Contactor Sizing 1

[StanleySmith]

Hi everybody,

I recently graduated and currently work as an entry level electrical engineer and I wonder if you can give me some tips/rule of thumb when it come to sizing contactors.

Thank you,

 

[Marke]

Hello StanleySmith

Contactors are given AC ratings that help to categorize them for different applications.
The most common categories are AC1 and AC3.

AC1 ratings are based on no overload currents and are generally described as resistive ratings.

AC3 ratings are for short term overloads of up to 6 times the rated current and are usually applied to motor control applications.

The actual ratings are thermal and the temperature of the contacts is dependent on the square of the current through them. Factors that affect the temperature and life of the contacts, are magnitude, duration and frequency of overloads, and time at steady state current and time at zero current.

If you operation frequency is high, it is a good idea to upsize the contactor to reduce the average current squared and thereby reduce the contact temperature and prolong the life.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[mc5w]

If you are doing jogging duty ( frequent starting and stopping ) or plugging duty ( forced reversing of the motor while it is running ) you need to upsize the contactor to account for the current broken so that the life will not be too short. You should be able to get life versus current broken from the contactor manufacturers.

In AC-3 service the assumption is that the motor will usually be up to speed which means that breaking the current is very easy as compared to jogging duty where current might be interrupted when the motor is only at 3/4 speed and drawing more current. In AC-3 service a general purpose motors controller has a life of 1 to 1.5 million operations when sized at 100% of the motor rating and more like 10 million operations when sized at 300% of the motor rating. In AC-3 service a definite purpose contactor ) such as in HVAC units ) has a life of about 200,000 operations which for a central air conditioner is quite acceptable.

Once you have the life versus load curves you can decide how much money you want to spend. As stated above, the required life depends upon the application.

If a motor is going to be starting and stopping say 10 million times a year then you need to think about using some type of servo drive that uses a solid state servo amplifier or variable frequency drive as the motor controller.