Does the motor trip as soon as it starts? Don't rush to replace the circuit breaker! Perhaps the 'Star Triangle Start' has been wired incorrectly

2025-09-09 13:55:06

A 55kW water pump motor in a certain factory tripped with a loud bang when it started for the first time after major repairs. The electrician replaced the circuit breaker with a new one, but the result is still the same. After tinkering for a long time, it was discovered that the problem was not with the circuit breaker at all, but rather with a seemingly loose wire in the star delta starter box, causing a phase to phase short circuit when the motor started.

Why does the star delta starter trip when connected to the wrong wire?

To understand this issue, we need to first understand the principle of the White Star Triangle startup:

-Star start (Y): Reduce voltage, decrease starting current (only 1/3 of the delta connection), like starting a car in 1st gear, smooth but powerless.

-Triangle operation (Δ): Full voltage operation, providing full torque, like a car shifting into third gear and accelerating at full speed.

The consequences of connecting the wrong wire: If the motor winding is not switched from "star" to "triangle" in the correct order at the moment of starting, it will result in:

1. Phase to phase short circuit: A huge current directly impacts the system, and the circuit breaker immediately activates.

2. Torque shock: The motor vibrates violently, which may damage the load equipment.

The 3 most common wiring errors and on-site troubleshooting techniques

The following is a list of the culprits that caused the trip. You can investigate them one by one in this order:

Error 1: Improper setting of time relay (most common!)

-Fault phenomenon: The motor starts normally in a "star" configuration, but trips when switched to a "triangle" configuration.

-Reason: The switching time is too short (the motor has not yet accelerated to 70% -80% of the rated speed), which is equivalent to "low-speed high gear", and the instantaneous current is too high.

-Troubleshooting method:

-Check settings: Check the delay setting value of the time relay (KT). Simple calculation formula: Start time (s) ≈ power (kW) ÷ 10 ± 2 (for example, for a 55kW motor, the start time can be set to 5-7 seconds).

-Listen carefully to the sound of the motor, and wait until the sound of the motor running becomes stable and the speed increases before hearing the "click" sound of the switch. If the switching sound is too early and accompanied by a dull "wu" sound, it is because the time is too short.

Error 2: Welding or mechanical jamming of the main contactor contacts

-Fault phenomenon: The circuit breaker trips before pressing the start button after power transmission; The star shaped contactor (KM1) is not completely disconnected, and the triangular contactor (KM2) is pulled in, causing a power short circuit.

-Reason: The contactor contacts are fused together due to frequent start stop or overload, and cannot be disconnected.

-Troubleshooting method:

1. Power outage: Ensure complete power outage!

2. Manual pressing: Use your hand or screwdriver to press the mechanical mechanism of the contactor, simulate suction and disconnection, and feel whether it is smooth.

3. Use a multimeter to measure continuity: In the event of a power outage, measure the resistance between the upper and lower contacts of the contactor using the multimeter's resistance range. Under normal conditions, it should be infinite when not fully engaged. If the resistance is measured to be very low, it indicates that the contact has been welded and the contactor must be replaced.

Error 3: Wrong phase sequence connection of motor winding

-Fault phenomenon: Abnormal sound (shaking, weakness) when the motor starts in a star shape, and violent vibration and tripping when switching to a triangle shape.

-Reason: One or more of the six wires from the starter box to the motor were connected in the wrong position, resulting in a chaotic phase sequence.

-Troubleshooting method:

1. Refer to the drawings: Find the original star delta startup schematic and wiring diagram.

2. Line by line calibration: Use a multimeter or a shaking table with on-off settings, starting from one end of the motor junction box, to check the numbers and colors of the 6 cables one by one, ensuring that they correspond one-to-one with the Terminals of the starter box.

3. Remember the mnemonic: "Connecting the ends is a triangle, and connecting the ends is a star. Ensure that U2, V2, and W2 are short circuited during star startup.

How to prevent it? 3 things that must be done in operation and maintenance

1. Regular tightening: Tighten all Terminal screws in the starter box once a year during power outages. Vibration and thermal expansion and contraction can cause loose wiring, which is the cheapest preventive measure.

2. Regular replacement: For motors that start and stop frequently, proactively replace the time relay and main contactor every 3-5 years, which is much cheaper than the losses caused by an unexpected shutdown.

3. Make good markings: After debugging the motor and starter box, use a permanent marker to make clear phase sequence markings on both ends of the cable (such as U1, V1, W1, U2, V2, W2), which will be clear at a glance during the next maintenance.

summary

When the motor starts and trips again, don't blame the circuit breaker immediately. Think in the following order:

1. First ask: Does it jump when starting with a "star" or when switching to a "triangle"?

2. Listen again: Check if the sound of the motor and the sound of the contactor closing are clear and rhythmic.

3. Follow the sequence of "time → contactor → wiring" and investigate one by one.

Remember that 90% of star delta startup failures occur in the small secondary components in the control box, rather than the huge primary equipment. Careful inspection and maintenance are the key to ensuring stable operation.