PCB Design for BLDC Ceiling Fan

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Providing measurement suggestions.

Clarifying measurement guidelines.

Explaining measurement variations.

Clarifying voltage and measurements.

Outlining measurement guidelines.

Short answer: Your meter is reading a PWM’d, time‑varying inverter waveform between two motor phases (U and V). That voltage is not a steady DC, so a basic DMM will show a fluctuating, often misleading value (like ~14.7 V). This is normal.
Why the U–V reading varies
  • 3‑phase inverter output is PWM: It rapidly switches each phase to the DC bus or ground. The line‑to‑line voltage (U–V) is a train of pulses, not steady DC.
  • Commutation states change: At any instant, one phase may be high, one low, one floating (sensorless uses the undriven phase for back‑EMF). Your meter sees different pulse patterns every commutation step.
  • DMM measurement method: Many meters average or approximate AC, not true‑RMS at PWM frequencies. The reading drifts with duty cycle, speed, and load.
  • Load/back‑EMF: With the motor connected and spinning, its back‑EMF adds/subtracts from the inverter voltage in a speed‑dependent way, further confusing a non‑true‑RMS meter.
What the number does and does not mean
  • 14.69 V between U and V is NOT the DC bus voltage.
  • It’s also NOT the true line‑to‑line RMS of the synthesized 3‑phase sine/trapezoid.
  • The maximum line‑to‑line RMS you can synthesize is roughly a fixed fraction of the DC bus:
    • ~0.707 × Vdc with space‑vector PWM (SVPWM)
    • ~0.612 × Vdc with sinusoidal PWM (SPWM) These are ratios; the actual RMS needs proper instrumentation to verify.
How to measure correctly (pick what you have)
  • Safest sanity check (DMM only):
    1. Measure the DC bus across the inverter’s bulk capacitors (the 35 V electrolytics on a 12–24 V design). That tells you the real supply the inverter is switching.
    2. Expect U–V on a basic meter to wander; that’s normal.
  • Better (true‑RMS DMM):
    • Use a true‑RMS meter with sufficient bandwidth and measure U–V. It will still vary with speed and load, but readings will be more meaningful.
  • Best (oscilloscope):
    • Use a scope with a differential probe (or two channels with math) and view U–V. You’ll see the PWM pulses and can determine duty and modulation.
    • If you only need the average for a check, add a temporary RC low‑pass (for example, 10 kΩ in series and 1 µF to ground on the probe) to visualize the envelope. Remove this RC after testing.
Tricks for a steadier DMM reading at low voltage (12–24 V systems only)
  • Add a small dummy resistive load U–V (for example, 220 Ω, 5 W) during measurement. This reduces floating intervals and stabilizes the reading. Do NOT do this on high‑voltage inverters.
  • If your goal is just “is the inverter output scaling with my knob,” watch the DC bus (should be fixed if you chose fixed‑bus control) and measure motor speed instead of U–V voltage.
What to check next
  • Confirm what you are measuring:
    • DC bus: measure across the 35 V caps (should be your 12–24 V setting).
    • Gate/logic rails: check 12 V and 3.3/5 V are stable.
    • Phase output: use a scope or a true‑RMS meter if you need quantitative U–V values.
  • If you are adjusting a potentiometer for speed: keep the DC bus fixed (recommended) and feed the pot to the MCU ADC. The inverter then changes PWM duty to control speed; your DMM on U–V will naturally change as duty and commutation change.
If you tell me:
  • Whether the motor was connected during your 14.69 V reading,
  • Which meter mode you used (DC, AC, or AC+DC) and meter model, I can tailor the measurement method and expected readings precisely for your setup.
Measure DC bus next
Check if PWM explains it
Use scope-friendly method
Verify inverter type
RMS expectation calculation
1h
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