A VFD enables fine control of AC induction motor speed, creating opportunities for increased efficiency, reduced wear and tear on the motor, and decreased operating costs. However, while VFDs are extremely useful, there are several common misconceptions about the maximum and minimum speeds they can provide in practical, real-world applications.

How Does a VFD Work?

Unlike DC motors, an AC motor's speed cannot be controlled by modifying the input voltage. Instead, it is necessary to change the AC power supply frequency, which determines the speed of the rotating electromagnetic field in the motor that turns the rotor shaft.

A VFD (or Variable Frequency Drive) achieves this by converting the AC power to DC. The VFD then sends out a DC pulse that acts as an AC sine wave to control intensity. Operators can select the desired output frequency to control the speed of the motor.

There are two main types of VFDs. Voltage/Hertz VFDs are the most common and cost-effective. They control the motor's speed by managing the relationship between the voltage and frequency to provide speed control, especially around the motor's base speed. Vector Control VFDs are more complex and expensive but enable very precise motor speed control over the entire speed range, even providing full torque at zero speed (holding torque). VFDs usually come with a frequency range that determines the speed range they can provide.

While VFDs provide great benefits, there are practical limits to the minimum and maximum speed that a motor can safely be operated at.

Soft Start

When full power is applied to an electric motor at standstill, the resulting current surge can damage the motor and electronics, and sudden torque loads can damage bearings, pulleys, and mechanical components. For this reason, it is recommended to increase the speed gradually, and a VFD is often used to provide this capability.

Because the speed does not usually need to be controlled precisely during a soft start, there are few additional considerations when selecting and using a VFD to soft-start an electric motor.

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Minimum Speed

VFDs enable an electric motor to be operated slower than its base speed, and Vector Control VFDs can even enable the motor to provide holding torque at standstill. However, especially for TEFC (Totally Enclosed Fan-Cooled) motors where the cooling fan is mounted on the motor shaft, motor cooling is proportional to the speed of the motor, and cooling is decreased when the motor slows down. Especially when operating with full torque at low speeds, heat stress can quickly build up in the motor, and over time will drastically reduce its service life and drive up repair and replacement costs.

Generally speaking, TEFC motors are not designed to operate at less than a 4:1 speed range, while some manufacturers make a 10:1 or 20:1 range motor. Most often, operating the motor slower than this requires an auxiliary cooling system. This threshold can be higher depending on the torque the motor is providing. Many manufacturers provide data on the relationship between cooling efficiency and operating speed for their motors, enabling a precise calculation of the acceptable minimum speed. Always check the manufactures data pack if your turndown exceeds a 4:1 ratio or 15 HZ.

When selecting a VFD for a motor you intend to operate at low speeds, ensure it can deliver the required frequency with a smooth and clean output. Very low speeds or holding torque applications will require a Vector Control drive or similar.

Maximum Speed

VFDs come with an upper limit on the frequency they can provide to the motor, which often produces a speed significantly higher than the motor’s base speed. However, just because a higher speed is achievable does not mean that it is practical. Most motors only create Constant Horsepower or CT when over-speed. This results in a loss of torque, and if running a fan or a pump, it can greatly overload your motor. Always check your torque requirements and calculate your motor torque when over speeding a motor past 60 HZ.

When the rotor of a motor is not perfectly balanced, the result is a vibration proportional to the motor's speed. In practice, rotors can never be perfectly balanced due to manufacturing limitations and tolerances. When operating a motor 50% faster than base speed, it can become a serious issue. Vibration increases mechanical and heat stress on the motor, gearbox and bearings and can significantly impact the service life of motor drive components. Always ensure that vibration is within acceptable limits to prevent high maintenance and repair costs.

There are other electrical and mechanical limitations to the maximum speed that an electric motor can reach. At high speeds, bearing friction and fan loading increase, which reduces the efficiency of the motor and increases power requirements and operating costs. Also, because the motor produces a back-EMF voltage proportional to the motor's speed, the voltage required to reach higher speeds while maintaining torque increases, eventually exceeding what the VFD can deliver. A full analysis of the application's required operating characteristics is necessary to determine if the motor and electronics can deliver.

For high-horsepower motors with a base speed of 3600 rpm or more, NEMA (MG1) regulations prohibit overspeed to maintain safety and prevent accidents. Check your national and local safety codes to ensure that you can overspeed your motor while remaining compliant.

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VFDs are incredibly useful and enable operators to fine-tune the speed of their electric motor applications to increase efficiency, cut costs and get more from their motors. However, it helps to understand the practical limits that govern the minimum and maximum speed that an electric motor can safely operate within.

eMotors Direct offers a wide range of motor speed controls that enable you to precisely manage your electric motor application and motor drives and accessories for all types of budgets and applications.