Wiring high current motors in parallel

[wilriker]

I just have a question more out of curiosity. I plan to switch my motors out for high current low inductance ones. I know that these usually are wired in series (double-Z motors). I just wonder if it would pose some kind of a problem to e.g. the driver chip if they are wired in parallel instead? This would at least allow for higher speeds.

I also know I could just separate them out to two different drivers (as I have E1 currently unused) but I explicitly want to know if wiring them in parallel will break anything.

Oh and by high current I mean something around 2A per motor.

[dc42]

@wilriker, if your motors are rated at 2A then you probably want to run them at around 75% current which is 1.5A. if you connect 2 in parallel then that's 3A, which is beyond the maximum output of the Duet.

[wilriker]

@dc42 Thanks for confirming.

That's what I thought/expected.

Actually these motors are even rated higher (2.4/2.5A depending on the modell) and I plan to run them at around 2A so that would become 4A which is definitely far off.

[wilriker]

@dc42 said in Duet Maestro Freefall:

[...] connecting them in parallel (instead of in series, which is what you get if you plug them into Za and Zb) and using just one of the Z motor connectors may solve the original problem and/or make spreadCycle quieter. Don't forget to double the motor current setting.

Especially about the last sentence: this is probably be being stupid when it comes to electricity but I did not double the motor current after wiring my two Z motors in parallel. They have been running at a little bit over 50% of their rated current before. Does that mean they now run at only a little bit over 25% each? Do I get this right?

P.S.: This is of course about my current setup with low current, high inductance motors.

[SupraGuy]

@wilriker said in Wiring high current motors in parallel:

Especially about the last sentence: this is probably be being stupid when it comes to electricity but I did not double the motor current after wiring my two Z motors in parallel. They have been running at a little bit over 50% of their rated current before. Does that mean they now run at only a little bit over 25% each? Do I get this right?

If you have wired them in parallel, then this is correct, you would be running them at about 25% of their rated current, since the drivers would split the current between the 2 motors.

[wilriker]

@supraguy Thanks for confirming.
I doubled the current now but interestingly I had no issues running them at this low current. Still better be safe.

[RCarlyle]

One minor thing to consider with parallel motor wiring is that there may be pretty significant variation in coil resistance/inductance from motor to motor (max +/-10% resistance or +/-20% inductance per spec typically, actual variation will be less than that) so current may not split exactly evenly between the motors. You could have 45%/55% for example. That's one reason why I tend to prefer series wiring for two motors.

Series motors always get the same current, but your top speed is cut in half because each motor effectively sees about half the supply voltage. So, if you want torque at lower speeds, use parallel, but if you want high speed at lower torque, use series.

[wilriker]

@rcarlyle Thanks for pointing this out. Always very interesting aspects!

Once I change to my new motors (if only I could decide ) I will go back to wired in series anyway. In the end I would prefer to only use one motor for my Z axis but the geometry of my printer won't allow for that easily and/or cleanly.

[wilriker]

Just a follow-up: I finally got my new motors (2.5A rated current). I installed them and to get the printer back up and running I just left the two Z motors in parallel wiring. I set the total current for both motors to 2A and they ran just fine. Actually a lot slower than EMF calculator said but for Z I don'r really care. Today I rewired this to series, set the current to 1A and still everything runs fine. But now something unexpected happened: the steppers are louder now. I don't know how much because I did not measure before the rewiring as I did not expect any changes in noise level but they are louder.

@RCarlyle any idea on why that happened?

[RCarlyle]

Wiring in series made them louder than parallel? That’s interesting; I hadn’t ever thought about it but kind of makes sense as something that can happen. The constant-current PWM switching has different frequency or harmonics with different combinations of supply voltage and load inductance. Series wiring has a higher load inductance, meaning the coil current changes slower, so the switching frequency will be lower, and thus get more noise into an audible band where your ear can potentially detect it.

(I’m not specifically familiar with the exact 2660 PWM scheme so I don’t know if the overall switching frequency is changing or just harmonics within the PWM switching waveform. Different drivers do it different ways. For example 8825s have a constant switching frequency of 30 kHz which is inaudible but very easily throw 7.5, 15, etc kHz subharmonics that can be audible.)

Your age matters a lot for this; older people don’t hear most of the higher frequencies commonly generated by stepper driver switching. Makes it really hard for @dc42 to work on optimizing driver settings for switching noise

[wilriker]

@rcarlyle said in Wiring high current motors in parallel:

Your age matters a lot for this; older people don’t hear most of the higher frequencies commonly generated by stepper driver switching. Makes it really hard for @dc42 to work on optimizing driver settings for switching noise

My age is still at a level where I can hear high pitched sounds quite well.
But I am talking about movement noise. In standstill they are basically silent. The noise is more in the low to mid-range frequencies when moving. They just hum louder. At first it was because I forgot to reduce the current and they were running at 2A each so they had just a more aggressive torque (which in my limited experience leads to a louder, harder, more aggressive sound... hard to describe). But it is still audible louder at 1A than before.

Usually I am all about it has to be the most silent way possible but Z axis is moving slow and rare so I don't care about this too much. I was just curious. And harmonics due to different switching frequencies that will induce vibrations could be an answer here.

[dc42]

Have you used the back EMF calculator to calculate the maximum speed you can run the motors at without losing torque, at the 1A current you set? If you exceed that speed then the current waveform becomes non-sinusoidal and the motors will be noisy.

[RCarlyle]

What @dc42 said makes sense. I could speculate about electromagnetic damping through the multiple paths w/ parallel coils, but I don’t know whether parallel vs series motor wiring makes a difference for that.

[wilriker]

@dc42 Yes, I did. I go nowhere without the Calculator - thinking about it, that's even true literally because I have a tab with it open on my smartphone all the time.

Anyway, EMF Calculator returns a maximum of a little over 17mm/s and I run them at 15mm/s.

P.S.: 800 steps/mm on an 8mm lead leadscrew with 2:1 gearing - in case this would be relevant.