Motor Controls, Starters and Variable Speed Drives
Control circuits and starters
Motor control is built from contactors, overload relays, pushbuttons, selector switches, auxiliary contacts, and interlocking. Two-wire control (maintained contact) restarts automatically after a power failure; three-wire control (momentary start with a sealing contact) requires a manual restart - a critical safety distinction. Overload relay classes (Class 10, 20, 30) describe trip time at six times rated current: Class 10 trips in 10 seconds or less, Class 20 in 20 seconds, and so on; high-inertia loads need a higher class. Manual and magnetic starters, reduced-voltage autotransformer, star-delta, soft starters and VFDs each trade starting current against starting torque.
Variable speed drives
A VFD rectifies AC to a DC bus, smooths it with capacitors, and inverts it back to adjustable-frequency AC using PWM. Below base speed the drive holds a constant volts-per-hertz ratio so torque stays roughly constant and horsepower falls with speed; above base speed the voltage is capped, so the field weakens and torque falls. Important practical points: shielded motor cable with the shield grounded at both ends, keeping motor leads short to limit reflected-wave voltage, separate the drive from sensitive electronics, and expect harmonic distortion on the supply.
Traps
Sizing overloads from the drive's output rather than the motor nameplate; assuming a VFD eliminates the need for a motor overload device; forgetting that a soft starter reduces voltage but not frequency; and mixing up two-wire and three-wire restart behaviour.
Worked example
A 60 Hz motor running at 30 Hz from a VFD at constant V/Hz produces about half speed and half horsepower but essentially full torque (if cooling allows). Commanding 90 Hz on a 60 Hz motor gives 1.5× speed with torque reduced to about 67% because voltage can no longer rise.