1. What is DC Motor Resistance?

The resistance of a DC motor's winding is a key parameter that affects its performance. It determines the current draw at a given voltage and influences torque production and efficiency.

2. How to Measure Resistance Using Ohm's Law

The calculator uses Ohm's Law:

Where:

• \( R \) — Resistance in ohms (Ω)
• \( V \) — Voltage across the motor (Volts)
• \( I \) — Current through the motor (Amperes)

Measurement Procedure:

1. Apply a known DC voltage to the motor terminals
2. Measure the current drawn by the motor
3. Calculate resistance using the formula

3. Importance of Resistance Measurement

Details: Knowing the motor's resistance helps in:

• Predicting current draw at different voltages
• Calculating power dissipation (I²R losses)
• Diagnosing motor health (unusually high/low resistance may indicate problems)
• Designing appropriate drive circuits

4. Using the Calculator

Tips:

• Measure voltage directly across motor terminals
• Use a DC power supply for stable measurements
• For best accuracy, measure at several voltages and average results
• Measure with motor at rest (locked rotor) for winding resistance

5. Frequently Asked Questions (FAQ)

Q1: Why measure resistance instead of using manufacturer specs?
A: Actual resistance may vary due to temperature, wear, or manufacturing tolerances. Measurement verifies real-world values.

Q2: How does temperature affect resistance?
A: Resistance increases with temperature (positive temperature coefficient). Copper windings increase about 0.4% per °C.

Q3: What's a typical resistance range for small DC motors?
A: Small hobby motors (3-12V) typically range from 1-50 ohms, while larger industrial motors may be in milliohms.

Q4: Why is locked rotor measurement important?
A: It measures pure winding resistance without back-EMF affecting the measurement.

Q5: Can I use a multimeter directly?
A: While possible, the low resistance of many motors may be below a multimeter's accurate range. The voltage/current method is often more precise.