Turndown ratio in AC motor speed control

When choosing a motor for an application, a primary consideration is the speed range it will be speed reducer gearbox operated in. Whenever a motor is run considerably slower than its rated base speed, a number of potential adverse effects may come into perform, including reduced cooling efficiency, reduced power efficiency and a modify in the motor’s acceleration and torque characteristics. To mitigate this problem, some motors and speed controllers have already been designed especially to drive lots at low speeds with specific 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 a credit card applicatoin operates at a constant speed, the thing that is required may be a gearbox or speed reducer that brings the engine speed down to the required level. Nevertheless, many applications require the swiftness of the electric motor to be various during operation.

This is usually achieved using a VFD or Variable Frequency Drive, which controls the speed by modifying the frequency fed to the motor. Selecting the most appropriate electric motor and VFD type depends upon a variety of factors, however, it is necessary to 1st look at the way the characteristics of a motor change when the swiftness is reduced.

A motor usually has a base speed, specific by the product manufacturer, that it is usually made to operate at. Nevertheless, if a engine is managed below the base speed, it may experience reduced efficiency of the coolant system. Especially with commonly used Totally Enclosed Fan Cooled (TEFC) and ODP (Open Drip Proof) motors, where in fact the cooling program consists primarily of a shaft-mounted enthusiast, a reduction in speed results in decreased airflow over the engine and loss of cooling, and high temperature buildup occurs. Especially when the engine is operated with complete torque at low speeds, heat can easily build up inside the motor to harmful levels.