Lowering Motor Speed with a Variable Speed Drive

17 September 2014
The electromagnetic coil of an electric motor. Source: National Renewable Energy Laboratory

Adding a variable-frequency drive (VFD) upstream of an AC motor can lower energy use, reduce peak demand and improve power factor in many applications. Reducing motor speed at startup and during operation can cut maintenance costs, increase uptime and extend equipment life. Lowering motor speed when possible can reduce stress on the motor and its connected equipment. Whether the application is a process pump, a material handling conveyor or a piece of automated equipment, maintenance can be reduced by calculating and limiting a system’s maximum necessary speed, and by smoothing acceleration and deceleration when starting and stopping.

Although the primary focus of this article is to show how reducing motor speed cuts maintenance and repair costs, using a VFD can also result in substantial energy savings in certain applications. According to the U.S. Department of Energy publication, "Adjustable Speed Drive Part-Load Efficiency," reducing rotating equipment speed by 20% can reduce input power requirements by approximately 50%. This is particularly significant in light of research from IHS Technology, which finds that for an average electric motor used in a factory, a full 96% of the motor’s total cost of ownership is accounted for by its lifetime electricity usage. Repair and maintenance costs account for another 2%, with purchase costs accounting for the remaining 2%.

Calculating an average return on investment for a VFD attached to a motor can be difficult, says Kevin Schiller, an analyst who covers rotating machines and controls for IHS Technology. Among the variables to consider when making such a calculation are motor specifications (including power rating, motor efficiency, motor type), application (pumps, fans and compressors are the most common), motor operation (including the frequency of motor start/stop, operating time under varying load percentage, number of days of motor operation, previous use of mechanical throttling) and other cost factors (such as rebates/incentives, price of electricity and VFD efficiency).

Schiller says additional benefits from drive attachment that can reduce the investment payback time include reducing motor stress and prolonging motor life, reducing inrush currents, and realizing regenerative savings and harmonics mitigation on drives that offer such capabilities.

In general, suppliers report that customers expect at most a 2-year return on investment when making motor system-purchasing decisions, Schiller says. Some suppliers have accordingly shifted strategies, with programs to supply the more energy-efficient equipment for free and collect payment from the energy savings realized after installation. This is more common in retrofit scenarios, especially in building automation applications such as HVAC equipment.

When considering the most frequent causes of motor failures, motor bearings and damaged stator windings rank at or near the top. Taken together, bearing and winding issues account for more than half of motor failures. Fewer motor rotations result in less wear, so reducing motor speed can increase rotor and bearing life. Operating at a lower speed also reduces dynamic and static loading, along with thermal, vibration and shock stresses.

Ramping up slowly from stop to operating speed and running motors at lower speeds increases motor life by reducing wear and tear on motor bearings, motor winding insulation and connected mechanical components. Theoretically, reducing motor speed to provide 50% flow on pumps and fans will reduce the horsepower requirement to 1/8 of what is needed at 100% flow. That’s a lot less work and, consequently, less wear and tear.

With less work, there also is less heat, which extends insulation life. Stator winding insulation life is adversely affected by heat and running a motor slower usually reduces heat load. At reduced speeds, winding life increases in variable torque load applications for all motor types. The same is true for most constant torque load processes using totally enclosed not ventilated (TENV), totally enclosed blower cooled (TEBC) or drip-proof force ventilated (DPFV) motors. When running totally enclosed fan cooled (TEFC) motors at a constant torque load, insulation life may decrease at low speeds due to temperature rise, so it is possible to run too slowly.

Every process, conveyor and equipment application has an optimum speed at any given point in time. This optimum speed may be expressed in gallons per minute, inches per second, parts per minute and so on. Maintaining speed close to this optimum point can help cut maintenance costs.

A VFD offers the ability to adjust the amount of work being done to match the load. Some processes require adjustment of flow or pressure. An AC drive with proportional integral derivative (PID) control can control the speed of a pump and consequently flow or pressure. This speed adjustment may be more efficient than a flow control valve dumping excess flow back to a tank. The ability of an AC drive-controlled system to reduce the total flow or pressure delivered also allows a smooth rate of change, reducing, for example, the water hammer effect that can stress motors, pumps, valves and process piping.

Unnecessarily high motor speed affects any equipment the motor is attached to. Mechanical systems and power transmissions benefit from lower loads and stresses. It is easy to see the effect of too much speed on a ball screw in a linear actuator as running it too fast can cause the screw to bow and oscillate. Other types of detrimental effects can occur due to excess speed in mechanical systems such as pumps, fans and transmissions.

Custom equipment, such as a multi-station rotating table, must meet cycle time requirements. Running the table and related actuators faster than necessary can increase wear to the motion control components such as couplings, bearings, rollers, ball screws and gearboxes. Controlling the rotating table motors to run at only the required speed can also improve long term accuracy and durability.

Dynamic stresses, such as high acceleration and deceleration rates, regardless of speed, cause a variety of maintenance problems such as bearing failures and breakage of couplings, belts and chains in production equipment.

Across-the-line starts are harder on the motor and the electrical system than a controlled start. Starting and stopping are the most stressful parts of motor operation as they cause high dynamic stress, and frequent starts and stops can increase motor temperature.

An AC drive not only can control operating speed, but it can also reduce motor acceleration and deceleration rates. VFDs can reduce inrush currents and thus starting torque by 30-75% compared to across-the-line starters. This reduces wear on belts, gears, chains, clutches, shafts and bearings. If high speed must be maintained with a lightly loaded motor, many VFDs can make use of built-in configurable voltage/hertz curves. With this feature, the output voltage is lowered in relationship to the output frequency. In lightly loaded applications, this reduced voltage cuts excitation loss in the motor and reduces motor current, decreasing thermal stress without sacrificing speed.

VFDs certainly aren’t a fit for every application, but where practical, their control of speed, acceleration and deceleration can reduce maintenance, increase uptime and extend equipment life.

Additional resources:

The low voltage drives research is accessible here.

Energy efficiency is also discussed in this IHS report on low voltage motors.

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