Bipolar drive of unipolar stepper motor?

[Don Lancaster]

I'm trying to puzzle out the stepper drive on a programmable wire cutter
I am refurbing.

The medium power fist sized stepper is clearly six wire and intended for
unipolar drive.

Typical 1.5 ohm low voltage windings, one on either side of a center tap.

The drive is a L298, which is a fancy high voltage 38 volt switchmode
current drive bridge.

The center taps of the stepper windings do not seem to be connected
anywhere else. And clearly not to each other.

Can you simply bridge drive BOTH bifilar windings at once by ignoring
the center tap, causing the bipolar stepper to become a unipolar one?

Presumably of double power.

The "wrong" current direction (caused by the left half bridge versus the
right half) through the second winding should aid the first one.

Can't seem to find any mention of what might be obvious anywhere else.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Tim Wescott]

I'm trying to puzzle out the stepper drive on a programmable wire cutter
I am refurbing.

The medium power fist sized stepper is clearly six wire and intended for
unipolar drive.

Typical 1.5 ohm low voltage windings, one on either side of a center tap.

The drive is a L298, which is a fancy high voltage 38 volt switchmode
current drive bridge.

The center taps of the stepper windings do not seem to be connected
anywhere else. And clearly not to each other.

Can you simply bridge drive BOTH bifilar windings at once by ignoring
the center tap, causing the bipolar stepper to become a unipolar one?

Presumably of double power.

The "wrong" current direction (caused by the left half bridge versus the
right half) through the second winding should aid the first one.

Can't seem to find any mention of what might be obvious anywhere else.

AFAIK, yes, you can.

To avoid overheating the motor you'd need to drive it at sqrt(2) of the
unipolar drive voltage, so you'll only increase torque by 40% rather
than doubling it. Depending on how the motor is designed you may or may
not have funny saturation problems. Of course you can always run the
motor at the unipolar current for double the power dissipation in the
motor -- if it's intermittent use you may even be able to get away with it.

I've never done this myself, so I can't lend anything more than some
ramblings based on theory.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[[email protected]]

The medium power fist sized stepper is clearly six wire and intended for
unipolar drive.

Typical 1.5 ohm low voltage windings, one on either side of a center tap.

The drive is a L298, which is a fancy high voltage 38 volt switchmode
current drive bridge.

Actually, it's pretty low end for serious motion control.

The center taps of the stepper windings do not seem to be connected
anywhere else. And clearly not to each other.

Can you simply bridge drive BOTH bifilar windings at once by ignoring
the center tap, causing the bipolar stepper to become a unipolar one?

Yes. This is quite common. One catch through is that you increase the
inductance, and for this reason you may want a higher power supply
voltage to supply your chop-mode current regulator if you need high
step rates. Good motors may be 8 wire so that you can run the half
windings in parallel rather than series (using high current drivers of
course).

Presumably of double power.

Not necessarily. With a good driver the limiations may actually be
either heat dissipation in the motor, or the field strength at which
you risk demagnetizing the permanent magnets in the motor. (BTW, don't
ever pull the rotor out of a stepper motor without using a flux keeper
sleeve)

Try asking on alt.machines.cnc or the yahoo group CAD_CAM_EDM_DRO - the
hobbyist CNC gang does a lot with stepper motors from those using L298
type drivers up to ones using discrete H bridges of N MOSFETs with
floating high-side date drivers, etc.

 

[Spehro Pefhany]

I'm trying to puzzle out the stepper drive on a programmable wire cutter
I am refurbing.

A Schleuniger?

The medium power fist sized stepper is clearly six wire and intended for
unipolar drive.

Yes. Maybe a NEMA 34 frame, depending on how big your fist is.

Typical 1.5 ohm low voltage windings, one on either side of a center tap.

The drive is a L298, which is a fancy high voltage 38 volt switchmode
current drive bridge.

Old-fashioned bipolar dual H-bridge design, gets really hot with
substantial current. ST makes them, Mouser and Digikey should carry
replacements if it's shot. It's only switchmode if you drive it so
(eg. with the sidekick sequencer chip).

The center taps of the stepper windings do not seem to be connected
anywhere else. And clearly not to each other.

That's fine.

Can you simply bridge drive BOTH bifilar windings at once by ignoring
the center tap, causing the bipolar stepper to become a unipolar one?

Yes, you can use the unipolar as a bipolar by ignoring the center taps
on each coil and driving the ends with an H-bridge.

Presumably of double power.

The "wrong" current direction (caused by the left half bridge versus the
right half) through the second winding should aid the first one.

Yes, but you are getting twice the heating with two coils energized.
Not sure the relationship is between torque and driving method, but
motor data sheets should have this information.

Can't seem to find any mention of what might be obvious anywhere else.

Best regards,
Spehro Pefhany

 

[Don Lancaster]

A Schleuniger?



Yes. Maybe a NEMA 34 frame, depending on how big your fist is.




Old-fashioned bipolar dual H-bridge design, gets really hot with
substantial current. ST makes them, Mouser and Digikey should carry
replacements if it's shot. It's only switchmode if you drive it so
(eg. with the sidekick sequencer chip).




That's fine.




Yes, you can use the unipolar as a bipolar by ignoring the center taps
on each coil and driving the ends with an H-bridge.




Yes, but you are getting twice the heating with two coils energized.
Not sure the relationship is between torque and driving method, but
motor data sheets should have this information.





Best regards,
Spehro Pefhany

Unit is a Schuleniger UC 3750 and will be up for sale once I complete
the refurb.

Presumably the original engineering with regard to heat and current
etc... will still be valid once I replace the driver. I'm just trying to
find out exactly what they were intending to do.

Besides being frozen and locked up (already repaired with new bushings,
etc), some junior G-Man plant electrician managed to replace a bridge
rectifier 180 degrees out of position, thus converting the +42 volt
supply to a MINUS 42 volt supply, and raising all holy hell in the process.

Most of the refurb is done. Blades appear perfect and rollers repolished
beautifully. I suspect the slightly smaller rollers might affect the
exact wire size programmed in, though.

Discovered after a dozen false starts that the way you replace an oilite
bushing is to use a Hex Allen Cap Screw! Between the 20 threads per inch
and another 18:1 or so of wrench gain, two or three pounds of twist
gives you half a ton of insertion/withdrawal force.

Sure has been an adventure restoring an obvious basket case.
Also willing to sell as is at 90 percent refurbed.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Sjouke Burry]

AFAIK, yes, you can.

To avoid overheating the motor you'd need to drive it at sqrt(2) of the
unipolar drive voltage, so you'll only increase torque by 40% rather
than doubling it. Depending on how the motor is designed you may or may
not have funny saturation problems. Of course you can always run the
motor at the unipolar current for double the power dissipation in the
motor -- if it's intermittent use you may even be able to get away with it.

I've never done this myself, so I can't lend anything more than some
ramblings based on theory.

For unknown 6 wire stepper motors,I temporarely glue
a stick to the shaft, then touch a mall voltage to each
of the four coil ends. the stick moves a bit for each ending,
and it shows you the order in which to step them(watch out
for bouncing across whole step(s)).
to control the motor then ,the centre taps go to the supply,
and you step through the four endings in succesion.
I get a higher momentum when i step (a+b b+c c+d d+a .......),
watch motor temperature when you do that(but the holding torque
is very high).

 

[Tim Williams]

(BTW, don't
ever pull the rotor out of a stepper motor without using a flux keeper
sleeve)

Alnico???

Thought that stuff hasn't been used for ages.

Tim