APPLICATION GUIDE
Overview
A standard motor starting circuit usually consists of three components: the circuit breaker is responsible for short-circuit protection and isolation, the contactor is responsible for on-off control, and the thermal relay is responsible for overload protection.
The three must be selected according to the motor power. Selecting any one alone may cause protection failure.
The most common problem in actual procurement is: the contactor is selected correctly according to the motor power, but the thermal relay setting range does not cover the rated current of the motor, causing the overload protection to fail to operate normally.
coordination steps
- Step 1: Confirm the rated power and current of the motor (look at the motor nameplate, do not use power inversion).
- Step 2: Select the contactor according to the AC-3 usage category - the AC-3 rated current of the contactor should be greater than or equal to the rated current of the motor.
- Step 3: Select the thermal relay - the set current range must cover the rated current of the motor, and the rated current of the motor should fall in the middle of the setting range, not the edge.
- Step 4: Select a circuit breaker - a motor protection circuit breaker (such as the GV series) can provide both short-circuit and overload protection; if an ordinary circuit breaker is used, an independent thermal relay needs to be retained.
- Step 5: Confirm that the coil voltage is consistent with the control loop, and confirm the required number of auxiliary contacts.
coordination reference
| Motor power (400V) | contactor reference | Thermal relay setting range reference |
|---|---|---|
| 2.2 kW | LC1D09 level | 5.5–8A range |
| 4 kW | LC1D09 / LC1D12 level | 7–10A range |
| 5.5 kW | LC1D12 / LC1D18 level | Grade 9–13A |
| 7.5 kW | LC1D18 level | 12–18A range |
| 11 kW | LC1D25 level | 16–24A range |
| 15 kW | LC1D32 level | 23–32A range |
Note: The above table shows common matching directions. Actual selection must be confirmed according to the actual nameplate current, starting method and ambient temperature of the motor, and is subject to official samples.
Frequently Asked Questions
Why can't contactors and thermal relays be directly matched based on the motor kW number?
The nameplate current of a motor with the same power will vary depending on voltage, efficiency, power factor and operating conditions. The contactor should be checked against the motor nameplate current according to the applicable usage category, and the setting range of the thermal relay must also cover the actual rated current.
After using the GV2 motor protection circuit breaker, is it necessary to install a thermal relay?
Not necessarily. Some motor protection circuit breakers themselves provide both short-circuit and overload protection, but whether an independent thermal relay is also needed depends on the protection architecture, coordination level and application requirements, which should be checked according to the manufacturer's coordination table and project design.
Does the contactor coil voltage depend on the main circuit or the control circuit?
Look at the control loop. Even if the main circuit of the motor is 380V, if the control power supply is 220VAC or 24VDC, the contactor coil should be selected according to the corresponding control voltage.
Can star-delta, soft start or variable frequency start also be directly applied to the matching table on this page?
No. The table is only used as a preliminary reference for common direct starting directions; the number of contactors, protection configurations and coordination methods of star-delta, soft start and frequency conversion solutions are different, and they need to be re-selected according to the specific starting architecture.
Need help confirming a part number or application?
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