Dual Y Axis Stepper Motor set up

As a complete newbe, I have questions on driving 2 Stepper motors with one driver. I understand one field must be switched in order to drive the same direction, but my concern is what voltage and amperage issues arise when driving both with one driver. Voltage is divided which would cause the Amps to double... Can the drivers on a Replicape handle two motors??

Thank you for your consideration
RBuster

I obviously placed this in the wrong category...

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RBuster
[...] Voltage is divided which would cause the Amps to double... [...]

If refering to having the motors in wired in series, then coil of first motor becomes in series with same coil of the other motor. Voltage does indeed divide among coils, but the amps do not double. Coils being wired in series its the same current circulating in both. So the amps also decrease, because the current finds it harder to circulate on both parts if they are in series coz their resistance (impedance) adds together forming a bigger value. So voltage divides between coils, and current gets lower (proportionally with the values of both serialized coils added together).

Stepper drivers could handle 2 motors wired with coils in series like that if the coil resistance (impedance) is small enough so when added together it remains "workeable". Basically the driver does not "see" two motors, but sees just one motor with a double coil resistance. First effect, the current will rise slower because of that, which at some extent can be mitigated with higher supply voltage, like using 24v instead of 12v. But otherwise take a look at phase resistance, if its about some motors which have 8 ohms coils, those wont work in series because 8 ohms is already too much, but effectively 16 ohms is well beyond acceptable. On the other hand some motors with coil resistance like 1-2 ohms will form together a resistance like 2-4 ohms which is still good. One note, the steppers are supplied at frequency so in the big picture the impedance counts, but practically its easier to talk and measure Ohms with a multimeter than it is to figure out impedance value at x freq. And coil resistance still matters in rise times, and its still linked good to inductance. In relative terms, a low coil resistance points to a low inductance, usually in same form factor low resistance means thicker wire hence lower turns, and aslo similarly the other way around, same form factor with higher resistance means thin wire and higher number of turns, sort of.

The drivers used for 3D printers are constant current ones, and the motors are normally chosen to have a voltage drop of no more than 4V at maximum current. So you can connect 2 motors in series without any loss of current in the steady state, using a standard 12V supply.

However, when the motor is moving, additional voltage is needed to overcome the inductance of the winding and to overcome back emf resulting from the movement. The higher the speed, the higher the voltage needed. So with 2 motors in series, the torque will drop off at a lower speed than with just one motor. Whether this matters depends on what speed you are aiming for. Using a higher supply voltage (if your printer electronics allows it) will allow a higher speed.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

The stepper drivers are called constant current because that is what they are "trying hard" to do, that is their aim/objective only, not because they are actually achieving exactly that. The way of working the current is anything but "constant". Basically it senses the current rise/fall and it interrupts when it gets too high (Ipeak) and lets it flow when its undervalued again. That means at any time a coil is energized, at any specific point in time, the current is either on a rise or on a fall edge. This is "not exactly" constant. On the contrarily sort of speaking it is as "agitated" as it can be - can not figure a way of "exercising" the current more than that. What this is achieving, is not a constant current literally, but to be more precise the constant value is the *average* of the current. Even if the current is anything but constant, at least the average of it is constant.
However the current itself is either rising or falling, its always transitory, which is not exactly what i get for "steady state". For example i believe the "steady state" of a 1 ohm coil with a 12v supply would be 12Amps, ohms law, a value in which the current would be "steady" and not naturally rising nor falling anymore, an operating point at which the system would stabilize itself naturally. When the current is on a rise edge, that is the value the current wants to get to, even if the driver will interrupt it, that point dictates how the rise curve looks and thats the final current value at which it aims. Thats why a 24v supply would help, it would make this final point 24a instead of 12a, and with final at 24a the curve is shaped more abruptly sort of speaking (at least more abruptly in the short term / first part which is of interest). But if it would reach what i refer to as "steady state", if the current would reach the operating point needed to stabilize itself, like in any dc voltage applied to any load, current would rise at final current value given by ohms law, and if that would happen to a bipolar stepper most likely the coils or driver would burn. Sort of speaking, my opinion is that it does not have a "steady state", coz the current is always transitory, at any point in time the current either rises or falls, just never "stays put" in this case.

If i would have a 8 wire stepper (each half coils "medians" separated), wired/used in unipolar mode, coil resistance of 5ohms, and i put the coil edges at 5v, and the (separated) "medians" at low side mosfets, i would open a mosfet gate from uC and let the 5v flow through coil, the current will rise and in the end i would get 1A through each coil, then this will be steady state, because the current would of reached its final value dictated by ohms law. It wont go anywhere else, the current transition from zero to its final value reached the final value, its stable at that point. This is different drive method, this one reaching steady state and probably mostly sitting at steady state. This means i need stepper motor with coil resistance high enough so i can make use of coil resistance directly without the need to add other external resistors to limit the current. It will be slow, at 5v the coils will energize painfully slow, but will work, just wont be fast. So maybe an area of applications where the motor justs sits there and hardly moves at all, like opening and closing windows, or shutters, etc. This is exactly the other type of stepper motors (high resistance, high inductance) that are at the opposite spectrum of what would be required in a motor for a printer.

NoobMan and dc42 - Thank you so much!! This is precisely the information I need to answer my question. My education in stepper motors has just begun!
RBuster

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RBuster
NoobMan and dc42 - Thank you so much!! This is precisely the information I need to answer my question. My education in stepper motors has just begun!
RBuster

I'm glad it helped. Just to be clear, by "steady state" in my post, I meant when the motor is not moving, just holding position. As Noobman pointed out, the electrical state is far from steady in this condition. But you don't need to concern yourself with this.

Edited 1 time(s). Last edit at 02/26/2015 12:17PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Really nps, thats what the forum is for, discussing topics and sharing points of view etc, and we are all glad to do it - thats why we are posting. Cheers.

Dual motors on Z are usually wired in series because its usually a screw involved and it does not have to move fast either. I imagine for Y it would be about some moving gantry type of machine, and that is a bit different. First off the X and Y accelerations and speeds, should be balanced among each other, as only the slowest one matters because X and Y work together and the slower one will be the real limiting factor in combined movements. And in a moving gantry the gantry will be the heaviest axis, hence it really requires more motor power than the other. Comparing to the other axis motor, a connection in series would make it certainly slower even at same load, not to mention the heavier gantry. I think perhaps this is why the moving gantry cinematic (used in many other areas), is not that well common among reprap printers which need high speeds. One axis is much heavier than the other, and moving mass is maximized instead of being minimal.

I imagine that on Y its about some moving gantry cinematic with 2 motors on Y, so you will need motors with extremely low resistance&inductance, sort of speaking in such a way that both coils combined would be like a normal motor spec (which for reprap these values are low already). Perhaps some coil resistance of 0.9 ohms or around that, maybe max 1.5ohms, so together coils would form something like 2-3 ohms ideally combined, maybe 4ohms worst case. This is just to throw some numbers. And/or also you could try use a 24v psu instead of 12v, this would really help making things snappier. Good luck.