If the motor jumps or turns abruptly at startup, check the STC’s two settings first: Initial Torque and Duration of Ramp. The controller limits voltage during startup to reduce torque and current surges, but it cannot compensate for an incorrect model, unsuitable motor application, or a ramp that does not match the load.
Check the STC model against the motor phases
- Read the controller’s complete catalog number and the motor nameplate phase information.
- Compare the controller model and its wiring diagram with the motor and starter circuit. STC versions are available for single-phase motors and for three-phase motors with one or two control phases; do not treat those arrangements as interchangeable.
- If the phase arrangement or wiring does not match the applicable documentation, stop adjusting settings. Correct the model or wiring decision before testing a start.
A phase mismatch is not a ramp-setting problem. Changing Initial Torque or Duration of Ramp will not make an incompatible controller arrangement suitable for the motor.
Observe the start before changing a setting
Run a controlled start and note whether the motor starts abruptly, accelerates too slowly, or fails to start. Record the existing Initial Torque setting and Duration of Ramp before making a change. Keep the same load conditions for comparisons; otherwise, a changed load can look like an improvement or deterioration in the controller setting.
| Observed result | What to check next |
|---|---|
| Abrupt initial movement or torque surge | Check whether Initial Torque is sufficient for the load without being higher than needed. |
| Motor starts, but acceleration feels abrupt | Check Duration of Ramp and compare behavior after a small, recorded adjustment. |
| Motor does not begin turning | Check model/application match and Initial Torque before extending the ramp. |
| Behavior varies between starts | Compare the load and starting conditions before attributing the change to the STC. |
Set Initial Torque before extending the ramp
Initial Torque sets the starting torque level; the available adjustment range is 10–80%. Duration of Ramp sets the ramp time; the available range is 0.5–5 seconds. These are the two stated commissioning adjustments. The controller reduces startup stress by limiting voltage, so too little initial torque can leave the motor unable to begin accelerating, while an unnecessarily high starting level can preserve more of the torque surge you are trying to reduce.
- With the motor stopped, record both original settings.
- Adjust Initial Torque within 10–80% to obtain reliable movement from the load. Change one setting at a time.
- Once the motor begins moving reliably, adjust Duration of Ramp within 0.5–5 seconds to shape acceleration.
- Test each change under the same operating conditions and record the result. Follow the controller’s applicable manual for adjustment and wiring details.
Do not begin by setting the ramp to its longest value. If the starting torque is inadequate, a longer ramp does not address the reason the motor fails to start. Likewise, raising Initial Torque to mask a phase, wiring, or load mismatch wastes time and may increase the startup surge.
Verify acceleration and repeatability
After tuning, make repeated starts under representative load conditions. Confirm that the motor begins turning reliably, accelerates without the original abrupt movement, and behaves consistently. Recheck the recorded settings if results change; also compare the load and starting conditions rather than assuming the adjustment has shifted.
The STC is a basic reduced-voltage starter for low-horsepower single- and three-phase squirrel-cage induction motors, not a general remedy for every startup problem. If the motor still will not start or accelerates unpredictably after confirming the model, phase arrangement, wiring, and settings, stop repeated adjustment and use the controller documentation and official manufacturer support to resolve the application.
Frequently asked questions
How do I adjust an Allen-Bradley STC for a smoother start?
Record both settings, then adjust Initial Torque (10–80%) and Duration of Ramp (0.5–5 seconds) one at a time. Test under the same load conditions after each change.
How do I choose the STC Initial Torque setting?
Set enough initial torque for the motor and load to begin moving reliably, then tune the ramp. The stated adjustment range is 10–80%; use the applicable product manual for setup details.
How do I choose the STC ramp time?
Adjust Duration of Ramp within 0.5–5 seconds after confirming reliable initial movement. Compare starts under consistent load conditions rather than selecting the longest setting by default.
Can an STC for a single-phase motor run a three-phase motor?
Do not assume so. STC versions include single-phase models and three-phase arrangements with one or two control phases; match the controller catalog number and wiring diagram to the motor.
What should I check if the motor will not start with an STC?
First verify the controller’s phase arrangement and wiring against the motor and applicable documentation. Then check whether Initial Torque is sufficient before changing Duration of Ramp.