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I am using stepper motor for my CNC machine. Its model number is EM284 STP-42D221-01 by Shinani Kenshi. I don't know any specs of this motor; neither voltage rating nor current rating. I don't even know its RPM. I am using 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 motors simultaneously.

Problem is when I use 12 V power supply with 4.2 A current rating, my motors are getting hot and also driver circuit is also getting hot. But the motors are running smoothly and are not missing the steps.

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 don't know where is the problem? What should I do? Either there is a problem in my driving voltage or problem in driving current, or some 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.

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  • \$\begingroup\$ they are getting enough torque not to slip with 4A, but not enough with 2A. 4A (2A each) is quite heavy duty for little stepper motors. Current will always generate heat, specially with 4A constantly through something like a motor and it's driver circuit. Have you thought about putting a 12V DC fan on it to help with cooling? \$\endgroup\$ – KyranF Oct 6 '14 at 4:45
  • \$\begingroup\$ That motor has a resistance of 3 Ohms per phase, and could draw up to 4A on 12V if wired unipolar. Are you providing any current limiting? Please post your circuit so we don't have to guess what it is. \$\endgroup\$ – Bruce Abbott Oct 6 '14 at 6:40
  • \$\begingroup\$ Stepper motors tend to run very hot, particularly with simple drive schemes which maintain full current all the time. The mere fact that it feels hot is not an indication of a problem. \$\endgroup\$ – user1844 Oct 6 '14 at 8:26
  • \$\begingroup\$ Can you add photo of that motor? \$\endgroup\$ – Kamil Oct 6 '14 at 9:37
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What kind of steppr driver are you using? Setting the current limit with the power supply is a bad idea - it's not designed to do that, unless you are using a lab supply! It would be a beter idea to invest in a proper chopper driver that uses a PWM drive on top of the actual stepping to set the current through the coils, and then use a properly-sized power supply. There should be a way to set the drive current with either a software seting or a trimmer pot. At any rate, the motor torque is determined by the coil current, and you will need to turn it up until it doesn't skip steps. At high currents you may also need to cool the motor. The drive transistors will certainly need a good heatsink, but they should disspate less heat than the motor.

Edit: Chris remided me of another very important advantage of chopper controllers: driving the inductive motor windings. Inductive coils will try to resist changes in current flow. As a result, when the motor steps, the coil current takes some time to 'ramp up'. If you want to step very quickly, you need to use a high drive voltage to so that the coil currents will ramp up faster. This means that you need to use regulators on each coil. Chopper controllers do this for you, and all you need to do is set the desired max coil current. Chopper controllers can generally also microstep the motors, interpolating the coil currents to hold the rotor between steps. If you use the proper controllers, then you can get away with cranking the supply voltage up to 24 or even 36 volts for better performance.

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  • 2
    \$\begingroup\$ Yes. The really key point is that while the idle impedance of a stepper comes from the coil resistance, the turning impedance comes increasingly from its inductance. A simple voltage supply will either overheat the motor when idle (resistance only) or loose steps at high rates (inductive reactance in play). This makes it preferable to use a higher voltage supply which can overcome the inductive reactance, and a per coil chopping current regulator. But motors may still run hot without cooling, and their magnets may be degraded by this. \$\endgroup\$ – Chris Stratton Oct 6 '14 at 15:34

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