Comparing induction motors, permanent-magnet motors, and servomotors

Designers and motor personnel advantage from discovering a supplier that's an skilled resource of info to assist in pragmatic motor choice. Involve application specialists as early as you possibly can, as they are able to assist create prototypes, custom electrical and mechanical styles, mountings, and gearboxes. This also reduces expenses related with shorter lead occasions and rush delivery.

Servo motors can offer higher performance, faster speeds, and smaller sizes. PM synchronous motors offer advantages on high-energy- consuming and high-dynamic applications, compared to induction motors. Variable frequency drives used with asynchronous motors also can be used with synchronous servo motors, producing higher efficiencies than an asynchronous motor, using perhaps 30% less energy in positioning applications.

Induction motor systems (lower cost, rugged, reliable, and well known) can offer an alternative to servo motor systems (the traditional, established solution) for certain applications. This, of course, is based on similar electronic controls being used (with the latest technology and approximately the same cost), leaving the cost of motors the differentiating issue.

Overview of the pros and cons of each motor type

Induction motor

SPEED - Less speed range than PMAC motors • Speed range is a function of the drive being used — to 1,000:1 with an encoder, 120:1 under field-oriented control
EFFICIENCY - Even NEMA-premium efficiency units exhibit degraded efficiencies at low load
RELIABILITY - Waste heat is capable of degrading insulation essential to motor operation • Years of service common with proper operation
POWER DENSITY - Induction produced by squirrel cage rotor inherently limits power density
ACCURACY - Flux vector and field-oriented control allows for some of accuracy of servos
COST - Relatively modest initial cost; higher operating costs

PMAC

SPEED - VFD-driven PMAC motors can be used in nearly all induction-motor and some servo applications • Typical servomotor application speed — to 10,000 rpm — is out of PMAC motor range.
EFFICIENCY - More efficient than induction motors, so run more coolly under the same load conditions
RELIABILITY - Lower operating temperatures reduces wear and tear, maintenance • Extends bearing and insulation life • Robust construction for years of trouble-free operation in harsh environments
POWER DENSITY - Rare-earth permanent magnets produce more flux (and resultant torque) for their physical size than induction types
ACCURACY - Without feedback, can be difficult to locate and position to the pinpoint accuracy of servomotors
COST - Exhibit higher efficiency, so their energy use is smaller and full return on their initial purchase cost is realized more quickly

Servomotor

SPEED - Reaches 10,000 rpm • Brushless DC servomotors also operate at all speeds while maintaining rated load
EFFICIENCY - Designed to operate over wide range of voltages (as this is how their speed is varied) but efficiency drops with voltage
RELIABILITY - Physical motor issues minimal; demanding servo applications require careful sizing, or can threaten failure
POWER DENSITY - Capable of high peak torque for rapid acceleration
ACCURACY - Closed-loop servomotor operation utilizes feedback for speed accuracy to ±0.001% of base speed
COST - Price can be tenfold that of other systems

In the end, all industrial motor subtypes have strengths and weaknesses,plus application niches for which they’re most suitable. For example, many industrial applications are essentially constant torque, such as conveyors. Others, such as centrifugal blowers, require torque to vary as the square of the speed. In contrast, machine tools and center winders are constant horsepower, with torque decreasing as speed increases. Which motors are most suitable in these situations? As we will explore, the speed-torque relationship and efficiency requirements often determine the most appropriate motor.