a battery.