Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics Part 16 Wound Rotor and Servos
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
Recently, a question was posed about how we handle our research: The theoretical part of our MCA research FOLLOWS the detection of faults in real-world environments by MCA users and our laboratory consists of industrial and commercial locations, motor repair facilities and utilities. Therefore, the materials presented in this series of lectures do not discuss the potential capability based upon theory, but the results of actual applications with the theories representing the best explanation for real-world results. The theoretical part of our work involves verifying repeatability for new findings, keeping up to date in the latest theories, definitions of physical laws and research in both engineering and physics. The real-world part of our work involves continued direct contact with MCA users, of all technologies, users of other technologies and interaction with manufacturers of other condition based monitoring equipment (such as infrared, ultrasonics, etc.).
Now for our next lecture:
As previously discussed, one of the abilities of MCA equipment is the ability to test across the airgap for mutual inductance. In fact, MCA induces a voltage and frequency equal to the frequency applied to the stator and the transformer ratio of the stator windings to rotor windings. The total effect depends upon the airgap of the motor between the top of the stator teeth and rotor core. This produces the effect of the measured AC properties of MCA to be able to read across the airgap.
Wound Rotor Motors and Synchronous Motors
In a wound rotor motor or synchronous motor, the transformer properties allow the detection of winding shorts across the airgap. In the case of machine tool motors that have permanent magnet rotors, the magnetic domains of the ferromagnetic material will interact with the airgap frequency can be audibly heard as a tone that varies by the applied frequency. Following will be short descriptions on the effect and how to evaluate the condition.
Wound rotor motors have, normally, wye connected windings which are connected to a variable impedance source (usually a resistor bank). By increasing resistance, the starting torque increases while the breakdown and full load torques decrease. This allows for either starting high inertia loads or speed control by increasing slip. By decreasing resistance, the starting torque is reduced while the breakdown and full load torque increases, allowing for speed control by decreasing slip. The resistor bank is both used to change the rotor circuit resistance and dissipate heat, in effect acting as a heat sink for the rotor. The rotor is AC. Winding shorts occur in a wound rotor the same as in a stator.
Synchronous motors have wound rotor fields which are used to carry DC voltage. The DC voltage is used to lock the rotor with the rotating magnetic fields so that the rotor turns at the same speed as the rotating field. A second winding, called the amortisseur winding is an induction winding with the bars mounted in the field poles and shorting rings. This is used to produce starting torque and to act as a torsional absorber to reduce faults in the DC fields due to induced AC. Rotor fields normally fail due to heat generated from the AC magnetic field and contamination. The faults, just as in a DC motor field, occur from the inside out.
When a fault is detected in the Fi and I/F test results, this may indicate that a fault exists in the stator or rotor. The quickest and simplest way to determine whether the fault is in the stator or rotor is to move the shaft and re-test. If the test results change position, then the fault is in the rotor windings, if the results remain in the same position, then the fault is in the stator windings.
Servo and Machine Tool Motors
Most servo and machine tool motors have permanent magnets in the rotors. The magnetic domains (permanently aligned magnetic dipoles) will have a direct effect on the inductance of the windings. If a rotor is held in one place and a set of readings taken on a permanent magnet machine, the inductance in one or two phases will be very high, which causes the impedance to follow. Therefore, there is, normally, an extreme inductance and impedance unbalance. In a few cases, the extreme can have some effect on Fi or I/F (but not both).
Winding contamination is still measured by comparing the impedance and inductance of each phase. Verifying the Fi and I/F, in a few designs that cause variance, will require an extra step, unless it is being trended (the difference between phases will be maintained regardless of rotor position). To verify condition, view real-time inductance until it is at the minimum reading for that phase, then perform winding tests. Repeat this action for each phase. The tolerance for Fi and I/F is then +/- 1 from the average between phases.
Conclusion
Extensive work and field (not lab) confirmation has gone in to the analysis of winding fault detection in the application of:
AC machines through 13.2 kV
AC generators through 250 MW
DC machines to over 8,000 hp
Synchronous machines to over 10,000 hp
All types and models of machine tool and servo motors
Locomotive and Automotive AC and DC traction motors
AC traction transmissions
Transformers, dried and oil filled, through 135 kV
Magnetic coils
Capacitors including power factor correction
Cables
In each case, MCA has performed as expected.