Testing a scooter motor with a multimeter: phases, insulation, and Hall sensors

Before replacing a motor or controller, a few measurements using a multimeter can help determine if the motor is functioning correctly. These measurements don't require opening the motor; you only need access to its wires: typically, three large phase wires, and usually a smaller bundle for the Hall sensors, and sometimes a temperature sensor. This guide provides a general method that is applicable to most hub motors. Rather than looking for absolute values, which vary from one motor to another, the focus is on finding a balance between the phases and identifying signals that clearly switch.

Before measuring: Safety and preparation

With the scooter powered off, placed on a kickstand or support, and the wheel in question raised, disconnect the three motor phases on the controller side, carefully noting their order with colored tape. If possible, also disconnect the battery. A powerful scooter battery can cause sparks and burns in the event of a short circuit: use insulated tools, remove any jewelry, and ensure that no measurement probe touches a battery terminal.

Equipment needed: A multimeter with resistance, continuity, DC voltage, and AC voltage settings. For the Hall sensors, fine probes or small needles can be used to take measurements at the back of the connector without disconnecting it.

Test 1: Resistance between phases

Using a multimeter, set it to measure resistance and select the lowest range. Measure the resistance sequentially between phases 1 and 2, then between phases 2 and 3, and finally between phases 1 and 3.

• What to expect: three very low and equal values. The winding resistance of a motor-wheel is so low that a standard multimeter often displays a value close to that of its own leads.
• Tip: First, bring the two probes into contact and note the displayed value, or use the relative measurement function if your multimeter has one. Subtract this value from each measurement.
• What should raise a red flag: a pair that displays infinite resistance (phase cut), or a pair that is significantly different from the other two (bad solder joint, damaged wire).

This test does detect a shorted phase, but a standard multimeter is not precise enough to identify a few shorted windings. For more precise measurements, the workshop uses a four-wire milliohmmeter. Test 3 partially compensates for this limitation.

Test 2: Insulation between phases and the chassis

Using a multimeter set to measure resistance, with the highest range selected, measure the resistance between each phase and the motor shaft. Then, measure the resistance between each phase and an unpainted metal point on the motor bell.

• What we expect: an infinite resistance, which is generally displayed as "OL" or "1".
• What should raise concern: any measurable value. This indicates a winding where the insulation has been damaged, often due to overheating, or a phase cable that has been damaged at its exit from the shaft. A motor in this condition should not be put back into service: the insulation fault can damage the controller.

A multimeter applies a very low voltage; what appears to be good insulation may fail under the voltage of the battery. An insulation tester, or megohmmeter, performs a more rigorous test, but its test voltage can damage the sensors: disconnect them before using it.

Test 3: Measuring the voltage generated when turning the wheel

A motor with magnets also functions as a generator. Disconnect the phases, use a multimeter set to AC voltage, and rotate the wheel by hand, as smoothly as possible, while measuring the voltage between each pair of phases.

• What to expect: At the same speed, you should see three similar voltage readings. Their value depends on the motor and its rotational speed; only the comparison matters.
• What should raise a red flag: a significantly lower or zero value. This could indicate a broken phase or shorted windings within a coil.

Also, perform the braking test by connecting the phases, as described in our guide on the motor that stutters: it complements this well. Keep in mind that a wheel spinning quickly generates a significant voltage on the phase wires: do not spin it too fast while you are holding the exposed wires.

Test 4: Hall sensors

The Hall sensors indicate the position of the rotor to the controller; their function is explained in our guide on position sensors. The wiring harness typically has five wires: a power supply, a ground, and three signal wires.

This test is performed with the sensors connected to the controller, the scooter powered on, the wheel raised, and the phases either connected or isolated. Many motor Hall sensors have an open-collector output: they only produce a high signal thanks to a pull-up resistor placed in the controller. When measured while disconnected, they do not provide any usable signal.

1. Using a multimeter, connect the black probe to the sensor ground and the red probe to the power supply. You should find a stable voltage of a few volts, provided by the controller.
2. Look for a red dot on the first signal. Slowly rotate the wheel by hand: the voltage should clearly switch between a value close to zero and a high value, then return. Repeat this process for the other two signals.
3. Compare: the three signals should toggle the same number of times per wheel revolution, and should never remain at an intermediate value.
4. On most motors, the sensors are offset so that the three signals are never all low or all high at the same time. If you observe this combination, one of the sensors is likely faulty.

A signal that is stuck, always low or always high, indicates a faulty sensor, a broken wire in the motor cable, or a defective connector pin. Start by checking the connector: this is the most common issue and the easiest to fix.

Test 5: The temperature sensor

Many motors have a temperature sensor embedded in the windings, connected by an additional wire within the sensor harness. This sensor is a resistor whose value changes with temperature. When measured between its wire and the sensor ground, with the connector disconnected, it should provide a stable and finite value that changes when the motor heats up. An infinite or zero value indicates a broken or short-circuited sensor, and the controller will no longer protect the motor.

Interpret the results