Rotating a Motor

Rotating a Motor

Rotating (or commutating) a motor in a known direction is accomplished by placing at least three windings, commonly referred to as phases, on the stator at an equal distance from each other. Sequentially activating windings on a 3-phase motor will cause the motor to move in a known direction. You commutate motors using an Microcontroller (MCU) to output digital signals to driver circuits, which in turn supply current to the windings.

The interval in which these signals are output is used to commutate the motor at a desired speed. Activating windings at certain time intervals does not guarantee the motor will maintain the intended speed. If the load on the rotor exceeds the power of the motor to move it the motor will “slip”. If these slips are persistent the motor will stall. If the slips are irregular the motor will run roughly.

Conversely, if the force exerted on the rotor is much greater than required to move the load the rotor may speed up arriving at the stator winding before the before the MCU activates the next winding. This will cause the rotor to repeatedly pause as it passes each stator winding. This lurching effect also results in the motor running roughly.

Matching the power and speed of a motor to the load being moved is a primary function of motor control

Modifying a Motor's Speed and Power

The power of the motor (torque) is a linear function of the force created by the current and the magnetic field. Modifying either the magnetic field or the current will modify a motors torque thus alter the speed of the motor.

Resize the winding or applying more current to the windings.

Resizing an inductor is not the preferred method of modifying torque. Appropriately sized inductors are essential but oversized inductors and high currents can cause emission issues as well as aggravate a condition known as torque ripple. Torque ripple is the variance in a motor's torque as the rotor moves through the stator windings. Quantified as a percent of change in a motor's power output, torque ripple can be of significant concern in applications where a smooth operating motor is essential.

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Increase the number of stator windings and poles on the rotor.

Rather than simply increasing the size of the windings to increase torque most motor manufacturers increase the number of windings. In conjunction with more windings, the rotors consist of multiple poles aligned with the additional winding sets.

It is not uncommon to find a motor with 4, 8 or even 12 sets of 3-phase windings. When referring to a motor with several sets of windings, developer’s often describe the motor’s mechanical rotation in relation the motor’s electrical rotation. Multiple sets of winding necessitate the need for an MCU to commutate through several electrical rotations in order to achieve one mechanical rotation.

Activate Opposite Windings.

On motors with an even-numbered set of stator windings and a rotor consisting of a permanent magnet, it is possible to pull the rotor from both magnetic poles. This is accomplished by applying a positive charge to one winding and a negative current on the stator windings which are 180 mechanical degrees apart. This will essentially double the torque generated by a single winding.

Modify the Magnetic Field

Varying the current flowing through the stator windings will alter the stator's magnetic field and influence the force on the rotor. Rather than activating each winding at full current, this approach concurrently applies a current to multiple windings in a sine wave. The magnetic field varies as the rotor passes by the windings. Properly done sinusoidal control can significantly reduce torque ripple and increase efficiency. Some motors are designed to exclusively rely on sinusoidal control (see Types of Motors).

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Sinusoidal control can require a significant amount of CPU resources. Not all motor types can be efficiently controlled with sinusoidal signals.

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