Servo and stepper motors are the “muscle” of any electromechanical solution. The motor’s torque, speed, and position can be controlled with extreme precision, allowing for incredibly complex motion. 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 & VFD (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 all times, 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.
Integrated Motion Solutions.
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.