FRA for Rotating Machine-Transformer Assemblies: Diagnosing Integrated Winding Systems
Motor-generator sets, variable speed drive (VSD) systems, and synchronous condensers often integrate transformers within the same enclosure or on a shared baseplate. The transformer's frequency response interacts with the motor/generator windings through capacitive and inductive coupling. A Transformer Frequency Response Analyzer, applied to the complete assembly, detects not only transformer winding faults but also interaction issues such as shaft voltage loops, bearing currents, and VSI-induced ringing.
Unique Aspects of Integrated Machine-Transformer Assemblies
Combined rotating and static windings create additional FRA complexities:
Capacitive coupling through the shaft: The motor rotor, even when stationary, couples capacitively to the stator and transformer. This adds a low-pass filter to the FRA response above 100 kHz.
Shared cooling and grounding: Common coolant loops (water, oil) and ground paths create parallel impedance paths not present in standalone transformers.
Variable speed drive (VSI) effects: IGBT switching transients couple through the transformer into the motor, producing reflected wave ringing. FRA can identify the transformer's contribution to this ringing.
Mechanical coupling (for rotating reference frames): For synchronously coupled machines (e.g., motor-generator), the mechanical angle affects transformer load through back EMF. FRA performed with rotor locked at different positions may vary.
Detecting VSI-Induced Ringing via FRA
When a VSI connects to a motor through a transformer, steep-front PWM pulses cause voltage doubling at the motor terminals if the cable and transformer impedance mismatch. FRA identifies the mismatch:
Measure FRA from VSI output terminals (transformer primary) to motor terminals (transformer secondary).
A peak in the transfer function at the reflected wave frequency (typically 100 kHz–2 MHz) indicates resonance that will amplify VSI pulses.
If the peak amplitude exceeds +6 dB, install output filters or adjust VSI rise time.
Case Example: Bearing Current Diagnosis via FRA
A 500 kW variable speed drive (480 V input, 690 V output through an isolation transformer) suffered repeated bearing failures. FRA performed from transformer secondary (motor side) to ground, with the motor stationary, showed:
Low-frequency (50–500 Hz) phase shift of 25 degrees between high-potential and low-potential motor terminals relative to ground
A high-Q resonance at 1.2 MHz with amplitude +12 dB
This indicated a capacitive coupling path through the transformer and motor stator that, during VSI operation, produced shaft voltages high enough to discharge through bearings. A shaft grounding brush and common-mode filter were installed. Follow-up FRA showed the 1.2 MHz resonance damped to +3 dB, and bearing failures ceased. Without FRA, the root cause would have remained hidden.
Testing Protocol for Integrated Assemblies
Follow this combined machine-transformer procedure:
De-energize entire assembly and lock out all supplies (VSI, exciter, auxiliary power).
For motor-generator sets, lock the rotor in a fixed angular position (use a mechanical brake or barring device).
Perform FRA on the transformer primary (VSI side) to secondary (motor side) as a transfer function.
Perform FRA from transformer primary to ground, and secondary to ground, to assess common-mode coupling.
Repeat FRA with rotor rotated 90° and 180° to detect angular-dependent coupling.
Using FRA to Validate Replacement Transformers in VSD Systems
When replacing a transformer in an existing VSD system, ensure impedance matching to prevent reflected wave damage:
Measure FRA of original transformer (primary to secondary) for a known good unit.
Measure replacement transformer using same test configuration.
If CC < 0.95 in the 100 kHz–5 MHz band, the replacement has different leakage inductance or winding capacitance. Consult VSI manufacturer for compatibility.
For variable speed drive and motor-generator systems, the Transformer Frequency Response Analyzer provides diagnostic visibility into the integrated winding assembly, detecting bearing current paths, VSI ringing hazards, and transformer-motor interactions that conventional testing misses.
