How Do Electric Motors Work?

How Do Electric Motors Work?

Electric motors convert electrical power into mechanical energy, usually in the form of rotational motion. This type of motor is the most commonly used type of electric actuator. It is used in a wide variety of applications, including appliances, electric drills and tools, household and commercial fans, automobiles, and disk drives. Electric motors may also be powered in reverse to act as generators, converting back to electricity the mechanical work they have done.

Electrical motors use magnetic fields to create the required force (torque) to drive linear or rotary mechanisms. The magnetic field can be produced either by coiled wire windings or permanent magnets, although the latter require complex and expensive arrangements to achieve a sustained rotary motion. Regardless of the type, every electric motors must produce an effective field to generate torque, and this requires a combination of factors, including core saturation or safe operating temperature rise, pole-pair number, excitation frequency, and air-gap flux density.

To produce the necessary magnetic field, a rotating coil of wire, called an armature, is wound around a laminated or “soft” ferromagnetic core. The armature is then energized by current flowing from brushes through a device called a commutator. The commutator switches the direction of the current passing through the windings at each half turn of the shaft. This flips the direction of the magnetic field being created, and the interaction between the two opposing fields drives the shaft to rotate.

How Do Electric Motors Work?

The exact nature of this force, known as torque, depends on the physics involved. It is determined by the vector product of the interacting fields and the core saturation constraints, which are generally set at a maximum value. The maximum shaft torque is also limited by the capacity of the rotor to sustain the required shaft speed, and it is reduced by core losses.

Some electric motors are designed for continuous operation, and this requires a specific core saturation point to avoid overheating. However, some applications require bursts of torque to accelerate a device from rest. As a result, the design criteria for the air gap, which determines the size of the electromagnetic core, varies across electric motor/generator types.

Most electric motors run on AC power, which is supplied for most fixed-speed applications from the power grid or for variable-speed application from a VFD controller. This is because AC induction motors are optimized to operate with three-phase sinusoidal or quasi-sinusoidal waveform power. This is because the rotor in an AC induction motor acts like a rotating transformer, with one of the three phases being supplied by the stator and the other two coming from the rotor. This allows the rotor to continuously spin as long as the motor is supplied with power. If the rotor loses power, it will stop spinning, but it will start again when the power is restored. A reversing valve is often built into some motors to allow for this reversal. This is especially important in regenerative braking with DC motors.

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