Stepper motor driving- Unipolar Vs Bipolar

[Bryan Hackney]

Hi

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

Thanks
Richard

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.

 

[Richard]

Hi

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

Thanks
Richard

 

[John Jardine]

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.

 

[John Popelish]

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

....for the same copper losses.

 

[John Jardine]

...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

 

[John Popelish]

news:[email protected]...

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.

 

[Tauno Voipio]

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

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

 

[John Jardine]

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.

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

 

[John Jardine]

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

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