Are three-phase contactors rated per pole or in total?

Question

Are three-phase contactors rated per pole or in total? For example, have a look at this datasheet for a random three-phase contactor: datasheet

In the datasheet it says that for AC-3 loads (1) the rated operational current [Ie] is 32 A, and (2) the motor power rating is 7.5 kW at 220..230 V. These numbers match, since 32 A * 230 V = 7360 W.

On the Technical FAQ pages of this manufacturer it says:

"Contactor current ratings are per pole. For example a contactor rated 40 amps AC3 could switch upto 40 amps AC3 on just one pole or on all the other poles." (see source)

What I'm confused about is that motor power rating is usually not declared per pole, but in total. So if I use 10 kW contactor for a 10 kW motor, my contactor if oversized by a factor of 3. Is this correct, or I'm missing something?

Answer 1

I think the calculation is not that easy because:

• the power rating for (industrial) motors is not given for the input (electrical) power but for the output (mechanical) one. If you want the electrical power, you have to consider the efficiency ;
• you have also to deal with the power factor which is not equal to 1.

I think the powers that are given in the datasheet you write about is the one you can find for standard motors. For example, here is a list of AC 3 phase motors from Emerson/Leroy-Somer:

For example, a 4kW motor at 415V (voltage difference between two phases) 50Hz has a rated current of 8.2A and a power factor of 0.81. We could calculate its efficiency:

$$ \eta = \frac{4000} {\sqrt{3} \times 415 \times 8.2 \times 0.81} = 0.84$$

Now, if we take the datasheet for contactors from Schneider (TeSys Green) (I think this is approximately the same products you gave the datasheet):

We can see that for a 4kW/415V motor, we have a 9A current. This is just above the 8.2A of the aforementioned motor.

Also, when you read motor datasheets, you will see that standard output powers are the same given in your document: 7.5kW 15kW 18.5kW...

Answer 2

You are correct. The current rating is amps per pole for the power poles. If there are auxiliary contacts, they will have a lower amps per pole rating. Some contractors have one or more normally-closed power contacts. Those may have a different amps per pole rating. There are likely other contactor configurations, but the rating specifications would be similar.

You are also correct about the motor power rating. Since motors are usually rated in output mechanical power, there can be only one rating. They are never rated in power per phase. If a contactor is rated for use with either single-phase or three-phase motors, it would have a single-phase and three-phase power rating. The power ratings would also be stated separately for each motor voltage rating for which it might me used.

Answer 3

Practically the kw & cont. current rating of the contactor must not smaller than kw & FLA of the motor. The rest of the detail may be available from the manufacturer.

Answer 4

You need to look at the peak current for each pole.

For a resistive load, this would be in time with the voltage peak; the other two poles split the return current (\$\cos 120^\circ = \cos 240^\circ = -0.5\$).

For an inductive load, there is a phase offset, but the peak height is the same.