Sizing Heater Elements for Motor Starters

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Another important step in installing or replacing a motor starter is to select the correct size of heating element so that it matches the current that the motor draws. The correct size of heating element will ensure that the motor starter senses the correct amount of overload current for a specific motor. From the previous tables one may have notices that 11 different sizes of motor starters are available (size 00-size 9). The motor starter size refers to the size of its contacts, which determines the amount of full-load motor current the contacts can safely switch on and off. For example, the size 1 motor starter is used to energize three-phase motors from 3hp through 10 hp. The 3hp motor typically draws 4.0 A and the 10 hp motor draws 12.4 A. The motor starter's contacts are large enough to handle up to 27 A. When a motor starter is manufactured, it's not known what size motor will be connected to it, so it may be shipped without heating elements. If the size 1 motor starter is to be used for an application that has a 3hp motor, the heating elements will need to be selected to set the overcurrent limit at 4 A to ensure that the overloads will trip at 4 A and the 3hp motor will not receive overcurrent. If the application used a 10hp motor, the heating element would be selected for 12.4 A so that the motor starter would trip its overload at 12.4 A. Notice that the selection of the heating element allows the motor starter to be customized to protect the motor that's connected to it for the exact amount of overcurrent the motor draws.

If you're technician you may be responsible for looking up the full-load amperage ( FLA) rating for the motor you are working with and selecting the correct heating element size to protect the motor. Ill. 1 shows one table for selecting heating elements for size 0 through size 4 starters. Other tables provide selection data for all of the motors and motor starters that one will encounter. If one is selecting the heating element for the 3hp power motor that's connected to a size 1 motor starter, one needs to look at the motor name plate to determine the exact amount of FLA for the motor. For this example, the motor name plate indicates the motor draws 4 A. From this table, notice that size W43 is the closest heater size since it's rated for 4.11 A when used in a size 1 motor starter. If one were selecting the heating element for the 10hp motor whose FLA is 12.4 A, one would select a size W55 heating element since its rating is 12.8 A. The heating element size is very seldom the exact size for the FLA of the motor, so one will need to select the next larger size heater to protect the motor.

Table for heating element selection for Allen-Bradley Type W heaters.
Above: Ill. 1: Table for heating element selection for Allen-Bradley Type W heaters.

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