The number that matters is the winding temperature reached during the 10-minute acceleration. Reduced current in wye does not make that interval thermally harmless: rotor speed remains low, motor cooling may be weak, and sustained current continues to deposit heat in the stator and rotor. Protect the 170 hp motor with a line-side motor-protection relay whose starting and running thermal behavior can be configured from motor-manufacturer data. First verify whether the existing delta-side overload already measures winding current in both connection states; the description of “delta windings shunted” may instead refer to the contactor that forms the wye point.
Thermal load during a 10-minute start
Motor heating follows approximately I²R. Cutting winding current in half cuts copper loss to one quarter, but maintaining elevated current for 10 minutes can still consume the motor’s available thermal capacity. A conventional overload selected for normal running duty may trip during that interval because its thermal curve interprets the long acceleration as an overload or stall.
Cooling also changes during acceleration. A shaft-mounted fan produces less airflow below rated speed, while a high-inertia load can hold the motor in this reduced-cooling state for most of the start. The protection decision therefore cannot come from horsepower and elapsed time alone. It requires the actual current-versus-time trace, the motor’s permissible stall or acceleration time, its thermal time constants, and the cooling method.
| Quantity or limit | Why it matters | Where to obtain it |
|---|---|---|
10-minute acceleration current profile |
Calculates accumulated thermal load and identifies a stalled or abnormally slow start | Recorded line current and speed or process feedback during a representative start |
| Locked-rotor current in wye | Sets the upper starting-current region for relay and contactor evaluation | Motor manufacturer or a controlled measurement |
| Permissible stall time | Defines the point beyond which current must be interrupted to prevent thermal damage | Motor thermal data or manufacturer-approved protection curve |
| Stator and rotor thermal time constants | Determine heating during a start and cooling between starts | Motor manufacturer |
| Starts per interval and hot-restart limit | Accounts for residual heat before another acceleration | Motor duty data and operating records |
| Line and star-contactor current duty | Checks whether each contactor can carry starting current for the full interval | Contactor datasheet and measured starting current |
Wye-delta winding and current relationships
A conventional wye-delta motor has three windings with both ends of every winding brought to the starter. It does not have a separate “wye winding” and “delta winding.” The star contactor joins three winding ends to form the wye point; the delta contactor reconnects the same windings for the running configuration. Confirm the motor terminal diagram before treating this installation as conventional because a custom or two-speed single-winding motor could use a different arrangement.
In wye, line current equals winding current. In delta, line current is √3 times winding current. At the same line voltage, each winding receives VLL/√3 in wye and VLL in delta. For the same standstill winding impedance, wye line current is approximately one third of delta line current, and starting torque is approximately one third of delta-connected direct-on-line torque.
Overload location changes the current that the device sees. A line-side device measures motor line current through both connection states. A device installed in a motor winding leg measures winding current, which remains related to delta line current by Iline = √3 × Iwinding. Compare the relay setting and curve with the current at its actual installation point; a setting based on nameplate line current cannot be transferred blindly to a winding-current location.
Symptoms and likely causes
| Observed condition | Likely mechanism | Deciding check |
|---|---|---|
| Standard overload trips before acceleration finishes | The overload curve does not allow the measured current for 10 minutes
|
Overlay the recorded current-time trace on the relay curve and the motor’s permissible thermal curve |
| No overload appears active while in wye | The protection may be bypassed, incorrectly located, or misunderstood because of the starter topology | Trace every motor lead and current-transformer or overload path in both contactor states |
| Star contactor overheats or its contacts deteriorate | Its current or duty rating is inadequate for locked-rotor wye current over the full acceleration | Compare measured current and duration with the contactor’s utilization and thermal-duty data |
| Motor reaches the transition point on one start but not another | Load inertia, process loading, supply voltage, starting torque, or residual temperature has changed | Trend current, voltage, acceleration time, transition speed, and motor thermal state |
| High current persists without increasing speed | The motor is stalled or the load torque exceeds available wye torque | Compare current with speed or process feedback and apply the manufacturer’s stall-time limit |
Protection approaches compared
| Approach | Starting protection | Main limitation | Use in this case |
|---|---|---|---|
| Rely on the existing delta-side overload | Potentially adequate if it continuously measures winding current in both wye and delta | Requires a verified wiring path and a curve that tolerates the legitimate start without exceeding the motor limit | Inspect before adding hardware; the apparent bypass may be a topology misunderstanding |
| Add a conventional overload at the star contactor | Adds current-based protection during wye operation | A standard curve may nuisance-trip during a valid 10-minute start or allow an unsafe thermal condition if oversized |
Use only when its curve can be coordinated with the motor’s starting limit |
| Install a line-side motor-protection relay | Monitors both connection states and can separate starting behavior from running protection | Needs motor thermal data, correct sensor scaling, and commissioning records | Recommended architecture for retaining the long wye-delta start |
| Convert to a VFD and operate the motor in its required running connection | Controls acceleration current and torque while adding electronic motor protection | Requires a motor, load, cooling, insulation, bypass, and process review | Evaluate when the existing start is mechanically or thermally marginal |
| Run without overload protection during wye | Only short-circuit protection remains active | A stall or extended acceleration can continue until another device operates or the motor is damaged | Not the recommended solution for an asset lacking a documented manufacturer-approved start curve |
A line-side motor-protection relay is the strongest fit because it observes the complete start and run cycle without depending on which contactor is energized. The relay must support separately coordinated starting and running thermal behavior, stall supervision, and retained thermal state between starts. A Multilin 469 is a candidate identified for this function. Before considering a Schweitzer SEL-701, confirm that its available protection elements and curves can implement the required separation for this motor.
Recommended protection architecture
Place the motor-protection measurement on the line side of the wye-delta starter so the relay sees current throughout wye acceleration, open transition, and delta running. Keep short-circuit protection separate from thermal-overload protection: the short-circuit device clears high-magnitude faults, while the motor relay limits accumulated heating, stalls, excessive acceleration time, and unsafe restarts.
Use motor-manufacturer limits as the protection envelope. The starting curve must ride above the highest legitimate current-time trace with measurement tolerance and normal process variation, yet remain below the motor’s permissible hot and cold thermal limits. Running overload protection must match the motor’s continuous duty rather than inherit a long starting delay.
The contactors need their own duty check. Verify that the line and star contactors can carry measured locked-rotor wye current for the full 10 minutes. Selecting the star contactor physically the same size as the line contactor is one conservative approach for this unusual duty, but matching enclosure size is not the deciding criterion; the published current, utilization category, operating duty, and thermal capability are.
Configuration and commissioning procedure
- Collect the complete motor nameplate, terminal diagram, load type, cooling method, permissible stall time, locked-rotor current, thermal time constants, hot-restart restriction, and permitted start frequency. Obtain the manufacturer’s starting-current or thermal-damage curve where available.
- Trace the power circuit from supply to all six motor leads. Record the locations of the line, star, and delta contactors, overload elements, current transformers, and every auxiliary contact used for transition logic.
- Verify which current the existing overload measures in wye and delta. Test the current path from drawings and conductor routing rather than from the physical position of the overload beside a contactor.
- Record line voltage, all three line currents, acceleration time, transition time, and speed or process feedback during a controlled start. Stop the test within the motor manufacturer’s permissible stall or acceleration limit.
- Compare the three phase currents for imbalance and plot current against elapsed time. A flat high-current trace without increasing speed identifies a stall condition; a declining trace shows acceleration but still requires a thermal calculation.
- Select and scale the line-side motor-protection relay for the installed current sensors. Configure starting and running curves from the manufacturer’s data, including retained thermal state and restart blocking where supported.
- Coordinate the relay with the short-circuit device and the starter control circuit. A motor-protection trip must de-energize the required contactors and block an immediate restart while the calculated thermal state remains above the permitted restart threshold.
- Check the line, star, and delta contactors against their measured currents and actual duty. Include the unusually long star-contactor conduction interval rather than applying a short, conventional wye-delta starting assumption.
- Perform a controlled functional test of current measurement, contactor state inputs, trip output, transition sequence, stall response, and restart inhibit. Record final settings, sensor ratios, test currents, and operating times.
Verification limits and transition checks
A successful start does not prove adequate protection. Verification requires two boundaries: the relay must remain stable through the worst acceptable start, and it must trip before the motor’s manufacturer-defined thermal or stall limit during an abnormal start. Test injection can verify relay logic without repeatedly heating the motor; use operational starts only within the permitted start frequency and thermal state.
Review the open transition separately. Confirm that the star contactor opens before the delta contactor closes and that the electrical and mechanical interlocks prevent simultaneous closure. Trend the current at transition because a large transient can indicate transition at an unsuitable speed, incorrect phase relationships, or a contactor sequencing problem. Set any transition or acceleration supervision from the measured process and motor limits rather than an assumed conventional start time.
After commissioning, trend start duration, peak and average phase current, voltage, transition behavior, thermal utilization, and time between starts. A rising start time at similar voltage points toward increasing load torque, mechanical drag, or process changes. Increasing current imbalance directs the investigation toward the supply, contacts, connections, or winding circuit.
Frequently asked questions
How do I protect a wye-delta motor during a 10-minute start?
Use a line-side motor-protection relay with starting and running thermal behavior configured from locked-rotor current, permissible stall time, thermal time constants, and the recorded 10-minute current trace. Retain separate short-circuit protection and make the relay trip all required starter contactors.
How do I know whether the existing delta overload protects the motor in wye?
Trace the overload or current-transformer conductors through both contactor states and identify whether they measure line current or winding current. In wye, line current equals winding current; in delta, line current equals √3 times winding current.
When should I stop testing and contact official motor support?
Stop when the motor’s permissible stall time, thermal curve, cooling duty, terminal arrangement, or hot-restart limit is unavailable, or when current stays high without increasing speed. Also stop if the measured start exceeds the manufacturer’s limit or the contactor duty cannot be verified. Escalate with the nameplate, terminal diagram, current-time record, start frequency, load description, and starter schematic to the motor manufacturer’s official support channel.