In the world of CNC machining and industrial automation, the servo motor is the muscle, and the encoder is the nervous system. When your machine crashes or throws an alarm, knowing how to test a servo motor and encoder with a multimeter is a critical skill. It can save you thousands of dollars in unnecessary replacement parts and hours of diagnosing the wrong component.

Whether you are dealing with a standard industrial setup or specific systems like Fanuc or Mitsubishi, the fundamental electrical principles remain the same. This guide will walk you through the static and dynamic tests you can perform using a standard digital multimeter to determine if your servo motor or encoder is dead or salvageable.

Safety First: Preparation Before Testing

Before touching any terminals, safety is paramount. Servo drives contain capacitors that store high voltage even after power is removed.

• Power Down: Turn off the main breaker to the CNC machine.
• Discharge: Wait at least 5 to 10 minutes for the internal capacitors in the amplifier and inverter modules to discharge.
• Disconnect: Physically disconnect the power cables (UVW) and the feedback cables from the drive side to isolate the motor.

Part 1: How to Test Servo Motor Windings

The servo motor itself is a 3-phase AC permanent magnet motor. The most common failures are shorted windings, open windings, or a ground fault (short to the case). Here is how to check them.

1. Testing Winding Resistance (Phase-to-Phase)

Set your multimeter to the lowest Ohms (Ω) setting. You will be measuring the resistance between the three power leads: U, V, and W.

1. Measure resistance between U and V.
2. Measure resistance between V and W.
3. Measure resistance between W and U.

The Verdict: The readings should be very low (usually between 0.5 Ω and 5.0 Ω depending on motor size) and, most importantly, balanced. If you read 1.2 Ω on U-V but 40 Ω or “OL” (Open Loop) on V-W, the motor windings are burnt or broken. If you need a replacement, browse our catalog of servo motors.

2. Testing Insulation (Phase-to-Ground)

This checks if the internal insulation has melted, causing the copper winding to touch the motor casing.

1. Set your multimeter to the highest resistance setting (Mega Ohms) or use a dedicated Megohmmeter for better accuracy.
2. Place the black probe on the Motor Case (ground) or the green ground wire.
3. Touch the red probe to the U, V, and W terminals sequentially.

The Verdict: You want to see “OL” or infinity. A reading below 10 Megohms usually indicates broken insulation or contamination by coolant. This is a common killer of Fanuc servo motors in older CNC mills.

Part 2: How to Test the Servo Encoder

Testing an encoder with a standard multimeter is more limited than using an oscilloscope, but you can verify basic functionality. An encoder failure usually manifests as a “feedback error” or “position deviation” alarm on the control system.

1. Checking Voltage Supply

Most encoders require a clean 5V DC or sometimes 24V DC power supply to operate.

• With the machine powered on (but servo off/emergency stop active), locate the encoder pinout.
• Measure DC Voltage between the +5V (or VCC) and 0V (GND) pins on the connector.
• If you have 0V, the issue might be the encoder cable or the drive’s power output, not the encoder itself.

2. Incremental Encoder Signal Test

If you have an incremental encoder (common on older Yaskawa or generic setups), you can check the pulse output.

1. Set the multimeter to DC Volts.
2. Connect probes to Channel A (or B) and Ground.
3. Slowly rotate the motor shaft by hand.
4. You should see the voltage toggle between High (approx 5V) and Low (approx 0V). If the voltage stays constant at 5V or 0V while spinning, the channel is dead.

Note: Serial/Absolute encoders (like those found on modern Mitsubishi encoders) communicate via data packets. A multimeter shows a fluctuating average voltage (usually around 2.5V), but this does not guarantee the data is correct.

Don’t Forget the Cables

Often, the motor and encoder are fine, but the cables moving in the drag chain have snapped internally. Always perform a continuity test on your power and encoder cables from end-to-end while wiggling them to detect intermittent breaks.

Troubleshooting Summary Table

Frequently Asked Questions (FAQ)