Electrical thermal current rating (Ith) for contactors

What is a contactor?

Contactors, also known as motor starters, are electro-mechanical controlled switches, that control other electrical loads on and off manually or automatically, by applying a control voltage to its coil.

Contactors are controlled by  a lower voltage level than the switched circuit. Such as a 48 VDC coil contactor  controlling a 380 VAC motor.

Contactors are designed to be connected directly to high current electrical loads such as three phase induction motors, thermal loads and large industrial three phase loads.

Contactor main function

Contactor main function is to switch loads, like induction motors and lighting, on and off automatically or manually as the application requires.

Contactors need a control circuit to do its function properly. The simplest control circuit of a contactor contains a start/stop switch and a fuse for protection. 

Contactors can be a part of a complicated control circuit that is run automatically by means of sensors, programmable logic control units and smart control systems. But still the main contactor function is switching loads on and off, no matter what these loads are.

What is thermal current rating (Ith) for contactors?

It means the rated current of this contactor which can switch for thermal Load category AC1 is = Ith. Contactors have two current values, One is the current of thermal loads (AC1) which are loads that transfer electrical current into thermal energy. i.e all types of heaters.The other current value of the contactor (AC3) is all other inductive loads like induction motors.

This means that contactor current depends on load utilization category.

  • AC1 is the Contactor standard utilization category for resistive loads.
  • AC3 is utilization category of inductive loads.

What do contactor AC1 and AC3 ? Contactor standard utilization categories.

Contactor rated current is categorized according to the load type. Contactor standard utilization categories define the current making and breaking values, which depends on the load type to be controlled.

  • AC-1 category : all AC loads with a power factor of 0.95 : 1, or resistive loads, like heaters or lighting loads. Those loads requires no high starting current.
  • AC-3 category : applies to inductive loads like induction motors, where breaking occurs during the motor running. When contactor starting AC3 load, it makes the inrush current (5 to 7 times the rated current). While when contactor opening the load it breaks the rated full load motor current.

In other words: AC-3 category means contactor may switch highly inductive loads such as three phase induction motors which is very difficult compared to resistive loads like heaters. Resistive load (AC1) ratings have a lower “inrush current” needs, and are mainly based only on thermal current carrying capacity of conductive parts of the contactor.

Inductive load ratings must consider the effects of heating of motor starting current and heat dissipation.

Example of contactor ith:

 A contactor of 9 Amp. on AC3, And its thermal current (Ith = 20 to 25 A) (this means it can switch up to 20 or 25 A for loads of AC1 (thermal load).N.B: Contactor thermal current Ith is greater than I of loads with AC3 category.

I have written a detailed article about “Answering contactor important question” you can find it here.

What is contactor structure?

A contactor has three main parts, coil, moving and fixed parts and the contactor frame.

  1. Moving and fixed contacts which are the current-carrying parts of the contactor, This includes both power and auxiliary contacts. The power contacts are  a three phase contacts to connect loads power wires.
  2. The coil (or “electromagnet“) provides the driving force which moves and closes the contacts. Its voltage usually is lower than the load voltage.
  3. The enclosure is a frame body for the contacts and the coil.

Contactor NC and NO contacts

Contactors have two contacts types:

  • Normally open (NO) contacts which allow current in case the contactor control voltage is applied or the contactor is energized.
  • Normally closed (NC) contacts in the opposite of NO contacts , allow current in case the contactor control voltage is not applied or the contactor is de-energized.

Is contactor normally opened or normally closed?

Contactors are normally opened contacts by default, however they may have auxiliary contacts normally open and/or normally closed to help in the control circuit.

The main contactor 3 contacts are normally opened, they change to closed when the contactor is energized. This normally opened approach helps the contactor to switch off the load in case of overload issue. The overload disconnects the control voltage of the contactor coil so the contactor switches off the load.


Contactor principle of operation

Magnetic field is produced when current passes through the electromagnet  coil, which makes the moving contacts of the contactor to attract.

The force developed by the coil holds the moving contacts and the fixed contacts together.

When the control voltage of the contactor is switched off, a spring returns the core to its normal position and the contacts are opened.

Is there a single phase contactor?

Yes, there are single phase contactors with a coil, moving parts and fixed parts. Single phase contactors are suitable for single phase loads with light current like lighting loads and small single phase motor.

What is the difference between single-phase and three-phase contactor?

The difference between single-phase and three-phase contactor is the number of poles each of them has and the type of load they can feed. Three phase contactors can feed AC1 & AC 3 There is no single phase contactor for AC3 loads.

Can a 3 phase contactor be used for single phase?

Yes, you can use a three phase contactor to control a single phase load by connecting the power terminals through outer two poles of the contactor L1 and L3. Connect L3 to the load load through the overload terminal T3. Then reconnect the third middle pole of the contactor L2 from the over load first leg T1. Connect the motor to the overload T2. By this way the motor current will pass through the three legs of the overload. Below is a simple wiring diagram.

using 3 phase contactor as a single phase



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