Turndown ratio in AC motor speed control

When choosing a motor for an application, a speed reducer gearbox primary consideration may be the speed range it’ll be operated in. Whenever a motor is run considerably slower than its rated base speed, a number of potential adverse effects will come into perform, including reduced cooling effectiveness, reduced power performance and a modify in the motor’s acceleration and torque features. To mitigate this issue, some motors and rate controllers have already been designed especially to drive a load at low speeds with exact control.

Most domestic and industrial motor applications use 3-phase asynchronous induction motors, which operate at a speed that is dependant on the frequency of the supply power. When an application operates at a continuous speed, the only thing that is needed could be a gearbox or rate reducer that brings the engine speed down to the required level. Nevertheless, many applications need the velocity of the engine to be varied during operation.

This is normally achieved utilizing a VFD or Variable Frequency Drive, which controls the speed by modifying the frequency fed to the motor. Deciding on the best electric motor and VFD type depends on a number of factors, however, it is necessary to 1st look at how the characteristics of a motor change when the speed is reduced.

A motor usually includes a base speed, specified by the manufacturer, that it is usually designed to operate at. Nevertheless, if a electric motor is managed below the bottom speed, it could experience reduced effectiveness of the coolant system. Especially with frequently used Totally Enclosed Fan Cooled (TEFC) and ODP (Open Drip Evidence) motors, where in fact the cooling program consists mainly of a shaft-mounted enthusiast, a decrease in speed results in decreased airflow over the engine and loss of cooling, and high temperature buildup occurs. Particularly when the motor is operated with complete torque at low speeds, heat can easily build up inside the motor to damaging levels.