Calculation

Calculations

Synchronous Speed

• Ws = 120 x f / P

• f = frequency (60Hz)

• P = Number of Poles

Locked Rotor Current

• Current drawn when rotor is not rotating

• Same as starting current

• 430.7(A)(8)

• Table 430.7(B) KVA per HP

• Used for sizing disconnects

Equations

• Single Phase: LRC = (LR KVA per hp x 1000 x hp) / E

• 3-Phase: LRC = (LR KVA per hp x 1000 x hp) / (E x 1.732)

• LRC = Locked Rotor Current in amperes

• LR kVA = Value from T430.7(B)

• 1000 = value used to convert kVA to Volt Amps

• hp = Motor Horsepower

• E = Motor Voltage

Using a 3-phase, 460V, 30 HP, squirrel cage motor, with a code letter F, determine the maximum LRC

• LRC = (5.59 x 1000 x 30) / (460 x 1.732)

• 210.48 Amps

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PF = (hp x 746) / (V x i x 1.732 x E )

• pf = Power Factor

• hp = Horsepower

• V = Voltage

• i = Current

• E = efficiency

What is the power factor of a 3-phase, 20 HP motor with an efficiency of 87%, connected to a 480V source and FLC is 25A?

• (20 x 746) / (480V x 25 x 1.732 x 0.87) =

• 0.82

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Manipulate formula to find FLC

• i = ( hp x 746 ) / ( V x E x 1.732 x pf )

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