stepper motor is getting heat up?????

hi every1, i am using stepper motor for my cnc machine...its model no is em284 stp-42d221-01 by shinani kenshi......i dont know any specs of this motor neither voltage rating nor current rating even i dont know its RPM.i am usig IRF640 mosfets to drive stepper motor along with 817b optocoupler.
i am running my stepper motor at 12 V.i have to do diagonal motion so i am running two stepper motor simultaneously.
Problem is when i use 12 V power supply with 4.2 A current rating my motors are getting heatup and also driver circuit is also getting heat up....bt the motors are running smoothly and are not missing the steps.
and when i use 12 v supply with 2 A current rating both motors are missing the steps but this time both driver and motor are not getting hot.

I dont know where is the problem????what should i do??? either there is a problem in my driving voltage or problem in driving current or is there any other issue???

the driver circuit for both motors are in parallel that means if i use 2A source then 1A current is going in each driver means in motor.

Please help me out!!!!!

The stepper motor normally runs hot. Maybe your lower amp supply simply just can not provide the set current in which case they dont get as hot as the other case, but they miss steps and such.

There is one important thing about it, do you have feedback loop, e.g. how does the system sense the coil current and how do you change the current level. Also i think the system is unipolar tho could have stated that.

Stepper motors (apart from very low power ones) should be driven by a constant current chopping controller, which is not what you are using. If the stepper motor is getting to hot, you need to either reduce the voltage it is getting (e.g. by using a lower supply voltage, or putting a resistor in series with each winding), or use a constant current stepper motor driver instead.

What size is the stepper motor, and what is the resistance of each winding? Those figures should give us a clue about what current it needs.

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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].

Using an undersized power supply isn't the right way to reduce current. You have to get some current limitation into place or reduce voltage.

That said, a constant current stepper driver is indeed a very good idea. Without current regulation you loose torque as soon as the motor starts to move.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Quote
dc42
Stepper motors (apart from very low power ones) should be driven by a constant current chopping controller, which is not what you are using. If the stepper motor is getting to hot, you need to either reduce the voltage it is getting (e.g. by using a lower supply voltage, or putting a resistor in series with each winding), or use a constant current stepper motor driver instead.

What size is the stepper motor, and what is the resistance of each winding? Those figures should give us a clue about what current it needs.

what do u mean by constant current driver??? i am using mosfet and obviously they would be constant current ......
nd resistance per coil is almost 3.9 ohms

Quote
arshia
what do u mean by constant current driver??? i am using mosfet and obviously they would be constant current ......
nd resistance per coil is almost 3.9 ohms

No, a mosfet driven as a switch is not a constant current device. This is the sort of thing you need: [www.pololu.com]. There are also many similar devices available on eBay, just search for A4988.

If the resistance per coil is 3.9 ohms, than that motor probably needs about 1A per winding if it is NEMA 17 size.

Edited 1 time(s). Last edit at 10/08/2014 03:03PM 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].

Maybe he has a feedback loop, and depending on how it uses it, it could actually have something that is called "constant current stepper driver" or chopper. Just a mention, mosfets are used as switches either way. The only other method to use mosfet would be as an amplifier which is not the case. In the syntagm "constant current driver", the coil current although called constant, its actually a sawtooth wave, not really constant per se, although if somebody zooms out far enough it appears to be.

Current device or voltage device, this syntagm refers to the type of input of the device, or how it is controlled. About what the output function depends on. E.g. mosfet input is gate, and the function of the mosfet depeends on the gate voltage, therefore mosfets are dubbed voltage devices, all of them. The old type transistors input is base, and the function describing the collector and emitter depens on the current that is being fed into the base, hence all those transistors (bjt, igbt) are dubbed current devices. An microcontroller which has logic level on voltage like 1.8v, 3.3v or 5v it is also a voltage device.

NoobMan, the OP said he is using mosfets and optocouplers. I infer from his use of optocouplers that he is probably driving the mosfets as simple on/off switches without feedback. I hope he will correct me if my inference is incorrect.

Edited 1 time(s). Last edit at 10/09/2014 07:26PM 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].

I on the other hand, automatically presumed he has a feedback loop, because otherwise things are quite ugly from my perspective.

The feedback loop job is to cut off the current when it reaches limit, and then let it fall down to zero (or not). Assuming a coil resistance of 1 ohm, the 12v psu / 1ohm, without a current limit and in condition longterm (inductor dissapears from circuit), it would go to ohms law and get up to 12Amps per coil which means coils will probably melt. No wander the coils getting hot. Instead, a bit of wander of how the test psus survived.

So without a feedback loop it has to be a much lower voltage psu, like 1v psu for a coil of 1 ohm, if we target a current of 1A. And again in this case the rise times will be bad and i expect motor dynamics also poor. Probably in an application where the motor just needs to sit there and just hold its position most of the times, and move slowly otherwise. Realm of some high inductance and high coil resistance motor (12ohms per coil) for that, which is contrary to the normal stepper type used in reprap.

Just read arshia mentioned 3.9ohms per coil. Then coil current would go to ~3amps. Quickfix could try 5v psu for the motors, something like an atx 5v line. Then current would go to 5v / ~4ohms = 1.25A which should be manageable and less heat. But still a 1.25A current would make quite some heat and we dont know how high the motor is rated (what does it say on the label?). If heat is too much, then could even try use the 3.3v line, who knows maybe that would work better with current to less than 1A.

Edited 1 time(s). Last edit at 10/10/2014 06:05AM by NoobMan.

About mosfets being used as switches, i think its a semantics difference of what that means literally. What the use of the words evocate / what expression is understood as. So ill try clarify what means to me. Mosfet has an elementary function of iDS=k/2*(vGS-VT)^2 which is normally a curve on the graph (^2), and *ONLY* when the mosfet is used as a switch (fully on/off) then this function can be approximated by considering the mosfet behavior similar to a resistor, and then instead of all those info, we can just simply use one rDS(on), e.g. resistance from drain to source when its on. So when datasheet proeminently mentions the on-resistance like rds or r(on), it menstions that because that mosfet is used as a switch. Stepper drivers have mosfets in a h-bridge and used as switches, datasheets gives miliohms on-resistance. Same with heater mosfets, and again the case of almost all power applications of mosfets, all around.

The alternative common area use of mosfet is as an amplifier in a small-signal domain, e.g. very low power. But again because its a curve function, its not all that good, that curve implies distorsion. In small signal use, we "zoom in" on a small segment of that curve untill it "appears" to be a line. Outside small signal, for amplification is better to use a regular transisfor which function is simply input*gain=out, because this one has no squared factor and being graphically linear it wont distort the output (so much).