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Low Current Motors with 5A/15 A Amplifier - Safety
#1
Hello, I have to drive a low current two phase stepper motor using the 5/15A PWM Amplifier.

The motor has a RMS current per phase equal to 220 mA. The Resistance is 7.4 ohms and the inductance is 2.1 mH.

I would like to know if there are recommendations about a safe Current Loop Tuning, talking about the starting PI gains and I2T configuration.

I'm using:

GLOBAL Ch6MaxAdc = 33.85;
GLOBAL Ch6RmsContCur = 0.17
GLOBAL Ch6RmsPeakCur = 2 *(Ch6RmsContCur);
GLOBAL Ch6TimeAtPeak = 0.1;

Motor[6].MaxDac = Ch6RmsPeakCur * 40132.44 / Ch6MaxAdc
Motor[6].I2TSet = Ch6RmsContCur * 40132.44 / Ch6MaxAdc
Motor[6].I2tTrip = (POW(Motor[6].MaxDac,2) - POW(Motor[6].I2TSet,2)) * Ch6TimeAtPeak
Motor[6].IdCmd = Motor[6].I2TSet/2;

The Current Step Magnitude I want to use is 150 bits (same for Rough Phasing) (less than the I2TSet value).

I have already damaged a same model motor, so it's important to do the current tuning process in a safe way, considering that is a low current and low inductance motor, that might be fragile over possible current peaks.

Any sugestions about driving low current and low inductance motors with the 5/15A amplifier? Starting safe gains?

Any condition that would bypass the I2T set to consider using external fuses?
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#2
Hi Matheus,

You have already discovered that it is not easy to configure small motors on big amplifiers! It is possible to do it but it requires care. We have many customers who have made it work.

I suggest putting a fuse in each phase to provide protection if you have an incorrect setting.

Start with a very small value for Motor[x].Iigain. Try 0.01 and a current step of 100 bits and see what happens. Make sure that the current loop is controlling properly without a massive overshoot.

I know all the units you have are 5/15A versions but there are also smaller versions (0.25/0.75 and 1/3) available if you need to run a lot of small motors.

Tony Jacobs (Faraday Motion Controls)
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#3
(09-02-2019, 01:43 AM)Tony Wrote: Hi Matheus,

You have already discovered that it is not easy to configure small motors on big amplifiers! It is possible to do it but it requires care. We have many customers who have made it work.

I suggest putting a fuse in each phase to provide protection if you have an incorrect setting.

Start with a very small value for Motor[x].Iigain. Try 0.01 and a current step of 100 bits and see what happens. Make sure that the current loop is controlling properly without a massive overshoot.

I know all the units you have are 5/15A versions but there are also smaller versions (0.25/0.75 and 1/3) available if you need to run a lot of small motors.

Tony Jacobs (Faraday Motion Controls)

Thank you by your advice.

I had success driving 150 mA and 400 mA motors before, so it should not be a problem.

But this time we suspect of the combination of low current and low inductance, that may cause higher current ripples.

Do you think increasing PWM frequency can help in this case? The currently frequency is 20 kHz.
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#4
You could increase the PWM frequency up to 30khz but I think the recommended 20khz should be sufficient. You need to use a current probe and an oscilloscope to see if there is a problem with current ripple.
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