Stepper motor driving- Unipolar Vs Bipolar

When you energize a coil in a unipolar motor, that's 25% of the motor,
but when you energize a coil on a bipolar motor, that's 50% of the motor,
so yes, you get more output.

 

Hi

Is it true that driving using bipolar method will provide more torque for
most motors as compared to unipolar method?

Thanks
Richard

 

Yes.
A bipolar drive benefits from less winding resistance and can give upto 40%
more torque.

 

....for the same copper losses.

 

Argghhh, Copper losses ...
John .
You're well au fait with the steppers. Any morsels you can cast in my
direction? ...
I finished a unipolar chopper design last month (1/2 to 6amps). Ran tests on
a collection of (11) motors and noticed they -all- overheat. This time round
I used a real temp' probe as against the usual spit and was surprised to see
temps of 90deg+ (and rising).
The makers usually specify a max case temp rise of 55degC over ambient. This
seems only qualifiable when a single motor coil is fed with the rated
voltage and -not- the rated current (=spiralling I^2.R loss due to +3900ppm
wire tempco). I've got no sense out of makers as to how they actually do
their motor temp' rating tests. Is there some kind of 'industry standard
constant-current derating factor', I've missed?.
(I'm worried that I can't in honesty say to a customer "this chopper will
drive a motor at it's rated current", knowing it could blow smoke if they do
so .
regards
john

 

Losses in stepper motors depend a lot on the details of the driving
circuit (pwm frequency, peak voltage, spike clamping, as well as
average current.)

I seldom run any stepper at its full rated current. If I really need
the full rated torque, I prefer to move up a frame size to keep the
temperature under control.

Can you send me a copy of your schematic. I might be able to suggest
some efficiency improving changes. But as a generality, using only
half a winding at a time is not the way to keep temperature rise low.

 

You may have met the other major loss: eddy and hysteresis losses in
the iron of the motor. For a fast PWM, you may get an advantage over
the high-frequency losses by using external filter chokes with
core material and structure fit to the chopper speed.

HTH

Tauno Voipio
tauno voipio @ iki fi

 

Thanks John. The points are appreciated. Nothing speaks better than the
voice of experience ;-)

As mentioned to Tauno, this aspect repeats at DC. Maybe it's why the motor
makers seem rather coy about clearly specifying their test ratings.
But If I may, I'll scan the circuit and send it through. A second pair of
eyes would be welcome.

regards
john

 

Indeed, ... These items were my first port-of-call but I lost further
interest having noticed the same temperature rise occurs at DC.
I.e apply rated voltage from a DC Power supply. Meter DC current. Sit
looking bored for 30 minutes whilst occasionally tweeking the supply voltage
to hold the current at it's rated value.
regards
john