Contactors and overloads are electrical components that are heavily utilised in motor control installations. A contactor can be used to switch power to a motor. An overload is used to sense if a motor is drawing too much current and thus being overloaded. These devices can be used in conjunction with each other to provide switching and protection in the same motor control circuit.
Contactor
A basic contactor has a coil which when energised closes a set of contacts (usually 3), when these contacts are closed they can deliver 3 phase power to a 3 phase motor. When the internal coil of the contactor is de-energised the contacts which are spring loaded open and stop power from being delivered to the motor. But what happens if the motor draws to much current? This is when the overload comes into play.
Overload
An overload can be connected to the contactor and it can sense when too much current is running through it. When it senses that there is an overload situation it opens a fault contact, but this contact is not part of the motor circuit directly and it does not itself stop power from getting to the motor. Instead what can be done is we can use this fault contact in the control circuit of the installation.
How They Work Together
So how does the overload help in a motor overload situation? Well we can wire the circuit that is responsible for energising the coil of the contactor through the fault contact of the overload first. That way if there is an overload situation and the overload opens its fault contact, control power will stop from getting to the contactor coil, it will de-energise the coil and the contactor contacts will open which will then stop power from getting to the motor. Preventing any damage to the installation.
To Recap
Contactors and overloads can provide switching and protection to to an electric motor installation. Contactors switch power to a motor and overloads can sense if a motor is drawing too much current.
Learn More
Fundamentals of Electrical Controls
3 Phase Motor Control Bootcamp
Learn Variable Frequency Drives