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MOTOR TESTING METHODS
No-Load Test (AC or DC)

Voltage, Current, Power, Power Factor, Speed (RPM) & Direction

The No Load test is performed by unloading the shaft of the motor as much as possible, or applying it as close to zero torque as possible.  A regulated AC voltage is applied to the motor leads and motor voltage, current, wattage and direction are monitored. A low voltage start test will also be done prior to these no-load tests.

Three types of no load tests can be provided: disconnected, inferred no-load and measured no-load.

Load Point Test (AC or DC)

Voltage, Current, Electrical Power, Power Factor Speed (RPM), Direction, Torque, Mechanical Power and Efficiency

As many different load steps as needed can be programmed for a given motor-under-test, or a continuous speed vs. torque curve can be generated.

To measure a specific Load Point, the dynamometer is set to control in either speed mode or torque mode, depending on customer preference. If the dynamometer is controlling speed (speed mode testing), then a speed is established by the dynamometer and torque is produced based on the capabilities of the motor. If the dynamometer is controlling torque (torque mode testing), then a load is established by the dynamometer and speed is produced based on the capabilities of the motor. In either case, once the desired point is established, the tester can measure speed, torque, voltage and current (amps) depending on how the individual tester is configured.

The most commonly used single load point is the full load point. This means that the speed selected is the motor’s rated speed (if speed mode testing is used) or the load selected is the motor’s rated torque (if torque mode testing is used). Then any data gathered is full load data. As many load points as desired can be gathered

Locked Rotor (AC)

Voltage, Current, Electrical Power, Power Factor, Torque, Direction

Another common load point is known as “Locked Rotor” or “Stalled Torque”. A full current is applied to a Hysteresis Brake, Dynamometer, or by simply clamping or “locking” the motor shaft and energizing the motor. More torque is produced by the Dynamometer/Brake than the motor can produce. In this state the shaft cannot turn, simulating the rotor being “locked” or “blocked”.

This test is very hard on a motor. There is a large amount of current that flows into the rotor, causing it to heat up rapidly. As a result, this test must be performed very quickly. A motor, with a locked rotor, draws up to six or seven times its rated current (sometimes more). The power supply used must be capable of regulating the motor voltage adequately during rapid changes in current to ensure the proper voltage is maintained when the data is being taken. ESW can suggest several levels of power supply performance to meet testing requirements in your budget.

Locked rotor torque testing is important. If the motor cannot produce enough torque to overcome the friction in the load, as it sits without rotating, the motor can be energized but it will not start the load. If the motor remains in this state for very long it will overheat and fail.

This criteria indicates whether or not a motor may be more likely to suffer from nuisance tripping during motor starts and determines whether the motor exceed the National Electrical Manufactures Associate (NEMA) locked-rotor current limit.  This performance measurement can help indicate the reliability of the motor.