Stepper Motor Measured Current Lower Than Expected

Using Arduino Motors, Mechanics, Power and CNC
#1

Hi, so I am using an A4988 stepper driver to control a stepper motor. I am trying to set the current limit to the right amount. When I measure the current in series for one of the coils while stationary I get a reading of 0.69-0.7A with the potentiometer turned all the way up on the driver. My motor is supposed to be rated for 1.5A and the stepper driver is supposed to be able to work for 2A. I bought both on Amazon so possibly their ratings are incorrect? My question is: should this be expected or possible, or does it possibly have to do with either the driver or the motor working incorrectly or being rated wrong?

Also, I understand that the stationary measured current is supposed to be like 70% of the full value. But even then that gives a max current of 1A I assume.

For reference this is the motor and drivers I purchased:

#2

Hi, so I am using an A4988 stepper driver to control a stepper motor. I am trying to set the current limit to the right amount. When I measure the current in series for one of the coils while stationary I get a reading of 0.69-0.7A with the potentiometer turned all the way up on the driver. My motor is supposed to be rated for 1.5A and the stepper driver is supposed to be able to work for 2A. I bought both on Amazon so possibly their ratings are incorrect? My question is: should this be expected or possible, or does it possibly have to do with either the driver or the motor working incorrectly or being rated wrong?

Also, I understand that the stationary measured current is supposed to be like 70% of the full value. But even then that gives a max current of 1A I assume.

For reference this is the motor and drivers I purchased:

#3

Which power supply are you using? Is it capable of continuous 1.5 Amps?

#4

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Thanks in advance for your cooperation.

#5

nesser:
When I measure the current in series for one of the coils while stationary I get a reading of 0.69-0.7A with the potentiometer turned all the way up on the driver.

Be VERY CAREFUL never to connect or disconnect the wires between the motor and the stepper driver while the driver is powered up. The driver will be instantly destroyed.

How is your multimeter connected?

What current do you measure when the potentiometer is turned to the other extreme?

What motor power supply are you using (volts and amps) ?

…R

#6

JCA34F:
Which power supply are you using? Is it capable of continuous 1.5 Amps?

Not necessary, power supply current draw is less than the motor current draw with a stepper driver.

The A4988 has an absolute maximum rating of 2A, so you'd never expect it to handle 2A, and in
practice the small modules struggle over 1A due to limited heatsinking.

The initial state at power on is at 45 electrical degrees, so that each winding should see 1/√2 times the
nominal set current at that time. So if set for 1A, you'd see 0.7A or so in each winding.

(Remember to never connect, disconnect or reconnect the motor windings with the power applied -
this usually destroys the device. This means that when measuring current you can't do handheld
multimeter measurements, you have to set up with the multimeter connected solidly into the circuit
before powering.)

#7

JCA34F:
Which power supply are you using? Is it capable of continuous 1.5 Amps?

I am using a 3S LiPo battery which can supply over 50A if needed.

Robin2:
Be VERY CAREFUL never to connect or disconnect the wires between the motor and the stepper driver while the driver is powered up. The driver will be instantly destroyed.

How is your multimeter connected?

What current do you measure when the potentiometer is turned to the other extreme?

What motor power supply are you using (volts and amps) ?

...R

I have my wiring set up on a breadboard with three wires for the motor connected directly to the motor and the fourth that connects to a header pin on the breadboard which I connect to another header pin via the multimeter using the amp measurement setting. The second header pin then is wired to the motor. I do not have any additional multimeter clips so I am just very careful to keep the leads pressed to the headers so that I do not disconnect the wires while running so as not to ruin my motor or driver as you stated.

When the potentiometer is turned to the other extreme it reads 0A.

Motor power supply is 3S LiPo which reads about 12.5V and can supply up to around 50A continuous current.

MarkT:
Not necessary, power supply current draw is less than the motor current draw with a stepper driver.

The A4988 has an absolute maximum rating of 2A, so you'd never expect it to handle 2A, and in
practice the small modules struggle over 1A due to limited heatsinking.

The initial state at power on is at 45 electrical degrees, so that each winding should see 1/√2 times the
nominal set current at that time. So if set for 1A, you'd see 0.7A or so in each winding.

(Remember to never connect, disconnect or reconnect the motor windings with the power applied -
this usually destroys the device. This means that when measuring current you can't do handheld
multimeter measurements, you have to set up with the multimeter connected solidly into the circuit
before powering.)

Right, so I turn my potentiometer all the way up and only get a reading of ~0.7A. Shouldn't that be 1.4A? The plan is to limit the driver to the motor's rated amps of 1.5A and use a cooling fan to cool the driver. However, I am measuring only 0.7A max, which would be 1A normally instead of 1.4A which would be 2A normally. Or am I misunderstanding how it works?

Additionally, the motor runs fine with the set up. I used the same wiring as described above with the multimeter hooked up and ran the motor through several different revolutions. While the amp measurement fluctuated as expected I did not see it go above the 0.7A as previously measured.

#8

nesser:
I have my wiring set up on a breadboard with three wires for the motor connected directly to the motor and the fourth that connects to a header pin on the breadboard which I connect to another header pin via the multimeter using the amp measurement setting. The second header pin then is wired to the motor. I do not have any additional multimeter clips so I am just very careful to keep the leads pressed to the headers so that I do not disconnect the wires while running so as not to ruin my motor or driver as you stated.

It's much to easy to misinterpret a textual description of wiring connections. Please provide a drawing.

...R

#9

Robin2:
It’s much to easy to misinterpret a textual description of wiring connections. Please provide a drawing.

…R

Attached a wiring diagram below which I used and made sure to follow correctly. The multimeter is wired up in series with one of the motor lines.

I am going to include my comment that I edited into my previous reply as it may have been missed due to a late edit:

“Additionally, the motor runs fine with the setup. I used the same wiring as described above with the multimeter hooked up and ran the motor through several different revolutions. While the amp measurement fluctuated as expected I did not see it go above the 0.7A as previously measured.”

A4988-Wiring-Diagram.png
A4988-Wiring-Diagram.png800×626 25.6 KB

#10

nesser:
I am using a 3S LiPo battery which can supply over 50A if needed.

And you have a fuse I trust?

I have my wiring set up on a breadboard with three wires for the motor connected directly to the motor and the fourth that connects to a header pin on the breadboard which I connect to another header pin via the multimeter using the amp measurement setting. The second header pin then is wired to the motor. I do not have any additional multimeter clips so I am just very careful to keep the leads pressed to the headers so that I do not disconnect the wires while running so as not to ruin my motor or driver as you stated.

That's precisely the sort of lash-up that's going to risk burning up a driver - solid connections only, and preferably not
in a breadboard either. Choc-block terminal blocks are great for this, you can clamp a multmeter probe into them too, not just wires.
I think you've been lucky.

When the potentiometer is turned to the other extreme it reads 0A.

Right, so I turn my potentiometer all the way up and only get a reading of ~0.7A. Shouldn't that be 1.4A? The plan is to limit the driver to the motor's rated amps of 1.5A and use a cooling fan to cool the driver. However, I am measuring only 0.7A max, which would be 1A normally instead of 1.4A which would be 2A normally. Or am I misunderstanding how it works?

Yes, you'd expect 1.4A if the unit you have has a current sense resistor appropriate to 2A full scale, which
it clearly doesn't, its 1A full scale. You may even be able to read the value of the current sense resistor on
the module, SMT resistors usually have the value printed on them.

Additionally, the motor runs fine with the set up. I used the same wiring as described above with the multimeter hooked up and ran the motor through several different revolutions. While the amp measurement fluctuated as expected I did not see it go above the 0.7A as previously measured.

It certainly should go above 0.7, as the home state is 45 electrical degrees, so you'll see 1/√2 times the nominal
current initially (sin(45) == 1/√2), then cycling up and down between +1 and -1 times the nominal current.

All assuming you are using a sensible microstepping setting (half steps or finer).

#11

MarkT:
And you have a fuse I trust?That's precisely the sort of lash-up that's going to risk burning up a driver - solid connections only, and preferably not
in a breadboard either. Choc-block terminal blocks are great for this, you can clamp a multmeter probe into them too, not just wires.
I think you've been lucky.Yes, you'd expect 1.4A if the unit you have has a current sense resistor appropriate to 2A full scale, which
it clearly doesn't, its 1A full scale. You may even be able to read the value of the current sense resistor on
the module, SMT resistors usually have the value printed on them.
It certainly should go above 0.7, as the home state is 45 electrical degrees, so you'll see 1/√2 times the nominal
current initially (sin(45) == 1/√2), then cycling up and down between +1 and -1 times the nominal current.

All assuming you are using a sensible microstepping setting (half steps or finer).

I don't have a fuse. Is it really needed?

Maybe I am a little lucky but I am quite careful with the connections I make. Haven't had anything go wrong yet in any of my projects to date so I think I'm doing something right.

Ok, so I assume that the drivers that I bought off of Amazon that call themselves A4988 are just modified versions that can only run 1A. The board definitely looks slightly different than the Polulu's A4988 page. The two resistors say R100.

So, assuming my drivers can only supply 1A then, would I still likely be okay running the stepper motors that are rated for 1.5A? They were turning fine with no load on them, I'm hoping they will still be able to do the job. Like how much will the performance be affected?

#12

Breadboard tracks burn if subjected to motor currents, so your luck is about to run out. The very fastest way to destroy a motor driver is through loose motor connections, so either use screw terminals, or solder the wires to the motor driver.

The A4988 data sheet has information on how to set the current limit, which in turn depends on the value chosen by the board manufacturer for the current sense resistor. Make sure that you set the current limit to 1 Ampere maximum, and you should be OK.

Pololu has a nice video on how to go about setting the current limit, but it applies only to their boards. Pololu engineers make high quality products, and very actively support them, so it is worth spending a bit more to buy from them.

#13

Image from Reply #8 so we don't have to download it. See this Simple Image Posting Guide

A4988-Wiring-Diagram.png
A4988-Wiring-Diagram.png800×626 25.6 KB

...R

#14

@nesser, the image in Reply #8 and made visible in Reply #12 is not a drawing that YOU made showing how YOUR Arduino and stepper driver is wired.

...R

#15