# 2-Phase, and 4-Phase steppers?



## 8ntsane

Hello Guys

This is probably just another on of my stupid questions again, but looking at different stepper motors Im seeing some listed as 2-phase, some as 4-phase.

I just have to ask, what is the difference, and why would you want one over the other? Meaning, would you pick one over the other for torque, or speed?

Not knowing anything about the two, leads to this post. Seems I ask alot of dumb questions lately.


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## DMS

I have seen motors labeled as one or the other, but I can't seem to find a clear distinction between the two. Some documentation seems to refer to them interchangeably, though I did find a trend of calling 6 wire unipolar motors "4 phase" motors. Which may make sense, given the have 4 windings (2 windings split in half in a lot of cases).

So... how does that help you.. Maybe it doesn't Jgedde seemed like he had quite a bit of experience with steppers, so maybe we can lure him out. 

If it were me, I'd go with the the 2 phase motors, as all the motors I have used have either been labeled "2 phase" or just "stepper motor".


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## jumps4

i thought it was a good question i have no idea the answer either
my quess was 4 wires 2 phase- 8 wires 4 phase but thats based on nothing just a guess
steve


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## jumps4

i looked up 4 phase on ebay and the ones i seen have 5 wires
then i found this 5 phase? 
:whiteflag:http://www.ebay.com/itm/LOT-OF-4-VE...t=BI_Control_Systems_PLCs&hash=item3377021fcb
i'm lost
steve


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## arvidj

Possibly more than you would ever care to know about steppers. Or maybe not.


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## Kennyd

Some good info here as well: http://www.geckodrive.com/support/step-motor-basics.html


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## jgedde

Let's leave 5-phase steppers out of the mix right now as they are a rather unique item manufactured by only a scant few companies...

OK,  Here goes...  The vast majority of stepper motors have two windings in them.  If the windings are NOT center tapped, this is a bipolar stepper motor.  If they are, it's a unipolar stepper motor.

Bipolar stepper motors are also known as two-phase motors.  They have the advantages of:
1) Ability to be microstepped.
2) More torque output for a given size because all the available copper (the windings) are being used at once.

They have the disadvantage of more complicated drive electronics as each wire needs to be either driven (+) or (-)

Unipolar motors, also known as 4-phase motors also have two windings, but they include a center tap.  Each center tap is either brought out separately (a six wire motor) or they are tied together internally (a 5 wire motor).  Either way, they are generally driven the same.  If a 6-wire case, the two center taps are usually connected together externally.

These have one big advantage...  Ease of driving...  The center taps are connected to the (+) supply.  The remaining wires (connected to the ends of each winding) are connected to (-) in sequence to move the motor.  ABCD is CW, DCBA is CCW.  

Of course the polarity could be reversed with the (-) connected to the center taps but using (+) common makes for an easier output stage as open collector transistors or open drain FETs can be used without any any level translation circuit.  OK.  I'm getting too complicated...

With any stepper motor, the holding torque with power applied substantially exceeds the running torque.  When in a detent with thr rotor not moving the stepper will "hold" its rated torque.  In other words, it will resist being backdriven until the rated torque is reached.

Running torque is another matter...  The maximum attainable running torque is generally 0.63 times the rated holding torque (if anyone is curious why I can explain).  The available torque falling off with increasing speed.

OK, so why use stepper motors?  

Let's say you have a simple brush or brushless DC servio motor and you want to move 7.2 degrees...  You ramp up the current, the motor starts to move.  As you approach your destination the motor slows down and you get where you need to be.  Does this happen by itself?  Nope.  You need some sort of position measuring device (encoder, resolver, pot, etc) to tell the controller you're there.  You also need a control algorithm.  Now what happens if the load tries to backdrive you to another position?  Well, torque needs to be throttled to maintain position.  This can yield an efficient solutuon since current (torque) is only taken when it's needed.  Also, basically infinite resolution can be obtained...

Enter steppers...  You need to move 7.2 degrees...  Let's say you have a 1.8 degree stepper motor (200 detents). Just step it 4 times (4 x 1.8 = 7.2 deg) and you've moved 7.2 degrees.  No fuss no muss.  If you keep current going after you get to 7.2 degrees the motor will resist being backdriving by its rated torque...  The downside?  You're drawing current to maintain position even if the motor is delivering no torque to counter being backdriven. 

A good start?

John


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## jgedde

jumps4 said:


> i looked up 4 phase on ebay and the ones i seen have 5 wires
> then i found this 5 phase?
> :whiteflag:http://www.ebay.com/itm/LOT-OF-4-VE...t=BI_Control_Systems_PLCs&hash=item3377021fcb
> i'm lost
> steve



Put your white flag down Steve.  It's not as bad as all that...J

ust forget you ever saw those 5-phase motors.  They are made by only a single company (that I'm aware of) called Vexta.  They require a special driver, nbut have a few interesting advantages for certain applications or where you need really fine step angles.

In our world, bipolar (two-phase) and unipolar (4-phase) are what we'd commonly use....

John


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## 8ntsane

Thanks guys for your replys

I have a few questions for John if he doesnt mind.

Im seeing talk of 5 and 6 wire steppers.
I know, probably another one of my all time dumb questions, but Ill ask anyway.:nuts:

Here is the links to 1200 oz/in steppers Im looking at for one of my own projects.

http://www.automationtechnologiesin...er-motor-1200-oz-in-12-single-shaft-with-flat

This one is the 4-wire, 2-phase, 6.0A current per phase.

http://www.automationtechnologiesin...or-1200-oz-in-half-inc-single-shaft-with-flat

The second link is a 1200 oz/in , but this is a 8-wire.
Now, looking at the bottom of the page, gives additional specs on each stepper. The first link with the 4-wire bipolar looks as though the wiring is simple.

The 8-wire can be wired 3- different ways. Parallel,
Series, or Unipolar. Now from what Im seeing listed, Bipolar can be wired Parallel,or Series.
Parrallel current amp-6
Series    current amp-3

This leads me to ask, what is the effect from wiring it one way, or the other?
More or less torque?
More or less speed?
Smoother operation? :thinking:

Allso noted, if wired for unipolar, the holding torque
drops from 1200 oz/in to 857oz/in.  Again, I have to ask what effect this mode of wiring has? I see it looses torque, but there must be some benefit in doing so. 

Maybe Im just over thinking all this 
Maybe I should just pour a triple shot of JD, and forget all about these steppers:nuts::thinking::whistle:, till I get straight again :rofl:


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## jgedde

8ntsane said:


> Thanks guys for your replys
> 
> I have a few questions for John if he doesnt mind.
> 
> Im seeing talk of 5 and 6 wire steppers.
> I know, probably another one of my all time dumb questions, but Ill ask anyway.:nuts:
> 
> Here is the links to 1200 oz/in steppers Im looking at for one of my own projects.
> 
> http://www.automationtechnologiesin...er-motor-1200-oz-in-12-single-shaft-with-flat
> 
> This one is the 4-wire, 2-phase, 6.0A current per phase.
> 
> http://www.automationtechnologiesin...or-1200-oz-in-half-inc-single-shaft-with-flat
> 
> The second link is a 1200 oz/in , but this is a 8-wire.
> Now, looking at the bottom of the page, gives additional specs on each stepper. The first link with the 4-wire bipolar looks as though the wiring is simple.
> 
> The 8-wire can be wired 3- different ways. Parallel,
> Series, or Unipolar. Now from what Im seeing listed, Bipolar can be wired Parallel,or Series.
> Parrallel current amp-6
> Series    current amp-3
> 
> This leads me to ask, what is the effect from wiring it one way, or the other?
> More or less torque?
> More or less speed?
> Smoother operation? :thinking:
> 
> Allso noted, if wired for unipolar, the holding torque
> drops from 1200 oz/in to 857oz/in.  Again, I have to ask what effect this mode of wiring has? I see it looses torque, but there must be some benefit in doing so.
> 
> Maybe Im just over thinking all this
> Maybe I should just pour a triple shot of JD, and forget all about these steppers:nuts::thinking::whistle:, till I get straight again :rofl:



Not so bad...  The crux is this:  Current times the torque constant equals torque.  Whether that current is split between two windings in series or flowing through two windings in series, the net result is the same.  Only the voltage required changes.  This gives you the opportunity to use one of two different voltages to get the same torque. 

The bipolar used in a unipolar case yields less torque because less copper is actively being used, but offers the advantage of a simpler driver.

In any case, the torque / speed curve remains the same whether it's wired series or parallel.  In addition nothing will be gained as far as smoothness of motion in either case.

From looking at your links, I suspect both motors are internally the same with the only difference being what wires are brought out.

John


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## 8ntsane

John

Thanks for anwering my questions. I dont mean to be a PITA, but inquiring minds just want to know.:thinking:

Thanks again


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