Test Methods for various common tests performed on motors
Many of these tests require a mechanical load to be applied to the motor so the motor has something to “push” against. Such loads are often either a mechanical brake, which converts the motor output energy to frictional heat loss, or a generator, which converts the mechanical energy to electricity which can then be used to drive a resistor load bank (convert to heat) or other means to dump the electrical energy.
For AC motors, use a CT and PT to measure the current (I) and voltage (V) applied to a motor to form the instantaneous power P=I*V.
For DC motors, rather than a CT, you measure the current with a shunt, and compute power as I*V.
The amount of power is somewhat proportional to the motor load so tests typically automatically ramp the load to form a curve of input electrical power to output mechanical power. The “no load” and “full load” (or “blocked”) conditions are often of interest because fundamental parameters of motor operation can be obtained. For example, the “no load” phase angle between current and voltage in each phase of an AC induction motors is of interest for equivalent circuit parameters.
Condition of Windings
Testing the resistance of each winding can be measured with a standard DMM. These tests help determine issues such as shorts to ground, shorts between phases, and broken windings. A high-pot tester can check for winding insulation breakdown.
Sensors used are listed in the table above and applied to various components used to construct the motor, such as the housing, heat sink fins, bearings, and so on. The type of sensor to use is usually chosen based on the required accuracy, with RTDs being the best. Careful calibration and precision signal conditioning is important to get the highest accuracy from temperature sensors due to their small signal output.
Bearing and Unbalance Vibration
For rotating motors, applying an accelerometer to a motor bearing mount can assess the mechanical integrity of the bearing. Excessive vibration indicates bearing failure and the frequencies of those vibrations are indicative of the types of faults, such as bad roller bearing elements or overall looseness due to worn out elements.
When two axes are measured simultaneously, say vertical and horizontal as defined by gravity, an overall mount movement can be detected if either the bearing or the mount is moving. These frequencies occur at rotational speed and the harmonics, caused either by shaft misalignment or motor unbalance. Asymmetric windings, whether mechanical or electrical, also cause rotational forces.
For rotating motors, placing an accelerometer in the direction of the motor shaft can detect looseness and misalignment in the shaft.
A more typical means of detecting motion of the shaft, due to run out or looseness, uses a proximity sensor. This type of sensor detects the distance between the sensor and the shaft and are almost always non-contact sensors, although some ride on the shaft surface. Non-contact version of these sensors usually use either eddy current or laser displacement methods, and one method may perform better that the other depending on the shaft material and condition. For example, the eddy current method will not work on a carbon fiber composite shaft.
A high-bandwidth CT or shunt measures the time response of current draw by the motor when first powered. Since the motor is not moving initially, the power source is driving a very low resistance due almost exclusively to the resistance of the windings. The current in-rush returns to normal levels after the motor starts moving. The shape and peak amplitude of the pull-in current can be used as a pass/fail disposition during manufacturing tests.
Run Up Time and Vibrations
The time required for a motor to reach the commanded speed can be an indication of proper motor construction. The typical sensor used for this measurement is an encoder or 1/rev (once per revolution) sensor. Motors that can be speed-controlled often have built-in encoders or 1/rev sensors which the motor controller can use to detect speed. Another method utilizes a non-contact sensor, such as a proximity or laser, to measure some aspect of the motor speed. For a prox, two notches at 180 degrees apart on the shaft (to maintain balance) can produce an oscillating signal the frequency of which is proportional to the motor speed. For a laser, a reflective yet lightweight piece of tape can produce an oscillating reflectance.
Also, since the run up to speed passes through all possible operating speeds of the motor, it is useful to monitor vibration during this run up in case any mechanical resonances are passed as the range of frequencies are invoked. In these situations, a slow run up is beneficial to allow build-up of low Q resonances.
Coast Down Time
The same test methods discussed for run up apply to coast down as well, with the one important fact that during coast down the motor is not being powered. Thus, any torque and other forces present during run up are absent in a coast down.