VA-TEK Engineering Motor protection testing proves that the relay protects the motor against overload, locked rotor, phase unbalance, earth faults and undervoltage without tripping on a normal start. The test is built from the motor nameplate and the starting duty, not from a generic relay menu, because the thermal model depends on the motor’s full-load current, service factor and start time.
Key takeaways
- Start with the nameplate: full-load current, service factor, rated voltage, CT ratio and start time set the thermal and locked-rotor settings.
- Test the thermal model: a cold overload test and a hot restart test prove the curve, not just the pickup.
- Prove the start inhibit: the relay must block a restart until the thermal capacity recovers.
Data to collect before testing
| Data | Source | Used for |
|---|---|---|
| Full-load current (FLA) | Motor nameplate | Thermal overload and locked-rotor reference |
| Service factor | Motor nameplate | Allowed continuous overload |
| Rated voltage and frequency | Motor nameplate | Undervoltage and frequency elements |
| CT ratio and class | CT nameplate and drawing | Primary/secondary scaling |
| Start time and locked-rotor time | Motor data sheet or starting study | Locked-rotor and start-inhibit settings |
| Relay settings | Setting sheet and relay readback | Comparison and test points |
| Starting method | DOL, star-delta, soft starter or VFD | Start logic and inhibit behaviour |
Record the relay model, firmware version and the settings read back from the relay. If the setting sheet and the relay disagree, resolve that before injection; a test against an unknown settings set proves nothing.
The test sequence
- Isolate and inspect. Open the motor starter, apply lockout/tagout, confirm the circuit is dead and secure the driven equipment against rotation. Inspect the CT circuits, auxiliary supply and grounding.
- Verify settings and metering. Inject a known three-phase current and voltage and compare the relay’s displayed values with the injected values, scaled by the CT and VT ratios.
- Test the thermal overload element. Inject a current above the FLA setting and record the trip time. Repeat from a cold state and from a hot state. Confirm the relay’s thermal memory and cooling behaviour.
- Test the locked-rotor element. Inject a current at the locked-rotor setting and verify the trip time against the motor’s permissible locked-rotor time. Confirm that the element is inhibited during a normal start.
- Test negative-sequence or unbalance protection. Apply an unbalanced current set and verify the pickup and time delay. Confirm the relay distinguishes a sustained unbalance from a brief starting condition.
- Test the earth-fault element. Inject the residual or measured earth current and verify pickup and time. Confirm the CT connection and the relay’s earth-current input configuration.
- Test undervoltage and restart logic. Apply a voltage step below the setting and verify the trip or alarm. Confirm the restart inhibit and the recovery time after the voltage returns.
- Test the start inhibit. After a thermal trip, attempt a restart before the thermal capacity recovers. The relay should block the start; after recovery it should permit it.
- Prove the trip circuit. With the motor isolated, trip the starter or breaker from the relay output and confirm the auxiliary contacts, alarm and indication.
- Run check. On a permitted run, compare the relay’s current, voltage and unbalance readings with a clamp meter or the starter’s metering. Record the load current against the FLA.
Thermal test: cold and hot
The thermal element is a model, not a single pickup. A cold test starts with the motor at ambient temperature and injects a multiple of the FLA. The relay integrates the thermal capacity and trips when the model reaches 100%. A hot test starts with the thermal memory already loaded and should trip sooner. After the trip, the relay should cool according to its model and permit a restart only when the capacity has recovered.
Test at least two current multiples, for example 1.5 and 2 times the FLA, and record the trip time at each. Compare the measured time with the motor’s thermal withstand curve and the relay’s setting. A single-point test can pass while the curve shape is wrong.
Locked-rotor and start inhibit
The locked-rotor element protects the motor when it draws starting current without accelerating. The setting is based on the motor’s permissible locked-rotor time, which is shorter than the thermal withstand time at starting current. The test should prove two things: the element trips when the current remains at the locked-rotor level beyond the set time, and it does not trip during a normal start.
The start inhibit is the second half of the thermal model. After an overload trip, the relay must prevent a restart until the thermal capacity has recovered. Test the inhibit by attempting a start immediately after the trip and again after the calculated recovery time. A relay that trips correctly but allows an immediate restart does not protect the motor.
Common mistakes
- Using primary FLA on the secondary side. The relay setting is usually in secondary amperes or in multiples of the CT-rated current. Convert with the CT ratio and state the basis.
- Ignoring the service factor. A motor with a service factor above 1.0 can run above its nameplate FLA. The overload setting must reflect the intended continuous duty, not an arbitrary margin.
- Testing only the cold curve. The hot curve is the one that matters after a real overload. A cold-only test can miss a thermal-memory error.
- Setting the locked-rotor time longer than the motor withstand. The relay must trip before the motor is damaged; the test should confirm the coordination, not only the relay operation.
- Forgetting the start inhibit. The thermal model is incomplete if the restart logic is not tested.
- Leaving the trip circuit isolated. Restore the trip circuit and prove the breaker or contactor operation before returning the motor to service.
Frequently asked questions
What is the difference between thermal overload and locked-rotor protection?
Thermal overload protects against a sustained current above the motor’s continuous rating. Locked-rotor protection responds to a high current with no acceleration, which is a much shorter thermal event. The settings come from different motor data and must be coordinated.
How do you test a thermal relay curve?
Inject at least two current multiples from a cold state and record the trip times. Repeat one point from a hot state to prove the thermal memory. Compare the results with the motor’s thermal withstand curve and the relay setting.
Why does the relay block a restart after an overload?
The thermal model retains the motor’s accumulated heat. A restart before the capacity recovers would add heat to a motor that is already near its limit. The start inhibit is a protection function and should be tested, not bypassed.
Can a three-phase test set test motor protection?
Yes. Motor protection is mostly a three-phase current and voltage problem. A three-phase set with independent current and voltage channels, binary inputs and binary outputs covers the thermal, locked-rotor, unbalance, earth-fault and trip-circuit tests.
Where VA-TEK test sets fit
The VAE-430 provides 3×0–40 A AC and 4×0–120 V AC, with 8 binary inputs and 4 binary outputs, which covers the three-phase injection, start-inhibit logic and trip-circuit checks in a motor protection test. Its 420 VA per phase at 40 A holds current into high-burden electromechanical elements, and the 1×0–120 A parallel mode covers single-phase high-current tests. The full range is listed on the relay test set hub.
For the overcurrent elements that sit behind the motor protection, see overcurrent relay testing procedure. For the injection basics, see secondary injection testing. For the commissioning sequence, see protection relay commissioning checklist.
Free whitepaper: the standards, test items and test-set requirements discussed in this article are consolidated in Relay Protection Testing: Standards, Test Items and Test-Set Requirements — a 29-page PDF, free to download without registration.
Standards and references
- DL/T 995-2016, field inspection items for relay protection.
- IEEE C37.233-2023, Guide for Power System Protection Testing: standards.ieee.org.
- NETA ATS-2025 / MTS-2023, Chapter 7.9: netaworld.org.
