Turndown ratio in AC motor speed control
by William V. Schrottky, Technical Writer, eMotors Direct Inc.
Choosing the right motor and VFD type depends on a variety of factors, however, it is necessary to first look at how the characteristics of a motor change when the speed is reduced
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When choosing a 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 rated base speed, a number of potential adverse effects may come into play, including reduced cooling efficiency, reduced power efficiency and a change in the motor’s speed and torque characteristics. To mitigate this problem, some motors and speed controllers have been designed especially to drive a load at low speeds with precise control.
Most domestic and industrial motor applications use 3-phase asynchronous induction motors, which operate at a speed that is determined by the frequency of the supply power. When an application operates at a constant speed, the only thing that is required may be a gearbox or speed reducer that brings the motor speed down to the required level. However, many applications require the speed of the motor to be varied 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. Choosing the right motor and VFD type depends on a variety of factors, however, it is necessary to first look at how the characteristics of a motor change when the speed is reduced.
A motor usually has a base speed, specified by the manufacturer, that it is designed to operate at. However, if a motor is operated below the base speed, it may experience reduced efficiency of the cooling system. Especially with commonly used Totally Enclosed Fan Cooled (TEFC) and ODP (Open Drip Proof) motors, where the cooling system consists primarily of a shaft-mounted fan, a reduction in speed results in reduced airflow over the motor and loss of cooling, and heat buildup occurs. Especially when the motor is operated with full torque at low speeds, heat can quickly build up inside the motor to damaging levels.
Heat stress experienced by a motor has potentially severe consequences for the health of all of the motor’s electrical and mechanical components. Of primary importance is the insulation of the motor’s stator and rotor windings, which can break down quickly at high temperatures. As a general rule, for every 10 degrees over the insulation rating that a motor is operated at, the service lifetime is halved. Different motors have different insulation properties and ratings, and NEMA has developed a standardized system of insulation ratings as a guide.
Some motors have been designed with special thermal management properties and cooling systems that allow them to operate at very low speeds without damaging effects, including better thermal insulation and separately driven cooling fans. The ability of a motor to be operated at speeds lower than the rated base speed is represented by the “turndown ratio.”
The turndown ratio of a motor is the ratio of speed relative to the rayed base speed that it can be operated at safely 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” – that is, torque at zero speed.
To determine the turndown ratio that is required for the motor used in a specific application, it is necessary to first determine the lowest speed that the motor will operate at for a significant period of time. Motors come with different turndown ratios, and, generally, higher ratio motors are more expensive. This means that it is important to fully understand the requirements of the application and choose exactly the right motor for the job.
There are two main methods for controlling the speed of a motor – scalar control and vector control.
Scalar control essentially involves maintaining the voltage to frequency ratio of the motor, which determines the torque it produces, so that torque theoretically remains constant even when the speed of the motor changes. However, this is not a fully accurate model because it relies on the assumption 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. Because of this, scalar control is usually recommended for motors with a turndown ratio of 5:1 or less.
Vector motor control, on the other hand, involves a more complex model of the motor characteristics which separates the management of the voltage and the frequency, providing greater control. Because vector control relies on a microprocessor and often also utilizes advanced sensors to provide feedback from the motor, vector control drives can be relatively expensive. However, for precise motor control at very low speeds (or holding torque at zero speed) it is usually required because scalar control methods are simply not accurate enough.
When choosing a VFD, it is important to choose one with a control method that provides sufficient precision and accuracy over the entire speed range that the motor will be operated at.
Operating an AC motor over a wide speed range requires an understanding of the effect of low-speed operation on motor characteristics. At lower speeds, thermal management becomes an issue, and it is necessary to choose a motor with a turndown ratio that covers the full speed range needed by the application. Additionally, the control method used by the motor’s speed controller has a great impact on the precision and accuracy of speed management at low speeds.
