Quick question, i don't fully understand contactors and was wondering if someone could give me a quick explanation,i get that they are used to help with the startup load of a circuit?? Also ive seen people wire them tons of diff ways ie putting L + N onto the coil and a few lighting circuits Lives going through the contactor.
Im a newly qualified spark and haven't had a great deal of experience with contactors so all help is welcome.
The contactor is also called a relay (according to size) form the main building blocks of any electrical system. In the old days there would be huge panels full of these devices and also timers and they would be wired to perform some quite complex tasks.
Today with the PLC (Programmable Logic control) the panels full of contactors and relays are things of the past.
Now the contactor is used in the main to control three phases from one small switch or button. They are still used in motor control to allow star/delta and even resistor or auto-transformer starting but are slowly giving way to soft start and inverter motor control.
With lighting they can allow rows of lights using multi protection devices all to be controlled with one small switch, timer or sensor. It is the use of timers in conjunction with contactors which reduces start load. This will allow a delay between each bank of lights although this is rare with lighting.
With motors either resistors, Auto-transformers or configuration of windings in star then delta allows the motor to start with reduced voltage to each winding. As the motor speed is increased the voltage to each windings is also increased. This may be simple two stage as with star / delta or multi stage as with resistor start often split into 6 stages. Each stage controlled by a timer to a contactor. Even pneumatic timers are expensive and as rule of thumb we considered 4 timers were about same cost as a PLC so where many motors are used it is common now to use PLC control even for star / delta starting. This also allows some extra control like holding in star for inching machines.
Normally motor control will also include some overload protection. There are two basic methods plus loads of specials. The standard overload has a small switch which controls supply to contactor. The overloads today are normally thermal. Years ago we had magnet overloads and to allow the motor to start there was a pot filled with oil with a piston and small hole and forcing piston through oil delayed the operation long enough to allow starting. Another type of thermal overload does the switching within the unit similar to a MCB and allow one to dispense with MCB.
However I can’t believe a “newly qualified spark” has not been taught all this at least in his college course even if not on the job. I will admit the first time I came across a resistor starter it did take some study of diagrams to work it all out. But many times when working on star/delta I have needed to re-wire mainly with items like pumps to take float control.
Also contactors are used for safety. Where irreversible damage to the body may result if something does not switch off it is required that everything is doubled up. So one main contactor will open and also the control will be dropped for all the small contactors. If some of the control contactors are semi-conductor type then often two main contactors are used.
How can any electrician work without that basic training? I bet after all this it’s a wind-up which I have fallen for?
