How to ohm out a motor

Overview

Ohm-ing out a motor is a fundamental diagnostic technique used to assess the integrity of its electrical windings. It involves measuring the electrical resistance between different points of the motor using a multimeter. This process helps identify common problems such as open circuits (broken wires within the windings) or short circuits (where insulation has failed, allowing current to bypass the intended path). By comparing the measured resistance values to expected values, technicians can determine if the motor's windings are functioning correctly or if they are damaged and require repair or replacement.

What is Resistance and Why Measure It?

Resistance is the opposition to the flow of electric current in a material. It is measured in ohms (Ω). In an electric motor, the windings are made of conductive wire (usually copper) wrapped around a core. These windings are designed to have a specific, low resistance. Measuring this resistance can reveal several types of electrical faults:

• Open Circuit: If a winding is broken internally, the multimeter will show infinite resistance (often displayed as 'OL' or '1'). This means there is no conductive path.
• Short Circuit: If the insulation between turns of wire or between different windings fails, the resistance will be significantly lower than expected, sometimes close to zero ohms. This indicates that current is taking an unintended, low-resistance path.
• Ground Fault: This occurs when a winding comes into contact with the motor's casing (ground). Measuring resistance between a winding terminal and the motor's metal frame can detect this. A low resistance reading indicates a ground fault.

Tools Needed

The primary tool required for ohm-ing out a motor is a multimeter. Modern multimeters are digital and offer a wide range of functions, including resistance measurement. Ensure your multimeter is in good working order and has fresh batteries. You'll also need the motor's specifications, if available, to compare your readings against. Safety gear, such as insulated gloves and eye protection, is also recommended.

Step-by-Step Guide to Ohm-ing Out a Motor

Follow these steps carefully to safely and effectively ohm out a motor:

1. Safety First: Disconnect Power

This is the most critical step. Ensure the motor is completely de-energized. Turn off the power supply at the breaker or disconnect switch. If possible, lock out the power source to prevent accidental re-energization. Verify that no power is present using a voltage tester before proceeding.

2. Identify Motor Terminals

Locate the motor's connection box or terminal block. The configuration of terminals depends on the motor type (e.g., single-phase, three-phase) and its design. For simpler motors, you might see two or three main power leads. For more complex motors, there might be multiple leads corresponding to different windings or speed configurations.

3. Set Your Multimeter

Set your multimeter to the resistance (Ω) function. Select an appropriate range. If you don't know the expected resistance, start with a higher range and decrease it if necessary. For most motor windings, the resistance will be relatively low, typically less than a few ohms, but this can vary widely.

4. Measure Resistance Between Windings

Place the multimeter probes firmly on the motor's winding terminals. Perform the following measurements:

• Between winding leads: For a simple two-lead motor, measure the resistance directly between these two leads. For a three-lead motor (common in three-phase motors), measure the resistance between each pair of leads (Lead 1 to Lead 2, Lead 1 to Lead 3, Lead 2 to Lead 3). All readings should be very similar.
• For multi-speed motors: If the motor has multiple speed settings, consult the motor's wiring diagram to identify the terminals for each winding and measure them accordingly.

5. Check for Ground Faults

Measure the resistance between each winding terminal and the motor's metal frame (the casing or ground connection). Ideally, the resistance should be very high, approaching infinite (OL). A low resistance reading here indicates a short to ground, which is a serious fault.

6. Interpret the Readings

Compare your measurements to the manufacturer's specifications. If specifications are unavailable, consider these general guidelines:

• Consistent, Low Resistance: If readings between winding pairs are consistent and relatively low (e.g., 1-10 ohms, depending on the motor), the windings are likely intact.
• Infinite Resistance (OL): If you measure infinite resistance between winding terminals, it indicates an open circuit in one or more windings. The motor is likely faulty.
• Very Low Resistance (Near Zero): Readings close to zero ohms between winding terminals suggest a short circuit within the windings.
• Low Resistance to Ground: If resistance between a winding and the motor frame is low, there is a ground fault.

Interpreting Results and Next Steps

The results of your ohm test provide valuable insights into the motor's electrical health. A motor with open or shorted windings, or a ground fault, will likely not run correctly and may pose a safety hazard. In most cases, motors with these types of winding failures are not economically repairable and will need to be replaced. However, for very large or specialized motors, rewind services might be an option. Always consult with a qualified electrician or motor repair specialist if you are unsure about your readings or the appropriate course of action.

Common Motor Types and Expected Resistance

The expected resistance values can vary dramatically based on the motor's size, type, and application. For example:

• Small Appliance Motors (e.g., fan motors): May have resistances in the range of tens to hundreds of ohms.
• Larger Industrial Motors (e.g., three-phase induction motors): Typically have very low winding resistances, often less than 1 ohm, especially for larger horsepower ratings.

The key is consistency between windings (for multi-lead motors) and comparison to known good values if possible. A sudden, significant deviation from expected values is usually indicative of a problem.