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Stepper Motors Explained
I am frequently asked the question, why do I need a 24V power supply when the stepper motor is rated at 3 to 5 volts. This seems a reasonable question when most normal motors run at the voltage it says on the label. However stepper motors and the way they are driven sets them completely apart from a normal motor. The ratings quoted on the label or datasheet refer to the static conditions of the motor holding power and is basically how much current it will pass for a given voltage to give the quoted torque rating of the motor. These static conditions are the DC resistance of the windings and using ohms law gives the simple equation of I = V/R. Therefore a stepper motor with 2 ohms resistance connected to a 3 volt supply will pass 1.5 amperes of current. I(1.5) = V(3) / R(2).
This is all well and good but we need the stepper motor to rotate. To accomplish this, a stepper driver board switches power from one winding to the next effectively causing the motor to rotate in fine increments, usually 1.8 degrees. After 200 steps of switching between the windings the motor will have completed a single rotation of its shaft. From this basic example it is easy to see that if you want the spindle to rotate at 10 times per second or 600 rpm you need to switch the windings at 2000 times per second or 2KHz.
As well as DC resistance, the windings have inductance and this is where the problems start, inductance has current lag. What this means is that when you apply a voltage to the winding the current builds up to maximum and the larger the inductance the slower it takes. Even with low inductance windings it can take hundreds if not thousands of microseconds for the current to build in the windings. Now remember that you are switching the windings on and off at a very fast rate. It does not take a genius to work out that at 2000 pulses per second for our 600 rpm movement that there is not enough time for the current to fully build in the wind before it is switched over to the other coil. The overall result is that the faster you rotate the motor the weaker it gets until it fails completely.
Now for the clever part. To significantly improve performance of the motor you can increase the voltage to the windings. Most consider a factor of ten times the normal voltage rating of the motor but can be anything from 5 to 20 times. With a higher voltage you of course get more current into the motor a lot quicker keeping up the power levels at higher rotation speeds. You are now thinking Ok, but that means at low speeds the motor will consume a massive amount of current. Well it would if we let it, but the modern stepper driver has built-in current limiting so that whatever speed the motor rotates the current stays no more that you set it. The other options you may have are serial or parallel operation. Quite a lot of motors have four windings, two for each phase, this enables you to connect up for series operation (needs higher voltage but less current) or parallel (needs less voltage but needs more current). Both options will give exactly the same results with holding torque or low speed operation but parallel modes tends to win over at higher speeds. You will also be restricted by the stepper driver board maximum working voltage or available current. Also remember you do not have to work the motor at its maximum capability, they will still work quite efficiently with less than the maximum power.
C R Harding
Further reading : http://en.wikipedia.org/wiki/Stepper_motor
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