When a three-phase asynchronous motor starts, the current flowing through the electronic winding is very large, 4-7 times the rated current. For large-capacity motors, if a normal full-voltage starting method is used, the excessive starting current will cause a significant drop in the power supply voltage, which may affect the normal operation of other equipment using the same power supply.
The solution to the above problems is to promptly perform reduced-voltage starting on the three-phase asynchronous motor, and then provide full voltage after the motor is running. Generally, three-phase asynchronous motors with a power supply capacity of 180kV.A or higher and a motor capacity of 7kW or less can use direct full-voltage starting; those exceeding this range must use reduced-voltage starting. Furthermore, because the current flowing into the motor during reduced-voltage starting is smaller, and the torque generated by the motor is also smaller, reduced-voltage starting should be performed under light load or no-load conditions.
There are many types of reduced voltage start control circuits. The following only introduces the more common star-delta (Y-Δ) reduced voltage start control circuit.
1. Wiring method for star-delta reduced voltage starter
The junction box for a three-phase asynchronous motor has six terminals: U1, U2, V1, V2, W1, and W2, as shown in Figure 1. When terminals U2, V2, and W2 are connected together, the internal windings form a star connection, as shown in Figure 2. When terminals U1, W2, and U2 are connected in pairs with terminals V1, V1, and W2, the internal windings form a delta connection, as shown in Figure 3. If the voltage between any two phases of the three-phase power supply is 380V, when the motor windings are connected in a star configuration, the actual voltage on each winding is 380/220V.
When the motor windings are connected in a delta configuration, the voltage across each winding is 380V. Since the voltage decreases when the windings are connected in a star configuration, the current flowing through the windings also decreases (approximately 1/3 of that in a delta configuration).
Figure 1. Terminal box for a three-phase asynchronous motor
Figure 2
Figure 3
The star-delta reduced-voltage starting control circuit connects the motor windings in a star configuration during startup, and then switches them to a delta configuration after startup, allowing the motor to run at full voltage. When the motor windings are connected in a star configuration, the voltage across the windings is low, the current flowing through them is small, and therefore the torque generated is also small. Therefore, the star-delta reduced-voltage starting circuit is only suitable for light-load or no-load starting.
2. Star-delta step-down starting circuit
The star-delta step-down starting circuit is shown in Figure 4. This circuit uses a time relay for automatic switching control. The working principle of the star-delta step-down starting circuit is analyzed as follows:
Figure 4. Star-delta step-down start circuit
Figure 5. Physical wiring diagram of star-delta step-down starter circuit components.
① Close the power switch QS
②Star-type reduced voltage start control
Press the start button SB1 → both contactor KM3 coil and time relay KT coil are energized → KM3 main contacts close, KM3 normally open auxiliary contacts close, KM3 normally closed auxiliary contacts open → KM3 main contacts close, connecting the motor windings in a star configuration; KM3 normally closed auxiliary contacts open, cutting off the power supply to KM2 coil; KM3 normally open auxiliary contacts close, energizing KM1 coil → KM1 coil energized, causing both KM1 normally open auxiliary contacts and main contacts to close → KM1 normally open auxiliary contacts close, allowing KM1 coil to continue being energized after SB1 is opened; KM1 main contacts close, energizing motor terminals U1, V1, and W1, starting the motor in a star configuration.
③ Triangle-shaped normal operation control
After the time relay KT coil is energized for a period of time, its delayed normally closed contact opens → KM3 coil is de-energized → KM3 main contact opens, KM3 normally open auxiliary contact opens, KM3 normally closed auxiliary contact closes → KM3 main contact opens, canceling the star connection of the motor winding; KM3 normally closed auxiliary contact closes, energizing KM2 coil → KM2 coil is energized, causing KM2 normally closed auxiliary contact to open and KM2 main contact to close → KM2 normally closed auxiliary contact opens, de-energizing KT coil; KM2 main contact closes, connecting the motor winding in a delta configuration, and the motor operates normally in a delta connection.
④ Stop control
Pressing the stop button SB2 → KM1, KM2, and KM3 coils are all de-energized → KM1, KM2, and KM3 main contacts are all open → The motor stops because the power supply is cut off.
⑤ Disconnect the power switch QS
【Note】When connecting cables on site, be sure to remove the jumper clips from all terminals inside the motor junction box!
Star-delta starting circuits are a circuit that electricians must master, and instrument technicians also frequently encounter them during on-site maintenance. Therefore, instrument technicians must be familiar with and master the principles, wiring, and troubleshooting of this circuit. Having an extra skill is always beneficial to everyone's work.
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