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MOTOR LOADING METHODS
Eddy Current BrakesNo Load Devices
These brakes are available in a wide range of torques and configurations, from fractional to hundreds of horsepower, air and water cooled. They provide smooth and controllable loads and operate without any physical contact of interactive members. As a result, with the exception of shaft bearing, there is virtually no wear on components. They are reasonably priced, however, they produce no torque at zero speed. This makes them ideal for applications where low speed control is not required. Note that locked rotor testing can still be accomplished using a conventional friction brake.
These brakes are available from very small fractional sizes up to about 15 horsepower. They are able to produce torque all the way down to zero speed and are available with speed ratings exceeding 20,000 RPM. They operate without any physical contact being made to the interactive members and they provide smooth and controllable loads. This results in virtually no wear on the components, with the exception of the shaft bearing. They are ideal for motor testing from zero speed (locked rotor) all the way up to full speed.
Hysteresis brakes often exhibit a minimum torque, so care must be taken to assure that the minimum torque is acceptable. In addition, dynamometers made from these devices can be expensive compared to other methods.
These drives, used as brakes, are the premium solution and are available from about one to hundreds of horsepower. They produce full torque at zero speed so they can be used for locked rotor testing. They have a very fast response (often needed for switch cutout testing) and, in some cases, sophisticated electronics that can determine shaft torque without the use of an additional instrument. Because the drive can act as a motor, it can help accelerate smaller motors being tested on systems that can accommodate much larger motors.
The motor shaft is disconnected from any load. This is the most accurate no-load test
The dynamometer or absorber is essentially "turned off" so that no braking torque is produced. With this method, zero torque is inferred (not completely achieved) as there will always be some drag torque on the shaft, even if the instruments read zero
An in-line torque transducer is used to measure the torque at the motor shaft. Then the dynamometer is controlled to run in such a way as to create zero torque, as measured by the torque transducer. This creates a situation where the speed to the dynamometer exactly matches the speed of the motor, which creates a no-load condition
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