Adjustable Speed Motor Drives PDF Print E-mail
Written by Washington State University Cooperative Extension Energy Program   
Thursday, 10 June 2010 12:33

Adjustable Speed Motor Drives

Energy Efficiency Factsheet

Washington State University Cooperative Extension Energy Program

Description: Most motors turn at nearly constant speed. However, much of the time the devices they drive may operate at less than maximum design speed.  This speed reduction can be accommodated by an Adjustable Speed Drive (ASD) that varies the shaft speed to the driven load. Slowing a pump or
fan in this manner reduces energy consumption much more effectively than allowing the motor to run at constant speed and then restricting or bypassing the flow with a valve or damper.

In addition to providing speed control, ASDs provide soft starting, whereby a motor starts slowly and then speeds up. This reduces the mechanical stress on both the motor and equipment driven by the motor. This also reduces the voltage sag that can occur when a large motor starts quickly. Voltage sags can dim lights and cause other equipment to shut down or restart.

The newest and most efficient adjustable speed drive is the variable frequency drive (VFD), sometimes called a variable speed drive. A VFD consists of an electronic power converter that converts constant frequency AC (alternating current) power input into a variable frequency output. The AC motor speed varies in proportion to the drive output frequency.

Applications: The most common applications of ASDs are for pumps and fans— to balance flows and meet changing system needs. For example, ASDs can be very cost-effective in retrofit or new construction of HVAC systems. Many HVAC systems were designed with constant flow pumps and fans that are throttled to meet changing operating conditions.

ASDs are also useful for loads such as elevators, water and wastewater pumps, boiler fans, cooling towers, cranes and conveyors. The speed range, required precision of speed control, and required torque at lower speeds dictate the type of ASD needed.

Performance/Costs: Savings from ASDs come from reduced load of the fan, pump, or driven device. With fans and pumps, power consumed is proportional to the cube root of shaft speed. If shaft speed is reduced by 10%, flow is reduced by 10%, while power consumption is reduced by 27%. If speed is reduced by 20%, power is reduced by 49%.
adjustable speed motor drives1
adjustable speed motor drives2
Compared to throttling as a means of flow control, speed reduction provides dramatic energy savings. Throttling to reduce flow in a fan or pump backs the device up on its operating curve, increasing pressure and often increasing power consumption.

The following graphs of a typical circulating pump (75% efficient) demonstrate the difference in energy use between throttling and ASD to reduce flow by 33%. The operating point for peak conditions is at the intersection of the pump curve and the system head curve (925 gpm at 42-foot head) and draws about 13 horsepower. Reducing flow through throttling to 600 gpm requires the pump to produce about 45 feet of head, drawing about 9 horsepower. Reducing speed with ASD reduces the head to less than 20 feet and draws less than 5 horsepower. This results in almost 70% reduc- tion in power consumed.

Basic ASDs cost about the same as the motors they drive, but they are highly dependent upon features and application requirements. Costs for basic variable torque drives start at around $520 per horsepower for a one HP drive and drop sharply to around $160 per horsepower for a 10 horsepower drive. Costs level off more slowly after that with 40 horsepower drives at about $100, and 500 horsepower drives down to about $70. Installation costs, electrical filters and special features for constant torque, special controls, or diagnostics can easily more than double the costs.

Availability: Electronic VFDs are readily available. There are many manufacturers producing units ranging from integral to hundreds of horsepower. DC drives are available from most major motor manufacturers.

For Additional Information:
Energy Star Purchasing Initiative
Contains purchasing and efficiency information for electric motors

ASDMaster software
A description and ordering information for the software on U.S. DOE’s Industries of the Future BestPractices website (scroll down to the ASDMaster entry under Software).

Motors—U.S. Department of Energy Industries of the Future BestPractices
This website contains tip sheets, case studies, and technical reports on motors and drives efficiency practices.

Variable Frequency Drives
FEMP Greening Federal Facilities, section 3.3.2

The Drive for Energy Efficiency: Variable speed drives can help facilities maximize HVAC operations
From Maintenance Solutions, June 1996 ©Washington State University

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