Wiring in a single direction of the motor - Solutions - Huaqiang Electronic Network

Circuit controlled by a knife switch as main protection

Circuit protected by air switch as main circuit

The working process starts when the start button SB2 is pressed, energizing the contactor KM coil. This causes the main contacts to close, allowing the motor to run. At the same time, the auxiliary normally open contact closes, creating a self-locking mechanism. The power flows through FU1 → SB1 normally closed → KM normally open contact → contactor coil → FU2. When SB2 is released, the KM remains energized. When the stop button SB1 is pressed, the normally closed contact opens, de-energizing the KM coil. This causes the main and auxiliary contacts to open, stopping the motor. The thermal relay FR is used for overload protection. If the motor becomes overloaded or experiences excessive current due to a fault, the bimetallic strip in the thermal relay heats up, opening the FR normally closed contact. This de-energizes the KM, stopping the motor.

Each component has a specific function, and it's important to select appropriate electrical devices based on the motor’s capacity.

Rated current of the switch (QS or QF):

(1) The knife switch is used to turn the power on and off. It should be selected with a rating three times the motor’s rated current.

(2) The air switch provides short-circuit protection and can be chosen to match or slightly exceed the motor’s rated current.

The fuse (FU) acts as a short-circuit protection device:

It should be selected within the range of 1.5 to 2.5 times the motor’s rated current.

The AC contactor (KM) controls the motor and provides voltage loss protection:

The contactor’s rated current should be between 1.3 and 2 times the motor’s rated current.

Thermal relay (FR) for motor overcurrent protection:

The thermal element’s rated current should be between 1 and 1.5 times the motor’s rated current. The thermal relay itself must have a rating equal to or higher than the thermal element.

The setting value for the thermal relay is typically equal to the motor’s rated current.

Thermal relay adjustment:

There are two types of reset modes: automatic and manual. Automatic reset is set at the factory. To use manual reset, insert a small screwdriver into the adjustment hole and turn the buttons counterclockwise.

After an overload, the thermal relay can be automatically reset within 5 minutes. In manual mode, the reset button must be pressed after 2 minutes.

Can the thermal relay’s reset mode be changed arbitrarily? Why?

No, the thermal relay’s reset mode should not be changed arbitrarily. Changing it may affect the restart requirements of the device. For example, switching from manual to automatic reset could cause startup errors.

Main circuit wiring: Select wires according to the wire selection port.

Control loop wiring: Use insulated copper wire with a cross-sectional area of at least 1.5 mm².

Control loop fuse (FU2): 5A or 10A fuses can be used, but 1.5A fuses are also acceptable.

The wires are protected by the air switch in the main circuit.

Example: Wiring a 7.5kW motor in one direction using various electrical components and wires:

1. The rated current of the 7.5kW motor is 15A.

2. Switch: HQ-60/3 rubber cover blade switch or HK-60/3 iron shell switch can be used.

3. Main circuit fuse: RClA-30/30 ceramic plug-in fuse or RL1-60/30 spiral fuse can be used.

4. AC contactor: B25, CJ20-25, or CJ10-20 can be used.

5. Thermal relay: JR16-26/3D can be selected, with a thermal element rated current of 22A or 16A, set to 15A.

6. Control loop fuse: RClA-5/3 or RL1-15/2 can be used.