[day1118]

Hi Guys, I am looking at buying 4 * 385 Oz motors at 50V and 2.8mH inductance. (2 for the X axis)
To drive them I am planning to get a G540.

Now I am trying to select a power supply. From my understanding:
The ideal voltage is 32*sqrt(2.8) = 53V however G540 can only handle 50V so I will go for 48V to be safe.

Ideal current is 4*3.5A = 14A, however the motors will only ever draw 2/3 of this. 14A * 2/3 = 9.3A. (Out of interest, can anyone explain where the 2/3 comes from?)
This lead me to thinking a 48V 10A supply would be ideal, but from what I have read, the G540 has a 7A fuse in it anyway, so is there any advantage to getting a power supply that can produce more than 7A (Despite math suggesting 10A)?

Will I need to put current limiting resistors in? And at what resistance/current to drive 4 * 3.5A motors from the G540?

Thanks
Anthony

[aarggh]

Hi Anthony,

You'll be very happy with the Gecko, they're beaut that's for sure. 48v is the norm for supplies, you have a fair amount of leeway, even if the calcs indicate a higher voltage requirement, within reason that is, it just means with a 48v you may not get quite the full calculated speed or torque out of the motors. For most somewhere around the 2.8mH sweet spot it's just about perfect.

Even though it is the default max rating, most people do seem to recommend putting in the limiting resistor at the G540 end of the plug, I'd recommend 1% tolerance resistors as close to 3.5k as possible, in 1% I think they are around 3.48k, 5% is 3.3k to 3.6k. Jaycar hopefully is a decent enough electronics store they'll have 1% (E96 range) resistors. From memory the resistor was required for the drives to go into standby mode to reduce the power consumption, while maintaining hold with lower heat buildup. I could be wrong there though, not sure if the newer ones are different now? You'll want the advantage of standby mode anyway.

The 2/3 current rating is a rule of thumb calculation with a built in safety buffer, based on the fact that 3/3 means all steppers are running full load, all at the same time, this is in fact very rare, and you'll actually find 2/3 is a fairly generous rating you'll rarely go close to for casual use, but it's always better to allow a decent buffer.

Picture machining something, when all three steppers are engaged, it's usually on arcs or something similar, most general machining uses 2 of the 3 axis most of the time, quite often not even at full load per axis.

cheers,
Ian

[day1118]

Hi Anthony,

You'll be very happy with the Gecko, they're beaut that's for sure. 48v is the norm for supplies, you have a fair amount of leeway, even if the calcs indicate a higher voltage requirement, within reason that is, it just means with a 48v you may not get quite the full calculated speed or torque out of the motors. For most somewhere around the 2.8mH sweet spot it's just about perfect.

Even though it is the default max rating, most people do seem to recommend putting in the limiting resistor at the G540 end of the plug, I'd recommend 1% tolerance resistors as close to 3.5k as possible, in 1% I think they are around 3.48k, 5% is 3.3k to 3.6k. Jaycar hopefully is a decent enough electronics store they'll have 1% (E96 range) resistors. From memory the resistor was required for the drives to go into standby mode to reduce the power consumption, while maintaining hold with lower heat buildup. I could be wrong there though, not sure if the newer ones are different now? You'll want the advantage of standby mode anyway.

The 2/3 current rating is a rule of thumb calculation with a built in safety buffer, based on the fact that 3/3 means all steppers are running full load, all at the same time, this is in fact very rare, and you'll actually find 2/3 is a fairly generous rating you'll rarely go close to for casual use, but it's always better to allow a decent buffer.

Picture machining something, when all three steppers are engaged, it's usually on arcs or something similar, most general machining uses 2 of the 3 axis most of the time, quite often not even at full load per axis.

cheers,
Ian

Thanks Ian, that's great.

However, I am slightly confused on one point. Don't stepper motors run at constant current? (ie use power when moving, and use power when still to produce the holding torque?)

Also, can someone please confirm that there is absolutely NO reason to buy a power supply that can deliver more than 7A at 48V due to the fuse?

Thanks
Anthony

[cnc_4_me]

Day1118, your math is correct for 4 drives at 3.5 amps = 14 amps and .67 of that = 9.38 amps max required to fully power a Gecko G540. Also your choice of 48vdc is a good one for maximum speed from the stepper drives and a fairly common voltage supply.

The above calculation is actually worst case thinking you will drive 4 stepper’s at full power at the same time. In actual practice you will be hard pressed to move more than 2 stepper’s at full power, and have the other 2 at reduced power for holding torque. But i personally would try to get close to the 9.38A power supply anyway.

I think the .67 current factor comes from the PWM (pulse width modulation) chopping the current to the motor. Good stepper motors have a low voltage rating of a few volts. To keep the current down when putting 48 volts on a 3 volt motor you have to chop up the power going to the motor so the average current is your 3.5 amp motor rating. If you go back and add up all of the “off” spaces of the waveform you find you are using around .67 of the motor current set point from the power supply. The power supply capacitor smoothes out the PWM pulses to give you a fairly constant voltage and current load to the transformer.

The G540 needs a current set resistor of 1K per amp for each motor, so for 3.5 amps you need 3.5K or 3,500 ohms for each motor in the DB-9 connectors pins 1 and 5.

[aarggh]

Hi Anthony,

The prior poster has explained that the PWM method is employed to drive the steppers, this allows the motors to actually be over driven to achieve higher speed/torque ratings than normal, but the on/off period averages out to usually less than what would be required from a constant current power source. So you have a lot of leeway, and in an ideal world a 7A would be fine in your case most likely.

But, and I'm not inferring by any means the Chinese supplies are inferior, but I usually take the stated ratings to be somewhat optimistic and mean, "absolute maximum possible rating, for a maximum period of use, in "optimal" conditions. This essentially means it may derate the output a certain amount for every degree celcius over ambient, which is usually based on 20 deg C in a controlled test environment, or it can output the max stated but only for a bursts. What it boils down to, is while a true constant output 7A suply will most likely deliver more than you'll ever use, it's the difference between working at it's limit, and working within it's limits, that will affect it's long term performance and life.

7A will generally do, but if you can get a bigger one, it gives your equipment a better buffer, and higher tolerances. If however the 8-9A is twice the cost of the 7A, I'd go the 7A.

cheers,
Ian

[harryn]

If I remember correctly, you don't actually need to use a resistor across the G540 pins if the motor can actually handle 3.5 amps. If the resistor is present, it activates the "current reduction at idle" mode, but it isn't mandatory.

It will certainly help with motor cooling through.

I think Geckodrives also recommend linear power supplies, not switching power supplies, although I am still trying to understand exactly why. I suspect it is because a linear supply can often go into overload for a period of time without damage and often have less ripple.

[cnc_4_me]

I think Geckodrives also recommend linear power supplies, not switching power supplies, although I am still trying to understand exactly why. I suspect it is because a linear supply can often go into overload for a period of time without damage and often have less ripple.

Switching power supply’s do not have enough capacitor on the output for return energy dump, so you have to isolate the output of the power supply with a diode and add a capacitor bank. If you do not add a diode, the switching power supply may become unstable with all of the extra capacitance.

[day1118]

Switching power supply’s do not have enough capacitor on the output for return energy dump, so you have to isolate the output of the power supply with a diode and add a capacitor bank. If you do not add a diode, the switching power supply may become unstable with all of the extra capacitance.

Thankyou. Is there anywhere that discusses this? or shows how many and what size capacitors to use?

Thanks