Useful Formulas
Formulas for Electrical Motors
| To Find | Direct Current | Single Phase | Three Phase |
| Horsepower | E x I x EFF 746 | E x I x EFF x PF 746 | 1.732 x E x I x EFF x PF 746 |
| Current | 746 x HP E x EFF | 746 x HP E x EFF x PF | 746 x HP 1.732 x E x EFF x PF |
| Efficiency | 746 x HP E x I | 746 x HP E x I x PF | 746 x HP 1.732 x E x I x PF |
| Power Factor | – – – – – | Input Watts E x I | Input Watts 1.732 x E x I |
| E = Volts | I = Amperes |
| EFF = Efficiency (decimal) | PF = Power Factor (decimal) |
| HP = Horsepower | R = Ohms |
Formulas for Electrical Circuits
| To Find | Direct Current | Single Phase | Three Phase |
| Amperes | Watts E | Watts E x PF | Watts 1.732 x E x PF |
| Volt-Amperes | – – – – | E x I | 1.732 x E x I |
| Watts | E x I | E x I x PF | 1.732 x E x I x PF |
| E = Volts | I = Amperes |
| EFF = Efficiency (decimal) | PF = Power Factor (decimal) |
| HP = Horsepower | R = Ohms |
| Ohms Law | Capacitance in Microfarads at 60 Hz | ||||
| Ohms = Volts/Amperes (R = E/I) Amperes = Volts/Ohms (I = E/R) Volts = Amperes x Ohms (E = IR) |
|
Temperature Correction of Winding Resistance
| Rc = Rh x |
(K + Tc) |
|
Rc = Resistance at temperature Tc (Ohms) Rh = Resistance at temperature Th (Ohms) Tc = Temperature of cold winding (°C) Th = Temperature of hot winding (°C) |
||||||||
| Rh = Rc x |
(K + Th) |
||||||||||