# Current rating for an 8 wire stepper motor with coils connected in series

1. ## Current rating for an 8 wire stepper motor with coils connected in series

Hi. I have an 8 wire stepper motor which I have connected the coils in series to run off an A4988 driver.

The rating on the back of the motor is 5V 1A

I am driving it with a 20 volt supply. To adjust the current I set the driver stepping once every 5 seconds and measured the current through one of the coil pairs and adjusted the A4988 until the current was 1.02 Amps.

All works wonderful except the motor gets too hot to touch. WHY?

I am wondering if for the case of connecting the coils in series, should I only be using half the rated current or something.

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2. ## Re: Current rating for an 8 wire stepper motor with coils connected in series

Hi,

Possible problems
* wrong temperature expectation (it's normal that they get hot. 1A is the upper limit)
* wrong wiring. Maybe too many coils get current (see datasheet about wiring, current limit conditions)
* wrong current measurement

Current:
Average current measurement in a stepper motor gives the information about expectable torque.
RMS current gives the information about coil heating.

With low ripple (almost DC current) average current and RMS current are almost identical.
If there is high ripple current, then RMS current will be (much) higher than average current.

I assume you do a kind of average current measurement. Then 1.08A average current will cause higher RMS current and thus higher temperature rise.

Mind: Let's assume the RMS current is 20% higher than rated (1.2A) .. this will cause 1.2 x 1.2 = 1.44 times dissipated power and thu about 1.44 time higher temperature rise.

Additionally higher ambient temperature, less air flow, bearing heating and loss in magnetic core will increase temperature.

Klaus

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3. ## Re: Current rating for an 8 wire stepper motor with coils connected in series

I am wondering if for the case of connecting the coils in series, should I only be using half the rated current or something.
I guess yes. But without a detailed datasheet, it's impossible to decide clearly how the 5V/1A specification is meant.

I guess, the purpose of the "8-wire" configuration is to allow either unipolar or bipolar connection. If so, bipolar operation would connect two windings in parallel, or apply half the current in series circuit.

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4. ## Re: Current rating for an 8 wire stepper motor with coils connected in series

Thanks Guys. Looks like there is no hard and fast rule. Can't find a datasheet so will work on the premise that if it is not too hot it is not too hot.

5. ## Re: Current rating for an 8 wire stepper motor with coils connected in series

Can't find a datasheet so will work on the premise that if it is not too hot it is not too hot.
Motors can run at high temp but this is actually limited by the insulations on the windings and they can get cooked at high temp (say much above 100C)

We can estimate the expected temp rise: the motor will dissipate at worst condition 5W (5V @1A) and the heat will be dissipated (mostly by radiation) from the surface.

If the motor is physically small (5V 1A motors are not really large) the case temp may rise upto 100C (too hot to touch) but this will be the worst case.

Perhaps the 20V is too much (even if you regulate the current); are you driving it too fast or too slow (both are extreme and make lots of heat).

What is the DC coil resistance for each section? Does the 8 wires come from 8 individual coils?

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6. ## Re: Current rating for an 8 wire stepper motor with coils connected in series

Originally Posted by c_mitra
Motors can run at high temp but this is actually limited by the insulations on the windings and they can get cooked at high temp (say much above 100C)

We can estimate the expected temp rise: the motor will dissipate at worst condition 5W (5V @1A) and the heat will be dissipated (mostly by radiation) from the surface.

If the motor is physically small (5V 1A motors are not really large) the case temp may rise upto 100C (too hot to touch) but this will be the worst case.

Perhaps the 20V is too much (even if you regulate the current); are you driving it too fast or too slow (both are extreme and make lots of heat).

What is the DC coil resistance for each section? Does the 8 wires come from 8 individual coils?
Thanks c_mitra.

It was an 8 stepper with 4 coils each a DC resistance of 5 ohms. So with them connected in series there are two coils of 10 ohms each.

It is a stepping motor so not sure if speed is a factor. The coils are on all the time with a constant current of 1 amp whatever the load. Except when the current direction is being switched

The higher voltage of 20 V is used to decrease the time it takes for the coil current to reach 1 amp.

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