1. Basic Motor Formulas

The fundamental formulas for motor calculations are derived from Ohm's Law and power equations:

Where:

• \( I \) — Current in amperes (A)
• \( P \) — Power in watts (W)
• \( V \) — Voltage in volts (V)

2. Practice Problems

Problem 1: A motor draws 5A from a 120V supply. What is its power consumption?
Answer: P = V × I = 120V × 5A = 600W

Problem 2: A 750W motor runs on 240V. What current does it draw?
Answer: I = P/V = 750W/240V = 3.125A

Problem 3: A motor consuming 1.5kW draws 12.5A. What is the supply voltage?
Answer: V = P/I = 1500W/12.5A = 120V

3. Sample Calculations

Example 1: Three-phase motor calculation
For balanced three-phase systems: \( P = \sqrt{3} \times V_{LL} \times I \times PF \)

Example 2: Efficiency calculation
\( \text{Efficiency} = \frac{\text{Output Power}}{\text{Input Power}} \times 100\% \)

4. Common Applications

These calculations are essential for:

• Sizing circuit breakers and fuses
• Determining wire gauge for motor circuits
• Calculating energy consumption
• Troubleshooting motor performance issues

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between real power and apparent power?
A: Real power (W) does actual work, while apparent power (VA) includes reactive power.

Q2: How does power factor affect motor calculations?
A: For AC motors: \( P = V \times I \times PF \). Lower PF means more current for same power.

Q3: Why do motors draw more current when starting?
A: Starting current (inrush) can be 5-7× running current due to locked rotor condition.

Q4: How do I account for motor efficiency?
A: Input power = Output power/Efficiency. A 90% efficient 10HP motor draws 10/0.9 = 11.11HP input.

Q5: What's the relationship between torque and power?
A: \( P = \tau \times \omega \), where τ is torque (Nm) and ω is angular velocity (rad/s).