How to Wire a Motor Starter
The Technical Support page on the AutomationDirect website is full of valuable information and is available 24/7. The following was referenced from the Technical and Application Notes section.
A motor starter is a combination of devices used to start, run, and stop an induction motor based on commands from an operator or a controller. In North America, an induction motor will typically operate at 230V or 460V, 3-phase, 60 Hz. A control voltage of 115 VAC or 24 VDC is normally used to actuate the motor starter. Several other combinations are possible and are easily derived from the methods shown in this article.
The motor starter consists of (at least) two components: a contactor to open or close the flow of energy to the motor, and an overload relay to protect the motor against thermal overload. Other devices for disconnecting and short-circuit protection may be needed, typically a circuit breaker or fuses. Short-circuit protection will not be shown in the examples that follow.
The contactor is a 3-pole electromechanical switch whose contacts are closed by applying voltage to a coil. When the coil is energized, the contacts are closed, and remain closed, until the coil is de-energized. The contactor is specifically designed for motor control, but can be used for other purposes such as resistive and lighting loads. Since a motor has inductance, the breaking of the current is more difficult so the contactor has both a horsepower and current rating that needs to be adhered to.
The overload relay is a device that has three current sensing elements and protects the motor from an overcurrent. Each phase going from the contactor to the motor passes through an overload relay current-sensing element. The overload relay has a selectable current setting based on the full load amp rating of the motor. If the overload current exceeds the setting of the relay for a sufficient length of time, a set of contacts opens to protect the motor from damage.
This article shows how to wire various motors using the FUJI series of contactors and overload relays sold by AutomationDirect. Other brands of contactors and overloads may be wired the same or similarly. Consult the manufacturer’s wiring diagrams for other brands.
There are four basic wiring methods:
a) Full-voltage non-reversing 3-phase motors.
b) Full-voltage reversing 3-phase motors
c) Single-phase motors
d) Wye-delta open transition 3-phase motors
You must supply a disconnect switch, proper sized wire, enclosures, terminal blocks and any other devices needed to complete your circuit.
WARNING! Use the instructions supplied for each specific device. Failure to do so may result in electrical shock or damage.
Here are representative images of the components used in the folowing schematics:
Full-voltage non-reversing 3-phase motors
The following diagram depicts 3-phase non-reversing motor control with 24 VDC control voltage and manual operation. We will use a contactor, an auxiliary contact block, an overload relay, a normally open start pushbutton, a normally closed stop pushbutton, and a power supply with a fuse. The start and stop circuits can also be controlled using PLC inputs and outputs.
Full-voltage reversing 3-phase motors
This diagram is for 3-phase reversing motor control with 24 VDC control voltage. It uses two contactors, two auxiliary contact blocks, an overload relay, a mechanical interlock, two normally open start pushbuttons, a normally closed stop pushbutton, and a power supply with a fuse. The forward, reverse, and stop circuits can also be controlled using PLC inputs and outputs.
Full-voltage single-phase motors
This diagram is for single-phase motor control. It uses a contactor, an overload relay, one auxiliary contact block, a normally open start pushbutton, a normally closed stop pushbutton, and a power supply with a fuse. The start and stop circuits can also be controlled using PLC inputs and outputs.
Wye-delta open transition 3-phase motors
The following diagram is shown for 3-phase motor control of a delta-star connection. It uses three contactors, an overload relay, one auxiliary contact block, a normally open start pushbutton, a normally closed stop pushbutton, an on delay timer of 0-20 seconds and a power supply with a fuse. The start, stop, and timing circuits can also be controlled using PLC inputs and outputs.
By Keri Schieber,
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