[gera229]

I am pretty sure Series Bi-Polar would not be used because it's probably the worst set up because first if offers the same beginning torque as Parallel Bipolar, but is much slower and loses torque more quickly throughout the speed range.
If there are reasons to use Series over Parallel, I am curious to know.

What about Half-Coil setups:
Half-Coil Bipolar offers about 70% of torque of a Parallel set-up. Correct me if I am wrong.
Right now I am wondering why someone would choose Half-Coil over Parallel for any reason if Parallel offers the best performance range? Or does it not?

Does Half-Coil Bipolar have a faster speed?
Does Half-Coil Bipolar have a faster maximum speed or the same as Parallel Bipolar? If so by approximately how much faster? In percentage?
Does Half-Coil run cooler?
Which would run quieter and generate less noise?
Which would run smoother?

[wildwestpat]

The motor connections are for connection to the drive electronics. The sophistication of the electronics permits all the ampere turns to be realised per step change by supplying bidirectional current flows in each coil as dictated by the step sequence. This complication has reduced in price over the years with the use of FET switches. So the answer to your question is consider the driver as the performance of the electronics is part of the torque / speed achievable from a given pairing.

The more important characteristics is the inductance of each winding. Obviously connecting the phase windings in parallel will give increased current flow at higher speeds and connecting the windings in series per phase will reduce the maximum speed. The motors run hot and the heat is mainly generated with the shaft at rest and is addressed in some driver circuits that reduce the current when not stepping.

Since the torque / stepping speed is not a smooth curve due to resonance in the mid band range. The better electronic drivers help suppress this problem but at increased cost. For example the Leadshine web site contains details of three types of driver and there are others. i have listed the Leadshine as they have been reasonably open about the differences in the design. http://leadshine.com/ProductList.aspx?Type=9 Look at the different specifications and the % differences in price. The price of electronics tends fall with time.

Regards - Pat

[gera229]

Can you answer the last 7 or so questions about Half-Coil setups if you know the answer to them?

Thanks.

[wildwestpat]

it is very simple. There are four coils. Each coil has a maximum safe current rating. Energising a coil results in ampere turns which equates to magnetic force on the rotor and results in the torque available.

Thus if only one coil in each of the two phases is energised the torque is half the motors potential torque.

There are three possibilities to use the full number of coils for each step each has its advantage and disadvantages:-
First possibility is to connecting the coils for each phase in series doubles the ampere turns and hence the torque is doubled BUT the inductance is increased four fold which means that less current can be pumped through the coil at speed. i.e suitable for slow speed operation.

Second is to connect the coils in parallel. This as in one above doubles the ampere turns but reduces the inductance so this connection gives the increased torque BUT the lower inductance means that it is easier to pump current into the coil so it is better for higher speed operation.

Third possibility is to supply the currents to the coils in phase and anti phase such that one coil has current flowing into it and the other coil has current flowing out of it. This is the bridge driver connection. This requires additional electronics in the driver.

Driver electronics has a large effect on the motor's capability and needs as much if not more consideration than the motor. For a mechanism that needs to move quickly the motor should have the lowest inductance available if there is a choice of motors.

The driver electronics in most modern drives uses pilse width modulation to limit the current flowing in the coil to the safe value (printed in the data sheet for that motor). The act of pulsing the current feed to the winding means that the inductance of the winding is going to limit the actual power that can be injected as the individual steps get shorter in an attempt to make the motor go faster. Obviously this presents a limit to how fast the motor could go. However there are iron losses and resonances in the magnetic coupling between the coil and the rotor which further limit the speed.

Keep in mind that stepper motors when over loaded lose steps but do not mind being stalled. This fact is taken advantage of to maintain the shaft at the desired location with out having to use a brake or uni drive gearing.

I can not emphasise too strongly that the driver electronics is where the focus of your enquiry about motor connections should be. It is also necessary to have the torque speed graph for you motor. Some times this data is difficult to obtain hence the rule of thumb low inductance is better.

You also ask about the heat generated. The heat is due to two main sources the resistance of the winding and the iron losses. Heat is at a maximum with the motor shaft stopped. and is simply volts x amps.

There would be little if any difference in noise or smoothness as these factors are a reflection on the design of the electronics (switching to microstepping ) and the mechanical resonances of the mechanism as seen by the rotor of the motor.

You need to investigate the link I have given you to see the differences between the main stream driver electronics and how these effect the motor power - smoothness etc.


Regards - Pat

[gera229]

Haven't seen you mention Half-Coil setups, but just wonder do Half-Coil setups have the least inductance?
While they are slightly less powerful than Parallel setups, do they have a faster speed before losing complete torque? Or do they have the same maximum speed?

Also why someone would choose Half-Coil over Parallel. Is it because of the driver and how it can handle it? That is, not all drivers can handle a Parallel set up?
What if the drivers were able to handle both, would there still be any reason to use a Half-Coil setup over a Parallel setup?

Does Half-Coil generate less heat than Parallel?

As for the link you gave me, I already looked at it.
Please consider these as questions in a general way rather than relating to specific drivers.

I do not see why someone would still choose Series over Parallel unless their driver does not permit. OR unless a Parallel has a minimum speed it has to run, but someone needs to go slower than that speed in which case they would choose Series. Correct me if I am wrong.

Gera

[george4657]

You would not run 1/2 coil on a 8 wire stepper as it would be a waste of 2 coils.
1/2 coil is an option with 6 wire motors to get higher speeds but lower power.
If you have a 6 wire motor two coils are joined inside the motor and brought out as a center tap for the coils. You can not just join the other two ends of the coils as they would be hooked up in the wrong direction.

George

[gera229]

Makes sense, but I'm still curious about this one:
Is the max speed of a Half-Coil setup faster than or the same as a Parallel setup (which has more torque obviously)?

If it's the same max speed, then a Parallel setup would win both other setups in every way, that's why a lot of people choose it. But correct me if I am wrong.

Reason I ask these questions is because now I feel guilty for buying a unipolar driver board instead of a bipolar. It seems like that little extra expense would have been worth it. I wanted speed, while maintaining torque, then I heard about Parallel setups which is why it bothers me because they have both speed and more torque and maintain it quite well as to beat unipolar (Half-Coil) throughout the range in every way. Plus I have 8-wire motors for my unipolar setup that are 425oz bipolar and 305oz unipolar with 3.2mH inductance rating.

[wildwestpat]

Don't beat yourself up about the board. If you look at the data sheets for stepper motors you will see that they have torque when still and this falls quickly as the step rate increases. Stepper motors are by nature low speed devices.

IMHO the answer to your question is in the ampere turns that can be exerted per step - energise only half the total winding per step will obviously give less torque. BUT do look at the torque step speed curve for your actual motors. You are discovering that better drive electronics cost more and deliver more so I would just get on and complete the design and if it still looks a goer then build it. Play about with one of the on-line calculators such as http://www.kollmorgen.com/en-us/serv.../motioneering/ or similar to get a feel for motor size and drive for the different types of mechanism including the gearing ratios.

Take comfort in the fact that anyone who claims never to have made a mistake has probably never built anything or is remarkably uncritical.

Many useful machines have been built using simple drivers.

Regards - Pat

[gera229]

Simplicity is why I got that driver board.
Either way a bit extra money and effort would hurt, but I guess I should be fine.
I've just heard of one person that had the same setup as me that switched to bipolar and couldn't be happier.

But I will see. I think I will still be happy even though I'm not taking the motors to their max possible potential torque, while I have speed. But then again I do not know how much torque I need anyway since I lack experience. Seems like Parallel takes more juice from the power supply too. Correct me if I am wrong.

[george4657]

As Pat said build your machine with what you have.
305 oz should be enough power

Looking at a motor matching your description I found
Unipolar rating is 4.2V 3A 305ozin
Bipolar parallel rating is 3.0V 4.2A 425ozin
Bipolar series rating is 5.9V 2.12A 425ozin

A good bipolar driver that will run 4.2 amps is not cheap.
Don't make the mistake of buying a cheap driver or you will be disappointed and end up replacing it as well.
Speed is also dependent on voltage. The higher the voltage the higher the speed available. Here again drivers capable of higher voltage are expensive.

George

[H500]

Is the max speed of a Half-Coil setup faster than or the same as a Parallel setup (which has more torque obviously)?

No. They will reach the same no load max speed. The parallel will give more torque and higher speed when heavily loaded.

If it's the same max speed, then a Parallel setup would win both other setups in every way, that's why a lot of people choose it. But correct me if I am wrong.

Yes, parallel will be at least equal and most likely better than unipolar if the same power supply voltage is used.

[DICKEYBIRD]

Pardon the thread exhumation but you learned gentlemen's discussion was very close to my current area of electronics confusion.

I'm adding a 3rd axis live spindle to my retro'd Denford ORAC lathe. The machine has a bipolar halfstep 2A/40V Smartstep 3 axis driver and I have an extra 6-wire Vexta stepper motor with the following specs:
Bipolar Series 1.4A, 5V, 3.6 ohm, 10mh inductance
Unipolar: 2A, 3.6V, 1.8 ohm, 2.5mh inductance

If I connect it in the bipolar half coil mode, I think it should be safe since the 2A rating matches my controller.

If I connect it in the bipolar series mode, I think the amperage goes down to 1.4A and as such I think it would probably overheat something since my control has no provisions for individual axis current limiting and I don't want to change its onboard current limiting resistor as I need as much power on the other 2 axes as I can get.

Is it possible to add current limiting resistors between the drive output on that axis only and the stepper motor? Would the holding torque actually increase by a factor of 1.414 as I read on the Probotix page? This axis will only be used for small positioning moves and will sit idle most of the time. I want max holding torque; speed isn't a factor.

Thanks!

[H500]

If you cannot change the drive setting, then your only option is to use bipolar half coil.

A resistor will not work. Your drive would simply work harder to force 2 amps through the resistor and burn out the motor.

Are you sure there is no provision for changing the drive's current setting?

[DICKEYBIRD]

Thanks! That relaxes my worried brow.

The drive does have an onboard soldered-in current limiting resistor and the manual specifies how to calculate it but it changes all three axes and not individually. Can't afford the loss of torque on the other 2 axes.

[Tego]

Can you answer the last 7 or so questions about Half-Coil setups if you know the answer to them?

Thanks.

I'm not an expert, but I've ran into two situations where I ran "Bipolar half-coil" to fit the situation. Consider you have a single power supply and a variety of motors. You need to add another axis
and have an 8 wire stepper on hand but the power supply voltage is greater than recommended for a bipolar parallel configuration.

You can run it in bipolar series but pay a penalty due to the increased inductance (4x unipolar or bipolar parallel).

However, you hook it up in half-coil mode then the rated voltage, impedance, current and holding torque are EQUAL to the unipolar configuration (can run at higher voltage and keep the lower impedance).

The motor can handle a higher voltage supply. In both my cases, (one with a 6 wire, the other an 8 ) I actually could run the motor at faster speeds without losing steps and
they ran much cooler.

NOTE: Several web sites state bipolar half coil is actually bipolar parallel and show wiring for bipolar-parallel. I'm talking about actual bipolar half-coil where only one coil per side is used.