June 05, 2005
9 Motor Testing and CBM
Evaluating the condition of electric motor rotors is not a new concept. Electric motor repair shops have been using techniques such as growler testing, single-phasing, infrared, vibration and/or dye testing as methods used through the repair process. In the field, vibration, analog current tests and inductive testing has been used. Over the past twenty years, a variety of motor circuit analysis and current signature analysis technologies have entered the market providing even greater accuracy in the detection of rotor bars.
VP, Electrical Reliability Group
T-Solutions, Inc.
Rotor Testing
Evaluating the condition of electric motor rotors is not a new concept. Electric motor repair shops have been using techniques such as growler testing, single-phasing, infrared, vibration and/or dye testing as methods used through the repair process. In the field, vibration, analog current tests and inductive testing has been used. Over the past twenty years, a variety of motor circuit analysis and current signature analysis technologies have entered the market providing even greater accuracy in the detection of rotor bars.
Growler testing is performed by applying power to a half-transformer with the rotor resting upon, or near, it. The induced current allows the repair person to check the rotor bars by using metal filings or magnetic paper. The term growler comes from its original purpose of detecting shorts in DC armatures. The power is induced into the armature and a hacksaw blade would be held just above the armature over each slot. If a short exists, the blade would begin to vibrate causing a ‘growling’ sound. The accuracy of growler testing on AC rotors is improved by heating the rotor in an oven to approximately 200 degrees F prior to testing in order to cause expansion of any fractures.
Single phase testing involves the application of approximately 10% of the motor voltage across a single phase (ie: T1-T2), in an assembled motor with a good winding, then rotating the shaft slowly with a current probe attached. If the value stays within 3%, as the rotor is turned, then it is in good condition. This test is considered potentially dangerous to the technician.
Infrared testing is performed by winding a coil of wire through, or around, the rotor and applying a voltage and high current. This causes the rotor to heat and will identify smeared laminations and loose or broken rotor bars. Hot spots, identified with infrared, greater than 10 degrees C above the ambient rotor temperature identify faults.
Vibration analysis is performed with the motor assembled and under load, usually at least 50% of rated load. Signatures of twice line frequency with pole pass frequency and peaks of the number of rotor bars times the running frequency, will indicate rotor bar problems. This normally requires some degree of experience by the operator.
Analog current meters on switchgear/MCC’s, or hand-held analog current meters, can be used by observing the meters for sharp pulsing of the current as the motor operates under load. This ‘ticking’ motion will occur at pole pass frequency and is a strong identifier of multiple broken rotor bars.
Inductance testing can be used by viewing a continuous measurement of inductance or periodic testing of inductance through an arc or full rotation of the motor shaft, of an assembled motor. A repeating pattern indicates a good rotor and impact of the peaks or valleys of a few of the patterns indicate a broken rotor bar problem, impacts on the slopes of the patterns indicate casting voids (in aluminum cast rotors). There should be one pattern per pole of the machine being tested.
Current signature analysis is a classical method for analyzing rotor bars. Side bands of twice slip frequency approaching ’35 dB down’ indicate severe rotor bar conditions, that must be addressed.
