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Stepper Fundamentals Slide 6

For the same motor frame size, a motor that is wound as a bipolar motor will be able to deliver about 40% more torque than the same frame wound as a unipolar motor. Basically this is because of better usage of the copper in the motor. One of the factors that limit the amount of torque that can be produced is the dissipation in the coil and the maximum allowable temperature in the coil. The dissipation in the coil is proportional to the square of the current in the winding and the coil resistance with the coil resistance being proportional to the number of turns, N. Torque, however, is proportional to the current times N. Driving the same coil as a bipolar motor, the current flows through both halves of the coil so the torque is proportional to I times 2N and the dissipation is proportional to I squared times 2N. If the maximum dissipation is set to be the same in each case, the bipolar motor can only drive 70% (actually 1 over the square root of 2) of the current in the unipolar motor. However, that current flows through both halves of the coil and therefore produces 40% more torque. The improvement in performance that can be achieved using the bipolar motor makes this more advantageous than the unipolar motor. Integrated driver ICs have greatly simplified the design for the bipolar motor drive circuits since they integrate both the power transistors and all of the level shifting and gate drive circuitry into one easy to use IC. Typically, for low performance applications, such as the paper feed in a desk top ink jet printer, unipolar motors are still commonly used with very simple drive circuits. For higher performance applications, bipolar motors are typically used.

PTM Published on: 2011-12-14