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Contactor Rating


bonny

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Sir,

 

In a contactor there are generally two readings 1) thermal current rating Ith, and 2) operational current rating

 

what is the significance of thermal current rating , and which one is considered while selecting a contactor for a particular motor feeder.

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Hello bonny

 

Welcome to the forum.

 

There are a number of ratings that can be applied to any given contactor.

The thermal current Ith is the maximum continuous current that the contactor can carry.

 

The rating of the contacts within the contactor is a thermal rating. As the current is increased, the temperature of the contacts increases with the square of the current increase.

If there is any overload current, the temperature of the contacts is driven upwards.

 

In many contactor aplications, there are regular overloads required. For example, Full voltage starting of induction motors requires an overload current of at least 600% for the duration of the start.

In order to keep the maximum temperature of the contacts at acceptable values, the rated current is reduced for applications where such an overload occurs.

 

Typically, contactors designed for controlling motors, are rated AC3 and are suitable for regular overloads of 600% current every time the motor is started.

For arduous starting conditions, use AC4 ratings. - Never use the AC1 (thermal current) for starting motors, but you can use use an AC1 contactor for bypassing a soft starter as it does not carry the starting current.

 

Look at the manufacturers ratings for contactors with the different ratings for AC1, AC3 and AC4.

 

Best regards,

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Hello bonny

 

Welcome to the forum.

 

There are a number of ratings that can be applied to any given contactor.

The thermal current Ith is the maximum continuous current that the contactor can carry.

 

The rating of the contacts within the contactor is a thermal rating. As the current is increased, the temperature of the contacts increases with the square of the current increase.

If there is any overload current, the temperature of the contacts is driven upwards.

 

In many contactor aplications, there are regular overloads required. For example, Full voltage starting of induction motors requires an overload current of at least 600% for the duration of the start.

In order to keep the maximum temperature of the contacts at acceptable values, the rated current is reduced for applications where such an overload occurs.

 

Typically, contactors designed for controlling motors, are rated AC3 and are suitable for regular overloads of 600% current every time the motor is started.

For arduous starting conditions, use AC4 ratings. - Never use the AC1 (thermal current) for starting motors, but you can use use an AC1 contactor for bypassing a soft starter as it does not carry the starting current.

 

Look at the manufacturers ratings for contactors with the different ratings for AC1, AC3 and AC4.

 

Best regards,

 

If, by chance, you are in North America and looking at NEMA design contactors, the same general rules apply but there are only 2 ratings: Resistive (which would be the Thermal rating or equivalent to AC1) and Inductive (which would be a motor starting rating but is equivalent to AC4). In NEMA designs, there is only one motor starting rating and it is taken at the worst case scenario, inching and plugging duty.

"He's not dead, he's just pinin' for the fjords!"
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