Classifications of Overload and Heater Elements for Motor Starters

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Overloads and heater elements for motor starters are classified by the amount of time the overcurrent condition can occur. A Class 10 overload is rated to trip after experiencing an overcurrent condition for 10 seconds. A Class 20 overload is rated to trip when the overcurrent exists for 20 seconds and the Class 30 is rated to trip when the over-current exists for 30 seconds. The amount of time the over-current is allowed to occur will change with the type of application for which the motor is used and the type of motor that's being used. In some cases the load will seldom cause an over-current and it will be of short duration, so the Class 10 is recommended. In other applications such as extruders for plastic presses and in hydraulic pump applications, it's not uncommon for an overload condition to occur for 10-20 seconds and then the motor current will return to normal. Since this over-current exists for a short period of time, it will not damage the motor, so the Class 2 or Class 3 overload will be used to provide the motor time to get back to normal overload current levels without tripping the overloads. If a serious problem occurs and the overload exists for more than the rated time for the overload, the heating element will send enough heat to the overload assembly to cause it to trip.

Exercise

You are connecting a 7.5 hp motor to a new size 1 motor starter and you must select the proper size heater element. The FLA rating for this motor on its data plate is 8.5 A. Use the table below to help you determine the proper selection.

Table

Solution

In the table above use the column for the size 1 motor starter and move down the table until one reaches the rating for 8.5 A or the next highest rating. Since 8.5 A is not listed, the next larger rating is for 8.78 A. The heater element with this rating is a size W51.

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