The test path runs from the 208 VAC three-phase source, through its breaker and the 15 kVA 480-to-240 VAC delta/delta transformer, into a temporary single-phase stator circuit, and back through the transformer. The 500 HP, 480 VAC, 3550 RPM, twelve-tap star/delta motor remains stationary except for controlled manual rotation. Follow the current through every connection: a voltage collapse or protection trip is a source-path problem; repeatable current change with rotor position is the measurement of interest.
What path carries the test current?
Start at layer one. The normal motor supply must be disconnected, isolated, and verified de-energized. The temporary source then crosses five distinct elements: source protection, transformer primary, transformer secondary, selected stator winding path, and squirrel-cage rotor coupling. A defect or limitation in any element can change the measured current.
| Path element | Required observation | Failure indication |
|---|---|---|
| 208 VAC source and breaker | Primary voltage remains stable during excitation | Breaker trip or primary-voltage sag |
| 480-to-240 VAC transformer | Secondary voltage matches the selected terminals before loading | Wrong tap selection, excessive heating, or disproportionate sag |
| Temporary motor connections | Connection matches the motor lead diagram and intended star or delta circuit | Unexpected current, torque, or unequal winding response |
| Motor stator | Stable current at each repeated rotor position | Variation caused by a connection, stator, air-gap, or rotor issue |
| Rotor and shaft | Shaft turns slowly through one mechanical revolution without an uncontrolled start | Unexpected motion, binding, or an unsafe torque reaction |
Check: Trace both conductors of the single-phase secondary circuit and confirm that no normal 480 VAC power path remains connected before selecting a test voltage.
What secondary voltage does a 208 VAC primary produce?
The transformer ratio predicts the unloaded line-to-line secondary voltage:
Vsecondary = 240 V × 208 V / 480 V = 104 V
The measured value was 106 VAC between X1-X2, X2-X3, and X3-X1. That is close to the ratio calculation and establishes that the primary connection and main secondary terminals are behaving as expected.
The center tap, X6, is the midpoint of one delta-secondary winding. Measurements were 53 VAC from X1 to X6, 53 VAC from X3 to X6, and 93 VAC from X2 to X6. The 53 VAC readings are half of the 106 VAC winding voltage. The high-leg value follows delta geometry: 106 V × √3 / 2 = 91.8 V, close to the measured 93 VAC. The 93 VAC terminal pair is not another half-winding source and must not be treated as one.
| Terminals | Measured voltage | Interpretation |
|---|---|---|
X1-X2, X2-X3, X3-X1
|
106 VAC | Reduced three-phase line-to-line output |
X1-X6 |
53 VAC | Half of the center-tapped secondary winding |
X3-X6 |
53 VAC | Other half of the same winding |
X2-X6 |
93 VAC | Center tap to the opposite delta vertex |
Check: With the motor disconnected, measure the exact terminal pair selected for the test and record its unloaded true-RMS voltage.
Can the source and transformer hold the test voltage?
The 15 kVA nameplate applies at the transformer's rated primary and secondary voltages. Reducing the primary from 480 to 208 VAC reduces secondary voltage but does not increase the allowable winding current. If primary winding current remains the thermal limit, the proportional capacity at 208 VAC is approximately:
15 kVA × 208 / 480 = 6.5 kVA
This derived value explains the observed source stiffness. In the 53 VAC delta trial, the secondary collapsed to 18 VAC while current reached 125 A. The loaded apparent power at that point was 18 V × 125 A = 2.25 kVA, but this value alone does not describe transformer heating because a single-phase load on a delta bank produces unequal winding currents.
In the 106 VAC star trial, voltage fell to 77 VAC at 85 A, giving 77 V × 85 A = 6.55 kVA. That is essentially the proportional 6.5 kVA capacity calculation. The installation ultimately required a 40 A, 208 VAC source. The earlier 15 A breaker arrangement could not be treated as adequate merely because the transformer carried a 15 kVA nameplate.
| Symptom | Likely location | Next check |
|---|---|---|
| 53 VAC falls to 18 VAC at 125 A | Transformer/source impedance or overloaded half winding | Measure primary and secondary voltage simultaneously |
| 106 VAC falls to 77 VAC at 85 A | Available transformer capacity reached | Compare winding currents with transformer ratings |
| Breaker opens during energization | Protection, conductor, inrush, or sustained-current limit | Review the complete source circuit; do not simply increase the breaker |
| Secondary holds unloaded but collapses on connection | Load current exceeds source stiffness | Inspect motor configuration and temporary leads before raising voltage |
Check: Energize only long enough to record primary voltage, selected secondary voltage, and current; proceed only if protection, conductors, transformer windings, and test leads are rated for the measured duty.
Should the motor be connected in star or delta?
The connection determines how the applied voltage divides among stator windings. With a symmetric star connection and single-phase voltage applied between two line terminals, two phase windings are in series. Applying 106 VAC therefore places approximately 53 VAC across each participating winding. With delta connected, applying 53 VAC across two line terminals places 53 VAC directly across one phase branch while the other two phase windings form a series parallel path.
Those circuits load the transformer differently, so 53 VAC delta and 106 VAC star are not interchangeable merely because both can place about 53 VAC across an individual winding. The observed results demonstrate the difference: the delta arrangement drew 125 A while collapsing to 18 VAC; the star arrangement held 77 VAC at 85 A.
A cited test guideline used 10% to 20% voltage, with less than 25% and an energization shorter than one minute offered as a heating boundary. The reference point for that percentage was not defined. For this motor, 10% to 20% of the 480 VAC nameplate is 48 to 96 VAC, while 25% is 120 VAC. Before applying that guideline, identify whether the procedure specifies motor line voltage, applied test voltage, or voltage across one phase winding. Use the motor manufacturer's test instructions when available.
Check: From the motor connection diagram, calculate the voltage across every energized winding for the selected star or delta circuit and verify it against the chosen test procedure.
How should the temporary circuit be prepared?
Do not rely on forcing contactors to establish the test connection. A mechanically forced contactor can defeat electrical interlocks and can change state unexpectedly. Configure the twelve motor leads directly according to the motor diagram, using temporary conductors and connections rated for the prospective current.
- Open, lock, and verify isolation of the normal motor source and control power.
- Disconnect the motor from its normal feeder so the test transformer cannot backfeed the plant circuit.
- Confirm the shaft can be rotated through one revolution and control any connected load that could move or return stored energy.
- Identify the twelve leads and build the selected star or delta circuit from the motor's connection diagram.
- Connect a true-RMS voltmeter across the motor test terminals and an appropriate current instrument in the excitation path.
- Place the disconnecting means or test switch where excitation can be removed immediately without approaching rotating or energized parts.
- Measure insulation and winding resistance as separate checks before applying the rotor test voltage; a rotor-position current test does not replace them.
Check: Perform a point-to-point continuity check from one transformer secondary terminal, through the intended stator winding path, and back to the other secondary terminal.
How is the rotor-position current test run?
Single-phase stator excitation produces a stationary pulsating magnetic field. Rotor-bar and end-ring condition changes the reflected rotor impedance seen at the stator. Turning the shaft changes the spatial relationship between the energized stator field and the cage; a repeatable impedance change appears as a current change when applied voltage remains constant.
- Mark a shaft reference and divide one mechanical revolution into repeatable observation positions.
- Set the test circuit to the lowest planned voltage and energize it briefly.
- Record voltage and current together. Current alone is misleading when the source sags.
- De-energize if voltage collapses, current exceeds the prepared circuit's rating, motion becomes uncontrolled, or any connection heats.
- Rotate the shaft slowly through one full revolution while recording current and voltage at each position. Keep total energized time within the approved test procedure; the installation described a roughly 30-second test.
- Repeat the revolution in the same direction to distinguish a rotor-position pattern from contact movement, meter response, or manual speed effects.
The proposed acceptance criterion was less than 5% current deviation. Treat that number as a job-specific criterion unless the motor manufacturer supplies it for this motor and test method. Calculate variation with a declared convention, such as (Imax − Imin) / Iaverage × 100%, and retain the simultaneous voltage record. A current change caused by voltage sag is not a rotor indication.
Check: Confirm that any current maximum or minimum repeats at the same shaft position while applied voltage remains stable.
What proves the end-to-end result?
The completed star-connected trial used 106 VAC unloaded, which fell to 77 VAC at 85 A. Current did not vary during one slow rotor revolution. That result found no rotor-position-dependent current signature under that specific connection, voltage, current, and test duration.
No variation is a screening result, not proof that every rotor bar and end-ring joint is intact. Sensitivity depends on magnetic excitation, source stiffness, air-gap uniformity, stator symmetry, measurement resolution, and repeatability. A severely sagging source can mask a small impedance change, while eccentricity, loose temporary connections, or a stator problem can imitate rotor variation.
| Verification record | Why it is required |
|---|---|
| Unloaded and loaded test voltage | Quantifies source sag and actual excitation |
| Current at indexed shaft positions | Identifies a repeatable spatial pattern |
| Star or delta lead arrangement | Makes winding voltage and current comparable |
| Total energized time | Documents thermal exposure |
| Repeat revolution | Separates rotor position from measurement noise |
Check: Repeat the same indexed revolution with stable voltage, then compare the recorded maximum, minimum, average, and calculated percentage variation.
FAQ
Can I use the 53 VAC center tap for the rotor test?
Yes, only if the chosen motor connection and transformer winding-current limits support it. In the recorded delta trial, 53 VAC collapsed to 18 VAC at 125 A, so that source path was unsuitable for a stable comparison.
Does 106 VAC apply 106 VAC to each winding in star?
No. With single-phase voltage applied between two terminals of a symmetric star connection, two windings are in series, so each carries approximately 53 VAC when the terminal voltage is 106 VAC.
Can unchanged current prove the squirrel-cage rotor is good?
It shows no detectable rotor-position signature under the test conditions; it does not prove every cage joint is intact. Repeat the indexed revolution while recording voltage and current together, and verify that the calculated variation remains repeatable with stable applied voltage.