Electric drive and motor selection factors

An electric drive is a system that uses an electric motor as the prime mover to control the speed, torque and direction of a mechanical load. It consists of the power source, a power modulator/controller (starter, converter), the motor, the transmission (shaft, gears, belt) and the driven load, with sensing and control units.

Supply → Power modulator → Motor → Transmission → Load
              ↑                          |
          Control unit  ←── sensors ──────┘

Factors affecting the selection of motor

1. Nature of supply: AC or DC, single or three phase, voltage and frequency available. Most industries use 3-phase AC induction motors because the supply is AC.
2. Load characteristics:
   • Speed–torque characteristic of the load (constant torque like conveyors, torque ∝ speed² like fans and pumps, constant power like winders).
   • Starting torque required (cranes and compressors need high starting torque; fans need little).
   • Speed range and need for speed control (wide control → DC motor or VFD-fed induction motor).
3. Electrical characteristics of the motor: starting current and its effect on supply voltage, running characteristics, speed regulation, braking (plugging, rheostatic, regenerative), power factor and efficiency.
4. Mechanical considerations: type of enclosure (open, drip-proof, totally enclosed fan-cooled, flameproof) suited to dust, moisture or explosive atmosphere; type of bearings; mounting (foot, flange); transmission (direct, belt, gear); noise and vibration.
5. Size and rating: rating chosen from the duty cycle: continuous, short-time or intermittent periodic duty. Under-rating causes overheating; over-rating gives poor pf and efficiency.
6. Service conditions: ambient temperature, altitude, humidity, frequency of starting and reversal.
7. Cost: initial cost of motor and control gear, running cost (efficiency, pf), and maintenance cost. Squirrel-cage motors are cheapest and almost maintenance-free.
8. Availability and standardisation: standard frame sizes and ratings make spares and replacement easy.

Example: a lift needs high starting torque, frequent starts, reversal and smooth speed control, so a VFD-fed induction motor (or DC motor) is chosen; a ceiling fan works well with a simple single-phase capacitor motor.