Turndown ratio in AC motor speed control

When choosing a electric motor for an application, a primary consideration is the speed range it will be operated in. When a motor is run substantially slower than its ranked base speed, numerous potential adverse effects will come into perform, including reduced cooling performance, reduced power efficiency and a change in the motor’s swiftness and torque characteristics. To mitigate this issue, some motors and quickness controllers have been designed especially to operate a vehicle lots at low speeds with precise control.

Most domestic and industrial motor applications use 3-phase asynchronous induction motors, which operate at a speed that’s determined by the frequency of the supply power. When an application operates at a continuous speed, the only thing that is required could be a gearbox or velocity reducer that brings the engine speed down to the required level. However, many applications need the rate of the engine to be different 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. Selecting the most appropriate motor and VFD type depends on a number of factors, however, it is necessary to 1st look at the way the characteristics of a motor change when the quickness is reduced.

A motor usually has a base speed, specific by the manufacturer, that it is designed to operate at. Nevertheless, if a motor is operated below the base speed, it may experience reduced performance of the coolant system. Especially with typically used Totally Enclosed speed reducer gearbox Enthusiast Cooled (TEFC) and ODP (Open Drip Proof) motors, where the cooling program consists mainly of a shaft-mounted fan, a decrease in speed outcomes in decreased airflow over the electric motor and lack of cooling, and high temperature buildup occurs. Especially when the engine is operated with full torque at low speeds, heat can quickly build up in the motor to harming levels.

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