Electronic – Stepper Motors getting very hot after use

You don't say it, but your problem may be that you're trying to start the stepper at full speed. If so, there is a maximum speed (which varies somewhat with load) beyond which a stepper will not accelerate, and this speed is normally well below what you can reach with a gradual increase in speed. Google on "stepper motor torque curve".

The problem is that, with a 4-phase stepper such as you are using, if the shaft angle lags more than 2 step angles behind the commanded angle, the torque reverses and the motor sits and vibrates and makes horrible noises. This is not, technically speaking, a stall condition, where the shaft does not move at all.

The torque-speed curve for your motor can be found at http://www.kelinginc.net/KL34H2120-42-8AT.pdf and indicates that the motor can be run at 5000 steps/second, which suggests that your problem is your attempt at fast start.

If, on the other hand, you've already tried a slowly increasing step rate while running, then you probably do need a heftier motor. However, my calculations for a .144 hp motor at 10 rps gives about 2 Nm torque, and the linked torque curve for your stepper is about the same, so I wouldn't expect a problem. Unless, of course, you've modified your mechanical setup somehow.

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.