A motor is the mechanical device that converts electricity into the rotational or linear force used to power a machine. A drive is the electronic device that controls the electrical energy sent to the motor. The drive feeds electricity into the motor in varying amounts and at varying frequencies, thereby controlling the motor’s torque, speed, and position. Here we will investigate a few common types often used in industrial automation and machine design.
AC Induction Motors & Variable-Frequency Drives (Inverters)
Widely used in industrial applications for over 100 years, 3-phase AC Induction Motors are rugged, reliable, and economical. Traditionally used as a fixed-speed device, modern control electronics have allowed for Inverters (or “VFDs”), which change the frequency of incoming power – and thus the speed of a motor’s rotation. This allows for significant energy savings for variable-torque applications like fans, pumps, and compressors. AC motors and drives run the gamut from very small (fractional horsepower) to the very large (multiple thousands of horsepower).
Stepper Motors & Drives
A stepper motor is a brushless DC electric motor that divides a mechanical rotation into a number of small, equal, discrete “steps” (commonly 200 steps/rev). Stepper Drives take an incoming control signal (often “Step and Direction” pulses from a controller) and energize the proper windings in the stepper motor to excite the commanded motion. Steppers most commonly run “Open loop” (without feedback such as an encoder) and for simple speed- or position-control applications, steppers are a cost-effective choice. Steppers are used extensively in small machines such as 3-D printers.
Servo Motors & Drives
Adding feedback to a motor and drive creates a Servo system, whereby the drive “knows” the position of the motor shaft at alltimes, and can very accurately control position, speed, and torque of the motor. Servo Motors are most commonly brushless DC motors (permanent magnet rotor, wound-coil stator) and are thus commutated by the feedback device, such as hall-effect sensors, encoders, or resolvers. Servos are more expensive than Steppers for a given size due to their additional complexity, but offer far greater performance, speed, and accuracy.
Un-rolling a rotary Servo Motor creates a Linear Servo – most commonly the stationary Track is made of permanent magnets, and the moving Coil is made of windings and controlled by the Servo Amplifier. This system also requires feedback for position and electrical commutation. Linear motors are used as a “direct drive” alternative to belt- or screw-drive systems, and can achieve very high speeds and accelerations, and very high positional accuracy.
The addition of a gearbox to a motor increases output torque at the cost of speed, which can be a very useful thing to apply a small, fast motor to a large, slow-moving load. Gear reductions also reduce reflected inertia to the motor, an important characteristic for tuning servo systems. Commonly in motion-control applications we use Planetary gearboxes for high efficiency and low backlash. A motor with an integrated gear reducer is known as a gearmotor.
Simple DC motors can be either “Brushed” or “Brushless” – a distinction based on where the electromagnets are placed, be they in the stator (housing) or rotor (spinning shaft). Brushed motors have rotor windings and commutate via physical contact with carbon brushes as the shaft rotates. These brushes are wear items and must be replaced at regular intervals, but the motors are very simple to operate – just apply DC voltage. Brushless motors take many forms, but all have the common trait of permanent magnets on the rotor, and windings in the stator/housing. Brushless motors normally require feedback for the drive to commutate.
Traditionally, motors and drives are separated by cables for power and feedback. Integrated motors package the amplifier (and sometimes controller) circuitry directly at the motor to simplify the package. This type of motor is convenient for smaller machines since it is possible to avoid control panels full of amplifiers and (frequently expensive) servo cables. The user only needs to run power (frequently DC) and communications or I/O to the motor.