Motor RPM Calculation

Calculate motor rotational speed from various sensor inputs including Hall-effect sensors, encoders, tachometers, and back-EMF measurements with advanced filtering and calibration options.

Hall-Effect Sensor (Digital Pulses)

Calculate RPM from digital pulse signals generated by Hall-effect sensors. Count pulses in a time window to determine rotational frequency.

Pulse Count

Time Window (seconds)

Pulses Per Revolution

Gear Ratio

Filtering & Calibration

Enable Filtering

Filter Alpha (0-1)

Lower values = more smoothing

Key Formulas

Frequency from Pulses:

RPM Conversion:

Angular Velocity:

Gear Calibration:

Motor Speed Measurement Methods

Hall-Effect

• Magnetic field sensing
• Digital pulse output
• Low resolution
• Cost effective

Encoder

• High precision
• Position & speed
• Quadrature output
• Industrial standard

Tachometer

• Analog voltage output
• Linear response
• Simple interface
• Legacy systems

Back-EMF

• Sensorless operation
• Motor voltage dependent
• Brushless motors
• Cost reduction

Signal Processing Workflow

1. Signal Acquisition

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2. Pulse Counting

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3. Frequency Calculation

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4. RPM Conversion

5. Filtering

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6. Calibration

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7. Output Display

8. Control Loop

How to Use This Tool

Step 1: Choose Your Sensor Type

Select the tab that matches your motor's feedback system:

• Hall-Effect: For motors with magnetic sensors (3-phase BLDC motors typically have 3 Hall sensors)
• Encoder: For high-precision rotary encoders with known PPR (Pulses Per Revolution)
• Tachometer: For analog voltage output proportional to speed
• Back-EMF: For sensorless speed estimation from motor voltage

Step 2: Enter Sensor Data

Input the values from your measurement system:

• Pulse Count: Number of pulses detected in the time window
• Time Window: Duration of measurement (usually 1 second for accuracy)
• PPR/Resolution: Pulses per revolution (check sensor datasheet)
• Voltage: Measured analog voltage for tachometer/back-EMF

Step 3: Configure System Parameters

Adjust for your mechanical system:

• Gear Ratio: Mechanical reduction/multiplication factor
• Kv Constant: RPM per volt for motors/tachometers
• Filtering: Enable smoothing for noisy signals
• Filter Alpha: 0.1 = heavy smoothing, 0.9 = light smoothing

Example Usage

Hall-Effect Sensor Example:

Motor with 3 Hall sensors, counted 24 pulses in 1 second:

• Pulse Count: 24
• Time Window: 1 second
• PPR: 3 (sensors per revolution)
• Result: 480 RPM

Encoder Example:

1024 PPR encoder, 5120 pulses in 1 second:

• Pulse Count: 5120
• Time Window: 1 second
• PPR: 1024
• Result: 300 RPM

Tachometer Example:

Analog tachometer outputting 2.5V:

• Voltage: 2.5V
• Kv: 1000 RPM/V
• Result: 2500 RPM

Quick Tips

✓ Use longer time windows (≥1 second) for better accuracy at low speeds
✓ Enable filtering when dealing with noisy or unstable signals
✓ Check your sensor datasheet for correct PPR/Kv values
✓ Gear ratio > 1 for speed reduction, < 1 for speed increase

Frequently Asked Questions

Which sensor type should I use?

Choose based on your requirements: Encoders for highest precision, Hall-effect for cost-effective solutions, Tachometers for simple analog interfaces, and Back-EMF for sensorless applications where wiring is limited.

How do I find my encoder's PPR?

Check the encoder datasheet or part number. Common values are 100, 360, 500, 1024, 2048, or 4096 PPR. For quadrature encoders, multiply by 4 if you're counting both edges of both channels (A and B).

What is the Kv constant?

Kv (velocity constant) represents RPM per volt. For motors, it's in the datasheet. For tachometers, it's the conversion factor from voltage to RPM. Common values range from 100-3000 RPM/V depending on the device.

Why are my readings unstable?

Enable filtering to smooth out noise. Use a lower filter alpha (0.1-0.3) for heavy smoothing. Also ensure good electrical connections, proper grounding, and adequate signal voltage levels.

How do I handle gear ratios?

If your sensor measures the motor shaft but you need the output shaft speed, divide by the gear ratio. For example: Motor = 3000 RPM, Gear ratio = 10:1, Output = 300 RPM. Enter 10 as gear ratio.

What time window should I use?

Use 1 second for most applications. For very slow speeds (< 60 RPM), use longer windows (5-10 seconds). For fast control loops, shorter windows (0.1-0.5 seconds) may be needed, but accuracy will decrease.

Can I measure bidirectional rotation?

This tool calculates speed magnitude. For direction sensing, you need quadrature encoders with A/B channels or directional Hall sensors. The phase relationship between channels indicates rotation direction.

What about very high or low speeds?

High speeds (>10,000 RPM): Use shorter time windows and higher resolution encoders.
Low speeds (<10 RPM): Measure time between pulses instead of counting pulses, or use longer time windows.