Motors are typically inexpensive but repairing can rack up high costs. The need for repairs is perfectly preventable if proper motor testing methods are applied. Keeping motor efficiently functional is possible once you know which tests to conduct.
These methods of motor testing are based on standard parameters, revealing potential risks and preventing failures that, when done regularly, can provide savings, safety, energy conservation, and increased uptime.
Types of Motor Testing
There are several ways to classify motor testing. Offline or static testing is done while the motor is turned off, and can be performed on-site, in a repair facility or during inventory. This test is scheduled once or twice a year and determines the insulation system’s integrity. Online or dynamic testing, on the other hand, is executed while the motor is on its average load and operational condition. This test collects current information to flag concerning results.
Motor testing can also be grouped either by electrical or mechanical methods. Most of these tests are done in conjunction with another to ensure optimal operability and are just some of the most common examples out of the many diagnostic methods, depending on the need and requirement.
Insulation Resistance Test
The windings of a motor are often subjected to damaging external factors like high temperature and moisture. Insulation resistance (IR) testing is vital when the motor has been stored for a long time. The IR rule of thumb is 10 Megohm with respect to ground.
This test is done using a hand-crank meter or an advanced tester that routinely checks IR and displays leakage current, voltage, Polarization Index ratios, and DAR. The IR test is often referred to as the dirt test, which means that particles like oil, grease, moisture and more can dwarf surface currents, thereby dropping Megohms. It’s mainly performed to check for direct shorts to ground.
The High Potential (HiPot) test is conducted using a high-voltage current made to flow through the insulation to determine whether it will break the integrity or cause other issues to the motor. Defective insulation can cause motor failure and increases the risk of shock to the operator.
The HiPot tester measures current leaks within the unit. It can also reveal crushed or nicked insulation, contaminants like dirt or moisture, stray wires, terminal spacing problems, or other manufacturing defects.
Polarization Index Test
Polarization Index (PI) tests, also known as dielectric breakdown tests, are performed to assess the health of the motor insulation, including any physical changes that may affect motor operability. It is done by introducing a positive charge to the conductors and a negative charge to the frame for about 10 minutes.
Graphs indicating changes in current over the timeframe will show good insulation, while an unhealthy one will stay constant. Induction motors should have a minimum value of PI at 2.0 to be functional. This test is typically an extension of the insulation resistance test.
Compared to the other tests previously mentioned that covers ground wall problems, surge testing identifies turn-to-turn weaknesses in the insulation. This early problem detection provides the opportunity to replace or repair damaged parts without having to suffer downtime.
The surge test is executed using a rapid rise-time and high-voltage impulse to the winding that will result in a voltage difference with an adjacent wire. If the voltage difference is high, or if the insulation weakens or gets damaged during the process, the wires will be sustaining an arc, which manifests as a change of pattern in the surge waveform on an impulse generator.
Mechanical or Rotor Tests
The growler test uses an armature growler, an iron core wrapped with a coil of wire and connected to an AC source current. This test is used to find any discontinuity of current flowing through the rotor bar, which may be due to loosened laminations, blown-out tires, or cracking. The rotor is separated from the stator during this test.
The growler induces AC power, much like an open-ended transformer, to check for shorted turns. A hacksaw blade or any feeler is placed above the rotor and growls or vibrates when there is a problem with the magnetic field produced.
Single-phase Rotor Test
The single-phase rotor test is conducted while the motor is not running yet still assembled. A low-voltage, several-ampere power is applied as the rotor is gradually turned. An ammeter is used to monitor the current in the supply circuit. The operator listens for a louder-than-normal electrical noise and cogging feel on the rotors.
Fluctuations in the stator current can indicate one or several cracked rotor bars.
High-current Rotor Test
The high-current rotor test is performed to a rotor separated from its stator by applying high current and thermal scans to search for shorted laminations. These issues are manifested via localized hot spots that may cause the rotor to lose its balance and bow. It may also result in premature cracking due to the stress from uneven rotor heating and can find loosely fitting rotor-to-shaft interference.
Vibration analyzers are utilized in a vibration spectrum analysis to determine any difference in frequencies of vibrations that can indicate malfunctions. Cracked rotor bars typically have an increased vibrational frequency amplitude as more load is added.
Other problems that can be pointed out by this test are shorted winding, elliptical stator core, rotor out-of-round, or unequal air gap between stator and rotor.
The Right Specialist Provider
Motor testing lengthens the service life of the unit. Regularly scheduling diagnostic methods can snip a potential problem at its early stages, ensuring that you get the best value for your money while also protecting the motor from further damaging the overall system.
Present Group is a specialist provider of electrical testing and commissioning, operations and maintenance, and other power technology services. If you need a company that is committed and driven to developing strong relationships with customers, contact us today.