Jaycar stepper motor

[Mikegw]

Here I sit with a jaycar stepper motor, part number ym2751. The jaycar
data sheet is kinda light on details. It is getting very warm. A few quick
questions....

1) It claims to be 12V operation with a 16 ohm coil resistance, current
400mA? Other steppers I have seen follow ohms law.

2)The whole circuit when running draws about an amp, with the motor off I
get effectively zero on that range, thus does the 400mA refer to per coil?
If so 0.4+0.4 is still shy of 1.

3) How hot should these run? Elektor claims the easiest way to tell a
stepper's operation voltage is raise the voltage till it gets hot to the
touch. If so is hot to the touch normal (as in I can touch, just. It but
it is definetly getting hot)?

Cheers

Mike

 

[John Tserkezis]

Mikegw wrote:

Here I sit with a jaycar stepper motor, part number ym2751. The jaycar
data sheet is kinda light on details. It is getting very warm. A few quick
questions....

1) It claims to be 12V operation with a 16 ohm coil resistance, current
400mA? Other steppers I have seen follow ohms law.

This is a bit of a grey area. Using constant voltage to drive stepper motor
is really just a compromise. You should be using constant current. The driver
coils are inductors remember? So the faster you're switching it, the less
effective current being passed through the coils.

If you're going to be using it at low speeds (or even static) use a lower
voltage, and if you're going to be cranking the speed up, use higher voltage.
If you're going to have a wide range of speeds (static and high speeds) there
will likely be no usable compromise on voltage, because it'll end up too hot at
static operation, and not run correctly at speed.

As I said, you should be using constant current.

2)The whole circuit when running draws about an amp, with the motor off I
get effectively zero on that range, thus does the 400mA refer to per coil?
If so 0.4+0.4 is still shy of 1.

They're probably making up an average for you. They apparently know how
you're going to use it before you do.

3) How hot should these run? Elektor claims the easiest way to tell a
stepper's operation voltage is raise the voltage till it gets hot to the
touch. If so is hot to the touch normal (as in I can touch, just. It but
it is definetly getting hot)?

It depends. If you know there's not going to be any load (or very little
load) on the motor, you can skimp on the power during static operation to save
power (and lower heat).

My opinion is that you drive with enough current to make it useable, and a
little more for reliability overhead. If that's too high (hot) while it's
sitting still, then you will need to setup a constant current source to each
coil, or at least have some kind of voltage controller to to lower the supply
voltage at low/static speeds.
--
Linux Registered User # 302622 <http://counter.li.org>

 

[Geoff C]

"Mikegw" <mikegw20@spam.com.spam.hotmail> wrote in
news:chmruj$67l$1@tomahawk.unsw.edu.au:

Here I sit with a jaycar stepper motor, part number ym2751. The
jaycar data sheet is kinda light on details. It is getting very warm.
A few quick questions....

1) It claims to be 12V operation with a 16 ohm coil resistance,
current 400mA? Other steppers I have seen follow ohms law.

2)The whole circuit when running draws about an amp, with the motor
off I get effectively zero on that range, thus does the 400mA refer
to per coil? If so 0.4+0.4 is still shy of 1.

3) How hot should these run? Elektor claims the easiest way to tell a
stepper's operation voltage is raise the voltage till it gets hot to
the touch. If so is hot to the touch normal (as in I can touch, just.
It but it is definetly getting hot)?

Cheers

Mike

It is true that most steppers are notated as ohms law type specs but most
steppers these days are driven with L/R type drives etc. It is common in
many applications to run steppers at quite high temps of 40 or 50 deg
above internal ambient. Most quality steppers with good bearings and a
decent class of wire insulation can easily do this. For a microstepping
drive which is always on there will be current in both coils contributing
to heating.

This motor is unipolar and my advice would be to follow the current
rating rather than the voltage rating. You can aim for a certain temp
rise above ambient and not let the case temp get above, say 60 deg or so.
I note that a 2000 hour life is specified and this is quite low, implying
this is really a shitty motor designed for intermittent use (low duty).

In short, go by an acceptable temp rise. The internal temp will be no
more than about 10 deg above case temp, from experience.

 

[mikegw]

Thankyou gentlemen. I have dropped the supply to 9V and all seems good. My
fingerprints are still intact.

Mike