How to Calculate AC Contactor Current Rating for Motor?

The AC contactor has found wide applications in the motor-controlled circuit. It is no exaggeration to say that the electric drive of most motors cannot be separated from the control of the AC contactor. Many electricians are very familiar with the AC contactor for their frequency use or maintenance of it. How should we calculate the AC contactor rating for the motor or other equipment and make a right choice? Let ATO Assistant take a look at it.

First, let's make a basic understanding of AC contactors.

• AC contactor coil
  The coil is usually marked by A1 and A2. Generally, there are two kinds of contactors, namely AC contactors and DC contactors. The AC contactor is the one that we use frequently.

• Main contact of the AC contactor
  L1-L2-L3 is connected with the three-phase power source incoming line; T1-T2-T3 is connected with the power source outcoming line, and can be used to get connected with the load line. The main contact of the AC contactor is all normally-open contact, which is mainly connected with the main circuit to control the start and stop of motors or other equipment.

• Auxiliary contact of the AC contactor
  The auxiliary contact can be divided into the normally-open (NO) contact and the normally-closed (NC) contact. Under general conditions, the NO contact is used for self-locking control of the contact and for transmission of operation signals. For example, the NO contact of the AC contactor is connected with a red indicator as the operation indicator of the motor. When the AC contactor is connected with the power, the NO contact turns off, and the indicator is connected to transmit operation signals of the motor or the circuit.

As shown in the below table, the rated power of the motor, working current and AC contactor, the three are one-to-one correspondence. For example, the working current of a 1.5kw motor is around 30a. We can use the cjx2-4010 AC contactor.

Next, let us take a look at the calculating method.

First, we need to calculate the working current according to the power of the motor. Here, two methods are provided.
1. Caculate the working current via the formula.

2. Estimating the working current directly.
Generally, it is 2A/kW for a 3-phase motor, and 4.5A/kW for single phase equipment.

It must be pointed out that the current of the AC contactor to be selected should be 1.5 to 2 times of the working current of the motor. A suitable AC contactor can be chosen according to the principle.

For example, how should we select a suitable AC contactor for 5.5kW 3-phase motor be equipped?

• First, the type of the AC contactor for the motor generally adopts the ac-3 series.
• Then, the working current of the motor is estimated: I=5.5*2=11A.
• At last, the working current of the AC contactor can be about: 11*1.5=16.5A.
• We can choose 18a, which is slightly larger, as the working current. This means that we adopt the CJX2-1810 AC contactor.

Both the formula and the estimation method are applicable. We can choose one according to personal habits. Of course, estimation based on the experiential value can obtain the result faster.