Heater Element Selection

[godfrey]

I need to select the right heater elements for a NEMA 1 starter. The motor is 480v, 3 phase, 7.5 HP, and FLA is 9.8. Is there a guide, rule of thumb or calc. for selecting the current rating?

 

[dash]

There should be a fairly thick section in the catalog for the manufacturer of the starter you are using to select the heaters for. I know that in the A-B catalog there is at least 5 pages describing the things you should consider when selecting a eutectic alloy heater. Basically you select the one closest to your FLA on the motor. You will also need to know what trip class you want, which ranges from 10 to 30. 10 for lighter duty starts (1.0 SF motors), 20 in between (most people use these) and 30 for heavy duty starts (1.15 SF motors). Then you may upsize or down size the heaters if there is a large temperature difference between the motor location and the starter (ie. panel is in temp controlled room & motor is in maufacturing area, which is not temp controlled). There should be graphs proveded by the manufacturer to help with this.

Also make sure you are looking in the correct chart for the starter you are using. I know on the A-B they use the same heaters in several starters, but they may be different sizes (part numbers) depending on which starter you are mounting them on.

Darren

 

[Lancie1]

godfrey,
The "rule of thumb" is to start with the motor nameplate full load ampere rating, 9.8 amps in your case, so you would pick an overload with a 9.8 amp rating. As dash said, then you consider all the other factors to see if you need to adjust it up higher for high starting current loads, or down for lighter loads. You want to keep the overload trip setting just slightly higher than the largest normal operating current (provided that your motor is sized correctly of course).

 

[elevmike]

According to my Square D slide chart, your overload trip current shoud ba about 14 amps. However you may want to check with the catalog or the starter manufacurer.