How to Replace Full Rated and Up Rated Motors


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On occasion you may have to search for a specific replacement pump motor only to find it unavailable. Then it happens.  The motor salesman will suggest substituting an available up-rated motor for the unavailable full rate (or vice versa dependnig on the region of the country and design preferences). When this happens, you will likely be sold an under performing motor.

Step by Step


Step 1

Consider the following scenario. You need of the full rated SQ1102, a one horsepower square flange replacement motor. Finding it unavailable, the up-rated USQ1102 - also a one horsepower square flage motor - is offered as a suitable substitute.

Step 2

What's the big deal some might ask? Same horsepower. Same manufacturer. Practically identical stock numbers. And the price - why, it is actually a little less to buy the up-rated motor. Sounds like a no-brainer.

Step 3

The problem is the motors are not equivalent. The SQ1102 actually has a total horsepower (THP) of 1.65, compared to a 1.25 THP for the USQ1102.

Step 4

That difference in total horsepower can be too much to overcome once installed, with the end result being an under-sized up-rated motor failing prematurely due to mis-application. You will be stuck with the motor given the warranty does not cover damage caused by mis-application.

Step 5

If following one simple rule, you could virtually eliminate any doubt or apprehension when faced with a potential full- and up-rated motor substitution. The key is to identify a truly equivalent substitute by calculating and comparing total horsepower. To do this, an installer need only multiply the nameplate horsepower by nameplate service factor.

Step 6

For the SQ1102 then, 1.0 HP multiplied by 1.65 SF equals 1.65 THP. And for the USQ1102, 1.0 HP multiplied by 1.25 SF equals 1.25 THP. Note that there is a significant difference - which should send up a red flag.

Step 7

Recognizing the disparity in total horsepower you should consider an up-rated motor with a higher horsepower for quick comparison. In this scenario the USQ1152 with 1-1/2 HP and 1.1 SF equals the required 1.65 THP and proves to be the ideal up-rated motor substitute for the full rate SQ1102.

Step 8

Avoid the frustration when subbing between full- and up-rated motors by remembering to multiple horsepower by service factor and then selecting a substiitute with the closest total horsepower (slighlty higher is better than lower).
Horsepower (HP) x Service Factor (SF) = Total Horsepower (THP)



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