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Motor selection depends on load inertia as well as the application. Excessive mismatches produce less than optimal response and can waste power.
Just because a motor's speed can be varied, it doesn't mean that it's a viable candidate for a motion-control positioning system. For example, an induction motor and inverter controller can run from about 300 rpm to a base speed of 1,800. But most positioning systems require speeds down to zero and often higher than 1,800 rpm.
However, induction motors are ideal for centrifugal fans, conveyors, pumps, and mixers because they are relatively inexpensive, simple and easy to use, highly reliable, and vibration free. Additional benefits include a peak overload capacity of 150% and direction reversal. And, they come with controls to preset speeds and program I/Os.
Vector motors
Still, don't dismiss induction motors for motion control altogether. Adding a feedback device such as a resolver or encoder transforms it into a vector motor, an adequate candidate for positioning systems with modest accuracy needs. Furthermore, closing the loop around the encoder, controller, and motor lets the motor run down to 0 rpm.
Vector motors with closedloop controls provide tight speed regulation and constant torque from 0 rpm to base speed. Operating in the constant torque mode lets the motor run up to three times the base speed. Such units include programmable features such as controlled acceleration, deceleration, and tuning.
When not operating in the constant torque mode, however, the vector motor still is limited to its base speed of 1,800 rpm. A typical 1-hp vector motor with an inertia of 0.052 lb-in.-sec^sup 2^ can accelerate to 1,800 rpm in about 0.37 sec while operating in the current mode and with a load inertia equal to the motor inertia.
Servomotors
When a motion-control system requires greater acceleration and velocity than possible with an induction motor, servomotors are a better choice. Servomotors can accelerate faster than vector motors because they have smaller diameter, lower inertia rotors.
A 1-hp dc servomotor, for instance, accelerates two times faster than a vector motor when considering inertia only. However, when...