Magnetic Starters 101

• The Normal State
• What does the start button do
• The normally open switch on the contactor
• How the thermal overload works
• Preventing catastrophic failure, overload reset
• How the stop button works on a magnetic starter

The magnetic starter is a very simple device that seems to magically control the power to a machine such as a tablesaw, shaper, or jointer. Here we will explore the inner workings of a magnetic starter and examine how this electrical device works. There are many ways to wire a magnetic starter, but we will examine the most common wiring diagram of a simple single motor hard start magnetic starter.

Above, we show a simple magnetic starter diagram. This is a two wire power source that will power a 2 wire motor. Normally, this will be used in an application that requires 220 volts to power a motor. The inputs are L1 and L2. Keep in mind that the state of L1 and L2 never change, they are always "hot".

NOTE: The dots are where wires are connected. If there is no dot where wires cross, there is no connection.

The Normal State

This is what the starter looks like in it's normal state. That is, no buttons are being pressed, and power is being supplied to it from the power source.

As seen above, L1 and L2 are "hot" supplied with voltage. With a magnetic starter, the objective is to "pull-down" the contactor so that ultimately the voltage will travel across the contactor, throught the thermal overload (or breaker) and to the motor, therefore turning the motor on. The way that the contactor accomplishes this is by energizing a magnet (coil magnet) and pulling-down or closing the connections across the poles, therefore supplying voltage to the motor.

In the picture shown above, we can see that one half of the magnet is hot, but the other half of the magnet is not. What we need to do to energize the coil is close the route to the other side of the magnet.

So what do we do to close the connection? We can press the start button.

The Start Button

As the picture shows, when the start button is pushed, voltage flows through the start switch, then the overload, and finally the normally closed stop switch. This supplies voltage to both sides of the coil and will pull-down the contactor. Keep in mind that the start switch is normally-open and in it's unpressed state will open the circuit, and not let voltage run through.

The normally open contactor

Now, even though the start button is no longer pressed, the coil is still energized. Why? Because the contactor itself is making the closing the loop. Voltage is flowing from L1 down to the normally open connection on the contactor, through the overload, through the stop switch (which is normally closed), to the coil magnet. Since the coil magnet is energized, the contacts on the contactor is closed and will remain closed until the magnet is de-energized. This is called a normally open connection on a contactor.

The coil magnet can be de-energized by a pair of actions. If you were to break (or open) the circuit that is energizing the coil, the contacts on the contactor would open, therefore permanently breaking the voltage that was being applied to the coil magnet. Notice there are only two ways to energize the coil, 1. press the start button, which closes the circuit and 2. close the contactor which closes the circuit.

There are two possible breaking points in the coil magnet circuit, the thermal overload which is a fail safe when too many amps are drawn through the power connections, and the stop button, which can be pressed when the operator wants to stop the motor.

Tripping the thermal overload

The thermal overload is open, therefore there is no voltage that can make it to the coil magnet. The magnet will de-energize and the contactors will open. The instant that the thermal overload is 'tripped' or opened, the starter will return to it's normal state.

The tripped thermal overload

Unlike normally open or normally closed switches, a thermal overload is either open or closed. In it's factory state, it is closed, and it will only open if an overdraw on the amperage exceeds the threshold of amp draw. If the thermal overload is tripped from an overdraw, the only way to close it is to press the reset button. So if the overload is tripped, you cannnot start the motor back by simply pressing the start button again, because the thermal overload switch remains open until someone investigates the problem the overload and decides to press the reset button.

The stop button

The stop button has been pressed, and it opens the circuit, therefore there is no volate that can make it to the coil magnet. Again, the magnet will de-energize and the contactors will open. The instant that stop button is pressed, the starter circuit will return to it's normal state.

After the stop button or thermal overload is opened, the starter circuit will return to it's normal state. Unlike the situation with the thermal overload, the stop button returns to the closed state when the button is released. Therefore it allows the start button to reactivate the starter circuit again by pressing the start button.

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Questions / Responses

portobello: Q: My contactor is not situated exactly like the one shown, are there other ways to wire the starter?

A: Actually yes, we picked the most common wiring scenario, in some cases, some of the buttons or overload may be moved to the L2 side. In addition, there may be other safety devices installed in the system such as door interlocks. The main thing to focus on is that the start button "jumps" voltage to one side of the coil, and the stop buttons, overloads, and safety devices break the connection to one side of the coil.

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Magnetic Component Examples

Here we show a typical magnetic starter with the cover open. This is a typical starter that we sell. This wiring diagram differs a bit from the examples shown on this page. Here the starter button is on the L1 side and the stop button and overload breaks the L2 side of the coil.

This is a typical magnetic contactor. This has 3 power lines and one normally open switch. Typically, this would be the contactor used in single and three phase operation.

This is a typical thermal overload. It has 3 power lines that are monitored for amp draw, if the amperage exceeds the threshold set on the dial, the circuit will open. This magnetic starter also has a built in normally closed stop switch.

In a normal magnetic starter configuration, the thermal overload uses the normally closed state. This starter is multi function and also has a normally open connection too.

This is a coil that has been removed from the starter. Coils are made for different voltages, such as 110v, 220v, and 440v. The different voltages on the coil is referred to as the "control voltage".

Other contactors

Above: Normally closed contactor, typically these are used in soft start applications.

Above: Contactor with normally closed and normally open connections. Typically, these are used with an indicator light to show power to a specific motor. In most cases a light will be wired to the normally open connection and when the contactor is pulled down, it will illuminate the light.

Above: A soft start magnetic starter. This starter box differs from the typical magnetic starter in that is uses contactors in series and parrallel to alter the actual motor winding paths to "soft start" the motor. Typically, this reduces horsepower initially to get a motor to optimal RPM and then switches over to full power after a specific time period. These are normally used in larger motors.

 

NOTE: Information provided on this page is informational only. Do not attempt to modify or troubleshoot electrical devices on your own unless you are a qualified electrician.