An AC motor isn't performing as it should. What do you do? Before replacing the motor, here are three effective ways to start troubleshooting your AC motor using a circuit tester.
AC motors are among the simplest types of motors to operate. Once power is connected, they run at a specific speed and stop when the power is turned off. When an AC motor fails to work, it's often due to a power or wiring issue. The key to identifying the root cause lies in isolating potential problems.
What is a Circuit Tester?
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A circuit tester (also known as a multimeter or multitester) is an electronic device that combines functions such as voltage measurement, resistance testing, and current detection. A typical circuit tester can measure voltage, resistance, and current in a circuit. |
First, Check the Basic Wiring.
If the motor is connected to a power supply but doesn’t work, the first step is to check the wiring and ensure all necessary components are functioning properly. Since there could be multiple components in the power circuit that affect motor operation—such as relays, switches, or controllers—it’s best to start with the most basic setup. This means connecting the AC motor and capacitor directly to the power source. If the motor works, you can gradually reintroduce other components to identify any faulty ones.
| TIP: Many AC motor issues stem from bad components or incorrect wiring. To simplify troubleshooting, start by checking the most basic circuit. Once confirmed, test each component one by one as you add them back into the system. |
3 Things to Test with a Circuit Tester
Here are three effective ways to diagnose an AC motor problem:
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1. Compare the applied voltage with the capacitor's terminal voltage.
2. Verify if electricity is conducted through the extension cable.
3. Check the motor winding resistance.
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Now, let's walk through how to perform each of these tests and what they confirm.
| TEST 1: Compare Applied Voltage with Capacitor Terminal Voltage |
If the motor is properly connected, the capacitor terminal voltage should be approximately 1.7 times the power supply voltage. This confirms whether the motor is receiving the correct voltage.
Using Circuit Tester A, connect the red probe to the exposed part of the red wire and the black probe to the exposed part of the black wire (ensuring good contact). Set the tester to AC voltage mode (indicated by a "V"). If correctly connected, Circuit Tester A will show the power supply voltage. In our example, a 100 VAC motor would display 100 V on the tester.
Using Circuit Tester B, connect the red probe to the red wire and the black probe to the white wire. If properly connected, the tester should read around 1.7 times the power supply voltage. In this case, it would show 170 V.
Compare the two readings. The capacitor terminal voltage (red/white) should be about 1.7 times the power supply voltage (black/red).
What If I Don't Have a Circuit Tester / Multimeter?
If you don’t have a circuit tester, you can manually rotate the motor shaft while the power is on. If no capacitor is connected, the motor should turn freely in the direction you apply force.
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| TEST 2: Verify if Electricity is Conducted Throughout the Cable |
A break in the motor’s circuit can prevent it from working. To check, use a circuit tester to verify if the circuit is closed or open.
Turn off the power. Using Circuit Tester C and D, test the continuity of the cables from the motor to the power supply. Connect the probes to the exposed ends of the wires near the motor and the power source. If the circuit is intact, the tester will beep.
| TEST 3: Verify Motor Winding Resistance |
Measuring the motor’s winding resistance and comparing it to the manufacturer's specifications can help determine its condition. Before measuring, remove all additional components like extension cables and capacitors. Switch the circuit tester to resistance mode and connect the probes directly to the motor’s lead wires.
Using a 100 V motor as an example (model: 2IK6A-JA), the resistance should read around 170 ohms. If the winding is broken, the tester will show several thousand ohms. If there’s a short, the reading will be lower than 170 ohms. For other motors, refer to the manufacturer for correct values, keeping in mind a tolerance of about +/-10%.
What Do I Do If My Motor Doesn’t Pass These Tests?
If the motor fails the tests, it might be due to a faulty component rather than the motor itself. Examples include bad switches, relays, cables, or incorrect voltage. These tests can help identify the issue. Here are some guidelines:
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If your motor doesn’t pass Test 1, check or replace the capacitor or wiring.
If your motor doesn’t pass Test 2, check or replace the cable or connector.
If your motor doesn’t pass Test 3, replace the motor.
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Mechanical issues, such as damaged bearings due to excessive load, can also prevent the motor from operating. To check, remove the motor and gearhead and try rotating the shaft in both directions. If it feels locked or makes unusual noises, there may be damage or misalignment. In such cases, replacing the motor or gearhead is usually the best solution.
Becoming a better troubleshooter involves understanding the product more deeply. Explore related posts to enhance your knowledge of AC motors or reach out to our technical support engineers for assistance.
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