Eddy-Current and Wound-Rotor Drives
An eddy-current drive consists of a fixed speed motor and an eddy-current clutch. The clutch contains a fixed-speed rotor and an adjustable speed rotor separated by a small air gap. A field coil, powered by a DC exciter, produces a magnetic field that determines the torque transmitted from the input rotor to the output rotor. The controller provides closed-loop speed regulation by varying clutch current, only allowing the clutch to transmit enough torque to operate at the desired speed. Once the speed is set, the drive control equipment receives shaft speed data from a tachometer on the output shaft, and any difference between set and actual speeds produces an error signal, which then increases or decreases the drive’s output of the exciter. Eddy-current drives offer excellent speed regulation, and they are electrically simpler than both DC and AC electrical adjustable speed drives.
Eddy-current drives are usually integrated with AC squirrel cage induction motors and are available from less than 1 HP to well over 1,000 HP. Speed ratios range up to 10:1 and the top speed cannot exceed the speed of the drive motor.
Eddy-current drives always have slip, so maximum rated output speeds typically run from 95 to 97 percent of the input speed. They dissipate heat losses through air cooling (up to 100 HP) and water cooling (100+ HP). They are best suited for variable-torque loads such as centrifugal pumps and fans.
Wound-rotor induction motor speed is controlled by varying the current through the motor’s rotating armature. Transistors control the current to provide smooth acceleration and deceleration.
Wound-rotor drives are available in ratings from 50 HP and higher. Maximum top speeds are about 95 percent of synchronous speed and minimum speeds are about 50 percent of synchronous speed. They are best suited for variable-torque loads requiring high horsepower and medium voltage.
Key Manufacturers of Adjustable Speed Drives We Offer
- Baldor (ABB)
- Boston Gear
- Hi-Lo Manufacturing
- SEW Eurodrive
- Speed Selector
- Sterling Electric
- TECO Westinghouse
- UST Tsubaki
Content on this page was created using excerpts from the Power Transmission Handbook (5th Edition), which is written and sold by the Power Transmission Distributor’s Association (PTDA). The Power Transmission Handbook is just under 400 pages and is a valuable resource for anyone involved or interested in the power transmission industry.