It is an electromagnetic mechanical switching device used in the power circuit to switch on and switch off the power to the electrical machine.
i.e. a switch that is operated by an electromagnetic phenomenon.
To put it simply Normal
piano key switch installed in your room switches on the supply when you press it from
bottom half and switches off when you press it from the top half.
An
electromagnetic mechanical switch operates when you supply electricity
to it and switches off when the electrical supply stops .
How a contactor actually works : A contactor consists of a coil which when charged attracts a core
downwards which is mechanically connected to three mutually insulated
strips that makes or break the 3 power contacts of the contactor.
In addition to the power contacts contactors may also have auxiliary NO and NC contacts that operate along with the contactor power contacts i.e. they are mechanically coupled to the power contacts of the contactor. the auxiliary contacts are used in the control circuit of the contactor for various applications.
the contactors are differentiated according to the rated voltage of the contactor coil .
it can be
24V DC
24V AC 50/60Hz
110V DC
110V AC 50/60Hz
220V AC 50/60Hz
Complete imagery of a dismantled contactor.
This is not the standard construction of a power contactor that all manufacturers follow, different manufacturers have there own unique i.e. "patented" way of making a contactor. but the basic components remains the same.
As the name suggests if an electrical machine (often a motor) is connected to supply directly with no provisions to limit the starting current drawn by the motor , the starting method is called the direct online starter, DOL starter is used for induction motors of power ratings up-to 5hp. the simplified form of most commonly used DOL starter circuit has the following components :
a normally open (NO) push button type switch.
a normally closed (NC) push button type switch.
A Contactor (follow the link for contactor info) with auxiliary NO switch.
The electrical drawing used worldwide is called the single line diagram , but before making our DOL circuit in single line diagram we will draw the whole schematic like an artist rather than the engineer , i.e. we will draw the whole step by step image of what the circuit would look like before converting it to proper electrical drawing .
1. Supply the contactor coil with rated volts to make it operate , the circuit would look like Now notice that the coil is supplied supply directly without any switch so the contactor is activated , with
the power contacts in closed state.
auxiliary NC open and Auxiliary NO closed.
2. Next, we add one NO push button switch in series with the coil supply circuit.
Notice the contactor operates when push button is pressed and returns to normal state when push button is not in pressed state. but we need the contactor to remain in the activated state even when push button is released. for that to happen we need a second NO contact which operates only when the contactor operates , that NO is placed in shunt with the NO push button so that
when we press the NO push button the contactor gets activated and so does the second NO contact, so now there are two paths for current to get to the coil and so on.
when the NO push button is released the second NO is still activated so the contactor is activated too.
Notice that this purpose can be served by the auxiliary NO contact.
the modified circuit would look like
STEP 1
STEP 2 STEP 3 This circuit is still missing one key component i.e. a provision to stop the machine manually . for that , we need to cut the supply to the coil that is achieved by adding one NC push button in series before the NO push button .
With power connections the circuit would look like:
This is the basics of DOL starter used in industries . But in actual applications other components for indications and protections like overload, over and under voltage , over temperature are installed .
the detailed circuitry and single line diagram of that can be viewed following the link bellow.
Direct online starter with complete protections installed.(link offline for now, will update soon!)
If you are an electrical maintenance engineer in any industry, be it chemical , steel or textile industry all you need to do is make the electrical machine run. yes that's it ! all other parts of the circuit are to make sure that the motor runs and stops according to the norms of the production.
So the basic idea is
electrical machine (often a motor) should start and stop when required .
The whole electrical circuit should be automated.
To achieve these two functions the electrical circuit consists of two component.
Power circuit.
Control circuit.
power circuit consists of :
A protection device called circuit breaker to switch off the power under fault conditions.
A Switching device to switch on and switch of the power as required under normal conditions.
The electrical machine to be operated .
Control circuit consists of the components to control the Switching device installed in the power circuit.
the whole function of the control circuit is to switch on and off this device under various normal operating conditions or fault conditions
The Switching device:
The switching device used in the power circuit is an electromagnetic mechanical switch, called a Contactor i.e. a switch that is operated by an electromagnetic phenomenon . to put it simply
Normal piano key switch installed in your room operates when you press it from bottom half and switches off when you press it from the top half. An electromagnetic mechanical switch operates when you supply electricity to it and switches off when the electrical supply stops .
Now we should directly delve into the most basic electrical control circuit called the Direct on line starter or the DOL starter, because learning about all the component even before they are used can be over whelming.
