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Three Phase Control Circuits
Large motors require a circuit to turn on and off. This may be as simple as a
single on/off drum-type switch or as elaborate as a VFD unit.
Typical three-phase motors use a large amount of current at a larger
voltage than most other motors. In these situations, it makes sense to
design control systems that isolate the operator as far from the dangerous
voltages as possible. This isolation may be in the form of a digital variable
frequency drive (VFD) or soft starter, but it may even be far more basic—
using relays and contactors.
Three Phase Control Circuits using Contactors
As learned in your previous course Electrical workshop 1, a contactor is a
device that is for making and breaking an electrical power circuit. For
example, we usually use a contactor for turning on and off an electrical
motor.
Fig 1.1: A Contactor
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Example 1: Draw and build a “Forward/Reverse” motor control circuit.
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Power Circuit
Control Circuit
Example 2: Draw and build a “Star – Delta” circuit.
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Power Circuit
Control Circuit
Example 3: In a soda bottles production factory, design a control circuit to control
the capping station as follows:
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When the start push button is pressed, the conveyer belt K1 starts moving till the bottle
hits limit switch LS1.
When LS1 hits the bottle, the conveyer belt K1 stops for 5 seconds and contactor K2
energizes to apply the cap on the bottle, so after the 5 seconds have finished, the cap
contactor K2 stops working and the conveyer belt 1 starts working again.
An emergency push button is used to stop the production line at any time.
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Control Circuit
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