Back Emf

Throughout this chapter, we have seen that electric motors and generators are closely related devices. A generator converts mechanical energy into electrical energy, whereas a motor converts electrical energy into mechanical energy. In fact, the internal construction of many motors and generators is nearly identical. The same coil of wire and magnetic field that allow a motor to produce rotation can also generate an emf whenever the coil rotates.

As a motor operates, its rotating coil continuously experiences a changing magnetic flux. According to Faraday's law of induction, this changing flux induces an emf in the coil. In other words, every running motor also behaves like a generator. The rotation may be produced by an external source, such as a belt drive, or simply by the motor itself as it performs useful work.

Lenz's law tells us that an induced emf always opposes the change that produced it. Therefore, the emf generated by the rotating motor acts in the opposite direction of the applied voltage that powers the motor. This self-generated voltage is called the back emf. Rather than helping the applied voltage, the back emf reduces the net voltage across the motor windings, as illustrated in Figure 51.1.

Because back emf is generated by the rotation of the motor, it is proportional to the motor's angular speed, [latex]\omega[/latex]. When the motor is first switched on, it is not yet rotating, so the back emf is zero. The full applied voltage therefore appears across the motor windings, causing the motor to draw its maximum current.

As the motor speeds up, the back emf increases. Since it opposes the applied voltage, the effective voltage across the windings decreases, reducing the current drawn from the power source. This automatic regulation explains why motors draw much more current when they first start than they do during normal operation.