Selecting Servo Motors

Permanent magnet synchronous, AC induction asynchronous, Dan Jones explores some options for servo motors.

A servo is not just a motor. It is a closed loop motion system composed of a controller, a drive, a motor and a feedback device, usually an optical or magnetic encoder.

The servo motor can be a synchronous machine with permanent magnets (PM) that is represented by a brush or brushless PM motor, or it can be an asynchronous machine based on an AC induction motor.

The permanent magnet synchronous motor possesses very high peak and continuous torques used to drive the servo system at high acceleration and deceleration rates in precision positioning systems. Torque is directly proportional to input current. Motor shaft speed is directly linked to input voltage. The higher the input voltage, the higher the motor’s speed. Torque versus speed curve is linear.

The permanent magnet structure is directly adjacent to the motor’s air gap. Looking at the brushless PM motor’s configuration, the two interactive magnetic structures, the moving rotor (with attached permanent magnets) and the stationary stator winding interact magnetically to create the motor’s torque and speed.

The three-phase stator field is sequentially energized and the PM rotor follows the rotating stator field in a synchronous fashion. A special electronic commutation system is used to detect rotor position to energize the stator windings. The brushless PM motor would be selected over all other motors in precision positioning systems with the exception of most automotive applications and systems using very large motors. The brushless PM motor is only a servo motor when it is used in a closed loop torque, velocity or positioning system.

Entirely different rotor

While the AC induction motor has the same physical stator as a PM brushless motor, it has an entirely different rotor construction. The squirrel-cage induction motor rotor consists of a series of conducting aluminum or copper bars laid into slots located in the rotor structure and connected at either end by large shorting rings.

These shorted rotor bars magnetically couple to the stator’s rotating fi eld and induce a new rotor fi eld that interacts with the stator fi eld to produce rotor motion.

There is a small difference between the synchronous stator and the slower rotor fi elds and the actual rotor speed. This speed difference is designated the “slip”. The input frequency determines motor speed.

For example, a 60 Hz, two-pole AC induction motor would have a no-load speed near 3,600 rpm, and a four-pole AC motor would run at below 1,800 rpm depending on the slip value. As the motor develops torque, slip increases and speed drops.

The AC induction motor will develop more torque at the expense of speed reduction until the load approaches the breakdown point where motor speed suddenly drops to zero. An intrinsic AC motor performance characteristic is that it has little starting torque and any load must be removed for the motor to start.

This motor intrinsic torque-speed performance was completely changed by the arrival of the inverter electronic drive in the 1980s. The inverter’s ability to change both voltage and frequency using adjustable or variable speed drives reshaped the torque-speed curve, opening use of AC induction motors as the speed-system king.

How to choose

Continued evolution of higher performance drives has brought the brushless PM and AC induction motors and associated drives closer to competitive conditions in many markets, but the brushless PM motor dominates position control applications. The AC induction motor is not presently designed for low inertia and high acceleration response.

Use a brushless PM motor for servo position systems except in 50 kW (67 hp) or larger, high power systems. And use AC induction motors more in constant or variable speed systems. Cross-over solutions are rare. Other motors hold promise, but without the success of the venerable AC induction motor or the up-andcoming brushless PM motors.

Brushless PM motors in speed controls have also made important inroads against DC brushed speed controls in the 1 kW (1.37 hp) and smaller power applications on the factory fl oor. The AC induction motor dominates most speed applications from up to 1 MW.

Dan Jones is President, Incremotion Associates (www.incremotion.com).

Siemens 1FK7 permanent magnet synchronous motor

Siemens 1PH7 AC induction synchronous motor