5250

This may be further expressed as follows:

rpm3

In USCS units Slip loss (hp) = -î—3 X load hp rpm3

Or rpm2 rpm3 rpm2

rpm3

In USCS units Slip loss 1 hp) =-X motor hp v ' rpm:

rpm3

rpmj where rpm1 = motor speed at designated load rpm2 = load speed at designated load rpm3 = slip speed at designated load T = load torque, ft • lb (N • m)

An eddy-current coupling must slip in order to transmit torque. The normal minimum value of slip for a centrifugal pump application is usually 3%, but values from 1 to 4% are common. The above formulas hold true regardless of the type of load involved.

The efficiency of an eddy-current coupling can never be numerically greater than the percentage that the output speed is of the input motor speed. This effectively takes into consideration only the slip losses, and a true efficiency value must also include frictional, windage, and plus excitation losses. The frictional and windage losses are constant for a fixed motor speed and therefore increase in significance with speed reduction. Excitation losses, on the other hand, decrease with reduction in output speed. The overall effect of these losses is an efficiency versus speed relationship that is somewhat linear with efficiency values anywhere from 1 to 4 points less than the output speed percentage.

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Survival Treasure

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