Testing Motor Condition with a Multimeter

This method is mainly applicable to three-phase asynchronous motors (the most common in industry) and single-phase AC motors (for household appliances/small equipment); the same principles can be referenced for DC motors. UN multimetro enables preliminary fault diagnosis by measuring winding continuity, resistance balance, and insulation to ground (a professional megohmmeter is required for accurate insulation testing, while a multimeter only serves for rapid screening).

Mandatory Safety Precautions

1. Completely power off the motor and disconnect all power lines (remove U/V/W for three-phase motors, and live/neutral/capacitor lines for single-phase motors) to prevent electric shock and measurement errors.
2. Discharge the windings of high-power motors (short-circuit both ends of the windings with a wire) to avoid damage to the multimeter from residual electric charge.
3. Clean oil, dirt and oxide layers from the motor terminals to ensure good contact between the multimeter probes.
4. Most Common in Industry: Testing Three-Phase Asynchronous Motors (Core Steps)

Impostare il multimetro sull'intervallo di resistenza (select based on motor power: 200Ω/2kΩ for low-power motors, 200mΩ/2Ω for high-power motors, and MΩ range for insulation testing).

Fare un passo 1: Test Continuity of Three-Phase Windings (Judge Open Circuits)

Touch the motor terminals U-V, V-W, W-U with the probes respectively, and measure the resistance between the three groups of windings in sequence:

✅ Normal: All three groups show a fixed resistance value (not infinite), and the resistance values of the three phases are essentially equal (deviation ≤5%). The higher the motor power, the smaller the winding resistance (a few tenths of an ohm for high-power motors, and a few to tens of ohms for low-power motors).

❌ Fault: Display OL/infinite for any group → open circuit in the winding (terminal disconnection or winding burnout); excessively large deviation in the three-phase resistance → interturn short circuit in the winding (local coil melting causes reduced resistance).

Fare un passo 2: Test Winding Insulation to Ground (Judge Leakage/Earth Fault)

Set the multimeter to the highest resistance range (MΩ). Touch one probe to a motor terminal (any of U/V/W) and the other to the motor’s metal housing (preferably the housing grounding screw for more reliable contact). Measure the insulation of U, V and W to the housing in sequence:

✅ Normal: Display OL/infinite (no resistance), indicating intact winding insulation and no leakage.

❌ Fault: Display a specific resistance value (especially several MΩ or lower) → insulation damage/earth fault of the winding (moisture or ablation causes the winding to connect with the housing, leading to leakage and tripping when powered on).

⚠️ Key Reminder: The MΩ range of a multimeter outputs low voltage (only a few volts to tens of volts) and only allows preliminary screening. Industrial specifications require insulation testing with a 500V/1000V megohmmeter, with a minimum insulation resistance of 0.5MΩ for low-voltage 380V three-phase motors.

Fare un passo 3: Auxiliary Judgment (Mechanical faults cannot be measured by a multimeter and require manual inspection)

Rotate the motor shaft by hand. Smooth rotation without jamming or abnormal noise → normal bearings/rotor; jamming during rotation → bearing damage or rotor rubbing against the stator, requiring disassembly and maintenance.

1. Household Appliances/Small Equipment: Testing Single-Phase AC Motors

A single-phase motor has a main winding (running winding, low resistance) and an auxiliary winding (starting winding, high resistance), and some are equipped with a starting capacitor. Multimeter testing includes winding detection + preliminary capacitor screening.

Fare un passo 1: Distinguish Winding Terminals (3 terminals in total: Common C, Main M, Starting S)

Fare un passo 2: Test Winding Continuity and Resistance Relationship

Set the multimeter to the 200Ω range, and measure the resistance of the three terminal groups: C-M (main winding), C-S (starting winding), M-S (main + starting winding):

✅ Normal: The resistance of M-S = resistance of C-M + resistance of C-S, and all three groups show a resistance value (not infinite); the resistance of the main winding is less than that of the starting winding (a fixed rule).

❌ Fault: Infinite display for any group → winding open circuit; resistance values not meeting the sum rule → winding interturn short circuit.

Fare un passo 3: Test Insulation to Ground

Same as the three-phase motor: set to the MΩ range and measure the resistance of terminals C, M and S to the housing respectively → infinite resistance is normal; a measurable resistance indicates insulation damage and leakage.

Fare un passo 4: Preliminary Screening of the Starting Capacitor (A multimeter only tests continuity; a capacitor meter is required for capacitance measurement)

Remove the capacitor from the motor, set the multimeter to the MΩ range, and touch the two poles of the capacitor with the probes:

✅ Normal: The pointer deflects slightly at the moment of contact (charging) and then slowly returns to infinity; reverse the probes and touch again, the pointer deflects more significantly and then returns to infinity.

❌ Fault: The pointer points directly to 0/conductive upon contact → capacitor short circuit; the pointer does not deflect and remains infinite → capacitor open circuit; the pointer deflects but fails to return to infinity → capacitor leakage (the capacitor must be replaced in all cases).

III. Simple Testing of DC Motors (Small DC Motors/DC Servos)

1. Test the armature winding (rotor): a small fixed resistance (not infinite or 0) between the two terminals, and infinite resistance to the housing (ground).
2. Test the field winding (stator, series/shunt excitation): a fixed resistance between the two terminals, and infinite resistance to the housing (ground).
3. Rotate the shaft by hand → smooth rotation without jamming; if equipped with carbon brushes, check that the brushes are not worn and make good contact.
4. Core Fault Judgment Table for Motor Testing with a Multimeter

1. Key Notes
2. A multimeter cannot measure motor speed, torque or micro interturn short circuits (professional instruments are required) and only serves for basic fault screening.
3. A low-resistance multimeter/milliohmmeter is required to measure the winding resistance of high-power motors (≥7.5kW); ordinary multimeters have large errors in low-resistance measurements.
4. A measurable insulation resistance may be displayed on the multimeter if the motor is damp. The motor can be dried and tested again; it can be reused if the resistance returns to infinity.
5. If the windings, insulation and rotation are all normal after testing → no fault in the motor itself. If the motor fails to rotate when powered on, check the power supply, contactors and wiring.
6. Supplementary Industrial-Grade Accurate Testing

For industrial equipment motors (380V/high voltage), after a normal preliminary test with a multimeter, professional instruments are required for accurate acceptance:

1. Insulation resistance: Test with a 500V megohmmeter (≥0.5MΩ); use a 2500V megohmmeter for 10kV high-voltage motors.
2. DC resistance: Test the three-phase winding resistance with a DC resistance tester (deviation ≤2%).
3. Interturn insulation: Perform a withstand voltage test with an interturn withstand voltage tester to prevent micro short circuits.