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Turndown Ratio in AC Motor Speed Control

December 10, 2020

The ability to slow down the speed of the motor is needed in some operations. However, if a motor runs substantially slower than its rated base speed, potential adverse effects may come into play. Here are some things to consider when controlling the speed of your motor.

Controlling Motor Speeds with VFDs

Most domestic and industrial motor applications use 3-phase asynchronous induction motors. These motors operate at a speed determined by the frequency of the power supply. When an application operates at a constant speed, the only required thing would be a gearbox or speed reducer that brings the motor speed down to the desired level. Many applications require the speed of the motor to change during operation. This is usually achieved by using a Variable Frequency Drive (VFD), which controls the speed by modifying the frequency fed to the motor.

Dangers of Operating Below Base Speed

A motor is usually designed to operate at a base speed set by the manufacturer. Motors that operate below the base speed may experience reduced efficiency in the cooling system. In Totally Enclosed Fan Cooled (TEFC) and Open Drip Proof (ODP) motors, the cooling system consists of a shaft-mounted fan. A reduction in speed results in reduced airflow over the motor and loss of cooling, causing heat build-up. When the motor is operating at full torque at low speed, heat can quickly build up inside the motor.

Heat stress experienced by a motor has severe consequences for the health of both electrical and mechanical components. An important factor is the insulation of the motor’s stator and rotor windings, which can break down quickly at high temperatures. The general rule of thumb is that for every 10 degrees over the insulation rating, the expected service life is halved.

How Slow Can You Go?

Some motors have been designed with special thermal management properties or separate blower fans, allowing them to operate at very low speeds without harm. A motor's ability to operate at speeds slower than the rated base speed is measured by the ‘turndown ratio.’

The turndown ratio is the lowest speed at which the motor can safely run relative to the rated base speed without suffering thermal damage. A turndown ratio of 10:1 means that a motor can be safely operated at 1/10th of the base speed. Common turndown ratios are 10:1, 20:1, and 1000:1, with some motors even designed to provide ‘holding torque,’ which is torque at zero speed.

Motors come with different turndown ratios. Generally, higher ratio motors are more expensive. It is important to fully understand the application requirements and choose a motor that has a speed range capable of handling that application.

Controlling the Speed of the Motor

There are two main methods for controlling the speed of a motor: scalar control and vector control.

Scalar Control

Scalar control involves maintaining the voltage to frequency ratio of the motor, which determines the torque it produces. The torque theoretically remains constant even when the speed of the motor changes. This is not a fully accurate model because it assumes that the voltage/frequency ratio fully determines the speed. In fact, motors experience a loss of efficiency at lower speeds, which often requires an additional voltage boost to be applied to the motor as it approaches standstill. Scalar control is usually recommended for motors with a turndown ratio of 5:1 or less.

Vector Control

On the other hand, Vector control involves a more complex model of the motor characteristics, which separates the management of the voltage and frequency, providing greater control. Vector control relies on a microprocessor and utilizes advanced sensors to provide feedback from the motor. Nonetheless, for precise motor control at very low speeds (or holding torque at zero speed), it is usually required because scalar control methods are not accurate enough.

Conclusion

When you have an application that needs your motor to run at various speeds, there are 2 major factors you need to consider. Firstly, the motor’s turndown ratio will determine the extent to which you can vary your motor speed without harm. Secondly, understanding the difference between scalar and vector controls will clarify the different types of controls out there.

Credit: William Von Schrottky

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