Motor RPM Calculation Formula

Motor RPM Formula (DC Motors):

\[ RPM = \frac{V - I \times R_m}{K_v \times \Phi} \]

Voltage (V):

volts

Current (I):

amps

Resistance (R_m):

ohms

Motor Constant (K_v):

RPM/V

Magnetic Flux (Φ):

Motor RPM:

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the Motor RPM Calculation Formula?

Definition: This calculator determines the rotational speed (RPM) of a DC motor based on electrical and mechanical parameters.

Purpose: It helps engineers, technicians, and hobbyists predict motor performance under different operating conditions.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ RPM = \frac{V - I \times R_m}{K_v \times \Phi} \]

Where:

\( RPM \) — Revolutions per minute
\( V \) — Applied voltage (volts)
\( I \) — Current draw (amps)
\( R_m \) — Motor resistance (ohms)
\( K_v \) — Motor velocity constant (RPM/volt)
\( \Phi \) — Magnetic flux (Webers)

Explanation: The numerator calculates effective voltage after accounting for voltage drop across motor resistance, while the denominator represents the motor's speed constant adjusted for flux.

3. Importance of RPM Calculation

Details: Accurate RPM prediction is crucial for motor selection, performance tuning, and ensuring motors operate within their designed specifications.

4. Using the Calculator

Tips: Enter voltage, current, motor resistance (default 0.5Ω), Kv (default 1000 RPM/V), and flux (default 0.001 Wb). All values must be ≥ 0 except voltage and Kv which must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What is the Kv (motor velocity constant)?
A: Kv indicates how many RPM the motor will turn per volt with no load (typically provided in motor specifications).

Q2: How do I find my motor's resistance?
A: Measure with a multimeter or check manufacturer specs. Small motors typically have 0.1-2Ω resistance.

Q3: What affects magnetic flux (Φ)?
A: Flux depends on magnet strength and motor design. Permanent magnet motors have relatively constant flux.

Q4: Why does current affect RPM?
A: Current causes voltage drop across motor resistance (I×R), reducing effective voltage available to produce rotation.

Q5: Is this formula accurate for loaded motors?
A: It provides theoretical maximum RPM. Actual loaded RPM will be lower due to mechanical losses not accounted for here.