The seamless working of motors is what guarantees hassle-free operation of the entire plant. Since the conception of the idea of variable speed controls in AC and DC motors, the production of shaft currents seems indispensable. How are these currents produced? When motor shafts are subjected to voltage that is varied through electronic speed controls, the shaft current is produced that either discharges to the ground or moves along the motor. This current is a potential threat to the working of motors. This current if accompanied with some of the risk factors, tends to fuel the chances of damaging the motors. These “Risk Aspects” include:
- Size of frames of motors and magnetic asymmetry in the windings of motors
- Transportation and handling of motors in a rough manner can dismantle the structure
- The improper grounding of motors causes some current to circulate through without passing over to the Earth
- Low-noise high carrier frequencies tend to induce shaft voltages thereby fueling the production of unwanted shaft currents
- Improper methods of usage without adhering to the methodology mentioned in the motor manual.
How does one test whether the shaft currents being produced are within the limits specified by IEEE standards? Performing this analysis is perhaps one of the most complicated and risky testing methodologies as the motors need to be functioning at an energetic level, and bare shafts are required. A carbon brush can be used along with an oscilloscope to fulfill the safety of the testing team. The process is as follows:
- The first step is the measurement of ambient noise level. The carbon brush is connected to a ground spot on the motor and oscilloscope is connected to the ground point on the motor frame.
- The next step is to place the carbon brush at the rotating shaft of motor while the oscilloscope still remains grounded. This step involved careful analysis of the voltage spikes produced in the graph. When analyzing the voltages at the peaks, the voltage in excess of 0.5mV can indicate trouble of improper shaft current production.
The deviation in the motor parameter values helps in identifying the areas that may be causing the trouble. If there is some problem due to mishandling, the motor can be replaced while highlighting the issue. Inappropriate ground can be handled by the test engineers. Insulated bearings or ceramic based bearings can also be used for avoiding such damage but these are quite expensive. The most feasible path seems to be the grounding rings. In this, the carbon brushes as used during the test are inserted onto the motor shaft and are connected to the frame of motor. This way, an alternate low impedance path for current is created that allows maximum current to pass through it rather than affecting the bearings.