Method for Testing the Condition of AC Contactor Coils with a Multimeter

The main faults of AC-Schütz coils are coil open circuit or poor coil insulation. These can be detected separately using the resistance range and insulation resistance range (megohm range) of a multimeter. Below are the detailed operation steps:

1. Safety Preparation
2. Power Off and Verify：Remove the AC contactor from the circuit, or disconnect the power supply of its control circuit. Confirm the absence of voltage with a test pen to prevent electric shock or damage to the multimeter.
3. Clean Terminals：Wipe the two wiring terminals of the coil with a dry cloth to remove dust, oil stains or oxide layers, avoiding measurement errors caused by poor contact.
4. Multimeter Selection and Range Setting
5. Resistance Range Measurement (Core Step, Testing Coil Continuity)

The AC contactor coil is a pure resistive load. Coils of different voltage levels (z.B., AC24V, AC36V, AC110V, AC220V, AC380V) have significant differences in resistance values. Je höher die Spannung, the more the number of coil turns and the larger the resistance value. Select the ohm range (Ω) of the multimeter, estimate the resistance value based on the coil voltage, and choose an appropriate range (select the 200Ω/2kΩ range for low-voltage coils, and the 20kΩ/200kΩ range for high-voltage coils).

– Touch the two wiring terminals of the coil with the red and black test leads of the multimeter respectively, and observe the value displayed on the multimeter.

Judgment Criteria

2. Insulation Resistance Measurement (Optional, Testing Coil-to-Ground Insulation)

The purpose is to test the insulation performance between the coil winding and the contactor housing (iron core) to prevent electric leakage.

– Select the megohm range (MΩ) of the multimeter. If the multimeter does not have this range, a dedicated megohmmeter can be used.

– Touch any terminal of the coil with one test lead, and touch the metal housing or iron core of the contactor with the other test lead.

– Observe the reading on the multimeter.

Judgment Criteria: The insulation resistance value should be ≥ 1MΩ. If the reading is close to 0Ω, it indicates poor coil insulation, which may lead to electric leakage or short circuit when powered on.

III. Common Abnormalities and Fault Analysis

1. Infinite Resistance: The most common fault, mostly caused by long-term overheating burnout of the coil or oxidation and detachment of wiring terminals. The coil or contactor needs to be replaced.
2. Excessively Large Resistance Deviation: Compared with the resistance of a coil of the same type of contactor, if the deviation exceeds ±20%, there may be an inter-turn short circuit of the coil, resulting in insufficient pull-in force and frequent pull-in and release of the contactor when powered on.
3. Low Insulation Resistance: The coil is damp or the insulation layer is aged. It needs to be dried or replaced; ansonsten, it is prone to causing electric leakage accidents.
4. Supplementary Instructions
5. When measuring resistance, there is no need to distinguish the positive and negative poles of the coil terminals, because AC coils have no polarity.
6. In the absence of a coil of the same type for comparison, empirical values can be referred to: the resistance of an AC220V coil is usually several hundred ohms to several thousand ohms, that of an AC380V coil is usually several thousand ohms to more than ten thousand ohms, and that of an AC24V/36V coil is usually several tens of ohms to several hundred ohms.
7. Multimeter measurement can only judge the electrical performance of the coil. If there is a fault in the mechanical parts of the contactor (such as the iron core and spring), manual pull-in testing is required.
8. Brand-Specific Instructions (Schneider, Chint)

– Schneider contactors (z.B., LC1 series): The coil terminals are clearly marked, and the resistance values have high consistency. If the measurement deviation is large, it is highly likely to be an inter-turn short circuit.

– Chint contactors (z.B., CJX2 series): Most coils are encapsulated. If the resistance is infinite, they are basically not repairable, and the coil or contactor should be replaced directly.

In addition to the multimeter, tools for testing the condition of AC contactor coils can be divided into three categories: specialized electrical testing tools, general auxiliary tools and on-site simple testing methods. The testing is focused on three dimensions: coil continuity, insulation performance and pull-in function. The detailed description is as follows:

1. Specialized Electrical Testing Tools
2. Megohmmeter (Insulation Tester)

Application Scenario: Accurately test the insulation performance between the coil winding and the contactor housing (iron core). It is more stable and has a wider range than the megohm range of a multimeter, suitable for industrial-grade insulation testing.

Operation Method

1. Disconnect all connections between the coil and the circuit, and clean the coil terminals and the contactor housing.
2. Select a megohmmeter of the corresponding range (commonly used 500V or 1000V range; select 500V for low-voltage coils).
3. Connect the “L” terminal of the megohmmeter to any terminal of the coil, und die “E” terminal to the metal housing of the contactor. Shake the handle of the megohmmeter at a constant speed (about 120r/min).

Judgment Criteria: An insulation resistance ≥ 1MΩ is qualified; if the reading is close to 0Ω, it indicates that the coil insulation layer is aged, damp or broken down, with the risk of electric leakage.

Brand Compatibility: Applicable to coils of Schneider LC1 series, Chint CJX2 series, usw., especially suitable for insulation testing of old equipment.

2. Clamp Ammeter

Application Scenario: Test the operating current of the coil after power-on to judge the inter-turn short circuit fault of the coil (the multimeter cannot directly measure the operating current).

Operation Method

1. Apply alternating current of the rated voltage to the coil (it must match the coil voltage, such as AC220V, AC380V).
2. Adjust the clamp ammeter to the AC current range, and clamp one power supply wire of the coil.

Judgment Criteria: A deviation of ≤ ±10% between the measured current and the rated current of the coil is normal; if the current is significantly larger (exceeding 20%), it indicates an inter-turn short circuit of the coil (the number of winding turns decreases, the impedance reduces, and the current increases).

Notizen: Ensure the accuracy of the coil supply voltage during testing to avoid misjudgment due to excessively high or low voltage.

3. Contactor-Specific Tester

Application Scenario: Batch testing and accurate judgment of coil performance. It can simultaneously measure parameters such as coil resistance, pull-in voltage and release voltage, suitable for quality inspection in electrical maintenance workshops or production lines.

Operation Method: Connect the coil terminals of the contactor to the corresponding interfaces of the tester, select the testing mode (coil testing), and the tester will automatically complete the continuity, resistance and pull-in function tests.

Judgment Criteria: If the tester displays “qualified”, the coil is normal; if it prompts “offener Stromkreis”, “abnormal resistance” oder “pull-in failure”, there is a corresponding fault.

Brand Compatibility: Most specialized testers on the market support mainstream brand contactors such as Schneider and Chint, and can directly select the corresponding model library to match parameters.

4. Insulation Resistance Tester

Application Scenario: High-precision insulation testing. It is more intelligent than a megohmmeter, can automatically record data and generate reports, suitable for occasions with high insulation requirements (such as explosion-proof and humid environments).

Core Advantage: It can set the test voltage and duration, avoid operation errors of manual shaking of the megohmmeter, and the test results are more reliable.

1. General Auxiliary Tools (Combined with Power Supply Testing)
2. Matching Voltage Power Supply + Step-Down Transformer (On-Site Simple Testing)

Application Scenario: Quickly judge whether the coil can pull in normally, which is the most commonly used non-instrument testing method on site.

Operation Method

1. Prepare a power supply consistent with the rated voltage of the coil (z.B., use 220V mains supply for an AC220V coil, and 24V switching power supply for an AC24V coil).
2. Disconnect the original circuit of the coil, and directly connect the positive and negative poles of the power supply (AC has no polarity) to the two terminals of the coil.

Judgment Criteria

If the contactor makes a clear pull-in sound and the iron core pulls in, the coil function is normal;

If there is no pull-in sound and the iron core does not act, the coil is open-circuited;

If the pull-in force is weak and there is a buzzing abnormal sound, it is highly likely that there is an inter-turn short circuit of the coil or voltage mismatch.

Notizen: The test time should not be too long (≤10 seconds) to avoid overheating and burnout of the coil due to long-term power-on; low-voltage coils are strictly prohibited from being connected to high-voltage power supplies.

2. Oscilloscope

Application Scenario: Analyze the voltage/current waveform of the coil after power-on to judge hidden faults (such as intermittent inter-turn short circuit and poor coil contact).

Operation Method: Connect the oscilloscope probe to both ends of the coil, apply the rated voltage, and observe whether the waveform is stable.

Judgment Criteria: The current waveform of a normal coil is a stable sine wave; if the waveform is distorted and has spike pulses, it indicates an inter-turn short circuit or poor contact of the coil.

III. On-Site Simple Testing Methods (Without Professional Tools)

1. Test Pen

Application Scenario: Roughly judge whether the coil is powered on. It cannot directly determine the condition of the coil, and is only used for auxiliary troubleshooting.

Operation Method: When the contactor control circuit is powered on, touch the two terminals of the coil with the test pen respectively.

Judgment Criteria: If both terminals light up (AC circuit), it indicates that there is voltage input at both ends of the coil; if the contactor does not pull in at this time, it can be determined that the coil is faulty (open circuit or short circuit).

Limitation: It cannot judge coil insulation and inter-turn short circuit, and can only assist in locating the fault point of “no pull-in when powered on”.

2. Acousto-Optic Electroscope

Application Scenario: Similar to a test pen, but with higher sensitivity, it can detect the power-on status of low-voltage coils (such as AC24V and AC36V coils).