3-PHASE ELECTRIC MOTOR CONTROL CIRCUIT

3-Phase Electric Motor Control Circuit

1. Power Circuit:

This is the main path that transmits power to drive the motor.

Power supply: The system uses a 440VAC - 3-phase supply (R, S, T) along with a protective earth (PE) wire.

Ensuring the PE wire is properly connected to the motor frame is a fundamental principle to prevent electrical leakage and protect human safety.

MCCB / Circuit Breaker (Q1): Protects the motor against short circuits and overloads.

Contactor (K1): Responsible for making and breaking the heavy current supplied to the motor upon receiving a command from the control circuit.

3-Phase Motor (M1): Converts electrical energy into mechanical energy.

2. Control Circuit:

The biggest plus point of this schematic lies here. Instead of using 220VAC directly for control, the designer used a conversion power supply (V1) to step down the voltage to 24VDC.

Why 24VDC? Because 24VDC is a safe voltage.

When the operator operates with wet hands or if there is an electrical leakage at the push buttons outside the panel cabinet, the risk of a dangerous electric shock is zero.

Control circuit protection: Uses a 1-phase circuit breaker (Q2) on the input side and a fuse (F1) on the 24V output side to protect all downstream logic components.

Safety Interlock (Q1 Interlock): Notice the auxiliary contact (13-14) of Q1 wired in series at the beginning of the circuit.

This means: If the main circuit breaker Q1 is not turned on (or trips due to a fault), the entire control circuit will be cut off and rendered useless. This is a very standard and robust design.

3. Detailed Operating Principles:

Standby state: Supply power, close Q1 and Q2. The 24VDC power is ready, flowing through F1, through the Q1 contact (already closed), passing through the stop buttons S1 and S2 (both Normally Closed - NC), and waiting at the Start button (S9) and the K1 contact.

When starting: Press the Start button S9 (Normally Open - NO), current flows through the coil (Bobine) A1-A2 of K1. Contactor K1 immediately pulls in, closing the power circuit for the motor to run.

Latching function: The moment K1 pulls in, the auxiliary contact K1 (13-14) in the control circuit also closes. At this point, even if you release the Start button S9, current still flows through the K1 (13-14) branch to keep the coil A1-A2 energized.

Status indication: Indicator light H1 lights up because it is wired in parallel with the coil, signaling that the motor is running.

When stopping: Press the stop button S2 (or emergency stop button S1). The circuit opens, coil K1 loses power, releasing the power contacts, and the motor stops rotating. The holding point K1 (13-14) opens, resetting the system to the standby state.

4. Practical Example & Equipment Selection

To help visualize, suppose we need to apply this schematic to install a control cabinet for a 15kW centrifugal exhaust fan, running on a 440V grid, power factor cos phi = 0.85.

Step 1: Calculate the rated current of the motor according to the formula: $I = P / \sqrt{3} / U / \cos(\phi)$

$I = 15000 / \sqrt{3} / 380 / 0.85 = 23.1 A$

Step 2: Select the equipment:

Select MCCB (Q1): Choose a rated current of about 2 - 2.5 times the motor rated current and based on the breaker characteristics type A, B, C, D, K. We can choose a 3-Phase MCCB - 50A.

Select Contactor (K1): Based on the AC-3 duty cycle, the load current of the Contactor must be >= the motor rated current multiplied by the safety factor. We choose a 40A Contactor.

Select power cable: With a current of 23.1A, you can use a copper cable >= 6.0 $mm^2$ (depending on the cable pull distance to ensure voltage drop).