See Motor PWM and Hall Signals with a USB Oscilloscope

Why this happens

A multimeter averages fast signals, so it cannot show you PWM switching or the sequence of Hall sensor pulses that tell a BLDC controller where the rotor is. An oscilloscope draws voltage over time, so you can see duty cycle change with throttle and spot a missing Hall pulse. A USB scope shares ground with your laptop, which makes grounding the main safety issue.

What you need

Step by step

1. 1

   Set up a bench-only rig

   Use a small hobby BLDC motor and a controller rated for a low supply voltage, powered by a bench supply with the current limit set low. Fit an inline fuse in the positive lead. Clamp the motor so it cannot move, keep fingers and loose clothing clear of the shaft, and work on a fire-safe mat. Do not connect the scope to a vehicle.

2. 2

   Understand the ground problem

   On most USB scopes, every probe's ground clip is connected to the USB ground and so to your laptop. If you clip a ground lead to a point that is not ground, you create a short through the scope. Run the laptop on battery with its charger unplugged, and clip probe grounds only to the controller's logic or supply ground, confirmed with a multimeter.

   Tip: Use one ground clip for both channels and leave the other ground clip unconnected.

3. 3

   Set probes and channels

   Set each probe to x10 and match the setting in the scope software, so voltages read correctly and the probe loads the circuit less. Start with a vertical scale that covers the logic voltage, a timebase of about a millisecond per division and the trigger on channel 1 rising edge. Use the probe's compensation tab to square up the waveform before measuring.

4. 4

   Look at the Hall sensor signals

   With the supply off, attach channel 1 to one Hall output and channel 2 to another at the controller's Hall connector, using hook tips rather than holding probes. Power on and turn the motor slowly by hand or at low throttle. You should see square waves switching between ground and the logic voltage, shifted in time from each other.

5. 5

   Check the Hall sequence

   Move a probe to the third Hall line and compare all three in pairs. On a healthy motor each Hall line switches in a regular repeating pattern, with no line stuck high or low and no missing pulses. Note the frequency at a steady speed. A line that stays flat while the motor turns points to a faulty sensor, broken wire or bad connector pin.

6. 6

   See the throttle and PWM

   Probe the throttle signal wire against logic ground and watch its voltage rise as you turn the throttle. If your controller board exposes a gate-drive or PWM test point on its low-voltage logic side, probe that instead of the motor phase wires. Watch the pulse width grow as throttle increases. Use the scope's measurement tool to read frequency and duty cycle.

7. 7

   Record and compare

   Save screenshots at several throttle positions and note supply voltage and current. These become your reference for a known-good rig. When something changes, such as rough running or a stall, repeat the same captures and compare. Power off and wait for the controller's capacitors to discharge, checking with a multimeter, before moving any probe.

How to tell it worked

You can capture clean Hall sensor waveforms and the throttle signal on the bench, read frequency and duty cycle, and tell a healthy sequence from a missing or stuck sensor.

Questions