What is a motor drive? Summary for design engineers

Some drives are wholly incorporated into the controller, so that the profile generation takes place in the controller as well as the torque command for the motor. On the other hand, a drive can also refer to the specific power electronic circuitry needed to drive the motor. Electric motors that drive industrial machines need some way to control motor speed. And at its most basic level, a motor drive controls the speed of the motor.

Some manufacturers refer to a controller and motor together as a drive system. However, from the electrical side of things, the drive is often specifically the electrical components that make up the variable frequency inverter itself. So drives are the interface between the control signals and the motor and include power electronic devices such as SCRs (silicon controlled rectifiers), transistors, and thyristors.

Matching the correct drive to the type of motor in an application is critical for getting the best fit for torque, speed, and efficiency. There are a wide range of drives available depending on the needs of the specific application and motor type. In general though, drive types typically fall into two categories; dc and ac drives.

DC drives control dc motors. A basic dc drive is similar in operation to an ac drive in that the drive controls the speed of the motor. For dc motor control, a common method is a thyristor-based control circuit. These circuits consist of a thyristor bridge circuit that rectifies ac into dc for the motor armature. And varying the voltage to the armature controls the motor’s speed.

AC drives
AC drives control ac motors, such as induction motors. These drives are sometimes known as variable frequency drives or inverters. AC drives convert ac to dc and then using a range of different switching techniques generate variable voltage and frequency outputs to drive the motor.

An adjustable speed drive is a general term used sometimes interchangeably with variable speed drive or variable frequency drive. Again, from an electrical perspective, all of these ultimately refer to the frequency converter circuitry.

An ac motor’s speed is determined by the number of poles and the frequency. Thus, as frequency is adjusted the motor’s speed can be controlled as well. A common way to control frequency is by the use of pulse width modulation (or PWM). A PWM drive sends pulsed inputs to a motor and by modulating the pulse width, making it either narrower or wider, increases or decreases the average dc voltage seen by the motor.

Another powerful kind of drive function is known as regenerative braking or regen braking. This is a way of stopping a motor’s rotation by using the same solid-state components that control the motor’s voltage. The energy generated from braking can be channeled back into the ac mains or into filter capacitors. Advantages of regen drives include the ability to run a motor in either forward or reverse direction without having to physically switch the polarity of the motor leads and without the need for reversing contactors or switches.