To identify 3-phase motor windings, measure resistance between terminals using a multimeter. Each winding (e.g., U1-U2, V1-V2, W1-W2) should show similar resistance, typically 1-2 ohms for a 7.5 kW motor. Infinite resistance indicates an open circuit. Check continuity to confirm winding groups.
Label or Mark Inspection
One of the most important things when checking a 3-phase motor is to pay attention to the labels or marks that will determine the proper identification of the windings. They usually are stamped on the nameplate or terminal box of the motor and allow the connection of windings in star or delta configuration. For example, a standard 3-phase motor rated for 5 kW with a voltage of 400V at 50 Hz will be marked at the terminals with U1, V1, W1 for the start of the windings and U2, V2, W2 for the finish. These markings not only make it easier to wire up but also provide compatibility with the voltage at which the motor is supposed to operate and the power supply.
An explained example can be found as follows. Let’s consider a motor designed for dual-voltage operation, such as 230V in a delta configuration and 400V in a star configuration. The terminal box in such a motor will usually contain six terminals, each corresponding to a specific winding. Set to run on 400V, the terminals U2, V2, and W2 would be joined to form the star point, while the U1, V1, and W1 would connect to the external power supply. Failure to decode such labels or failure to read them at all would lead to an incorrect connection of the motor, reducing its efficiency or causing immediate damage.
Accurate resistance measurements between terminals can confirm the correctness of the labels and their connection to the windings. Using a digital multimeter set to measure resistance you should find the resistance between U1 and U2, V1 and V2, and W1 and W2 are approximately equal. For instance, a motor with windings designed for high-voltage applications might show a resistance of about 1.2 ohms per winding. Any dramatic deviation from these values—a reading of 0 ohms would indicate a short circuit, while a reading of infinite could mean an open circuit—might signal a problem with the motor’s internal windings or connections.
Visual Examination
Visual inspection is a necessary first step to identify 3-phase motor windings. From this, one will have an immediate idea about the internal build and wiring connection of the motor. A motor intended to operate at two voltages, such as 230V/400V, may have a terminal box with six points of connection. Upon opening such a box, one may observe some connections inside as linking bars that show star or delta. As a general rule, when three terminals are interconnected by a copper bar or jumpers, one can assume star connection, for which the motor is suited for higher voltage applications, such as 400V; no common link among the terminals indicates delta configuration and is usually meant for lower voltage, typically 230V.
The color code of wires inside the motor can also provide some information. In most standards, three different colors such as red, yellow, and blue correspond with the three phases of the motor windings. This color coding is very prevalent in motors whose wiring is standardized. For example, on a 15 kW, 415V-rated motor, the wires connecting to the terminals U1, V1, and W1 are normally color-coded or marked differently to ensure the right phase sequencing. Colors and markings should be carefully compared, since any discrepancy in colors or markings may indicate that the winding arrangement has been previously altered, or that the wiring was not installed correctly and may require additional testing to validate.
Another important component of the visual check is an overall wire and connection condition check. In one case, an industrial 75 kW motor terminal lugs overheated because its connections were loose, showing discolorations and melting. These can also take a toll on the resistance and performance of windings. Similarly, frayed insulation with exposed wires can potentially cause short circuits. Identifying these problems visually before performing electrical tests ensures safety and prevents further damage to the motor or connected equipment.
Continuity Test
One of the best methods for tracing 3-phase motor windings and verifying their integrity involves a simple continuity test. A digital multimeter can be used to test between the terminals to ensure each winding is intact when set to the continuity mode. For instance, on a typical 5.5 kW, 400V-rated motor, you will have continuity between sets such as U1 and U2, V1 and V2, and W1 and W2. Now, if the multimeter beeps or shows a low reading in resistance—usually below 2 ohms for such motor sizes—the winding in that respect is continuous. If there is no continuity—infinite resistance—it would indicate a break in the winding and repairs are needed.
For motors designed for more industrial-sized applications, such as a 75 kW unit operating at 690V, the need for continuity testing becomes even more important because the motor windings have to bear higher electrical stresses. In addition, motors with larger ratings normally have windings of lower resistance, usually in the range 0.1 to 0.5 ohms. At the time of testing, ensure that the connections are clean and free from dirt or corrosion, as these factors can introduce resistance that might falsely indicate a break in continuity. A consistent reading across all three windings confirms that the motor’s internal wiring is intact and balanced, which is crucial for smooth operation.
Continuity tests can also be used in determining some of the winding groups within the motor. A dual-voltage motor rated for 230/400V has six terminals representing the start and end of three windings. Continuity tests between certain terminals will help pair them up correctly. For example, if there is continuity between the U1 and U2, both are on one winding. Likewise, pairs can be identified as V1-V2 or W1-W2. Inappropriate pairing might result in phase imbalance, which could reduce efficiency or cause excessive vibration during motor operation.
Resistance Measurement
It is very important in the identification of the 3-phase motor windings and their condition and balance. Using a digital multimeter set to measure resistance (ohms), one can test between winding terminals. For example, in a 3-phase motor rated at 7.5 kW and designed for 400V operation, the resistance between U1 and U2, V1 and V2, and W1 and W2 should typically be in the range of 1.2 to 1.8 ohms. If the readings are considerably different, such as one winding reading 0.5 ohms and another reading 2.5 ohms, this could indicate shorted turns or damaged insulation, conditions that would call for further investigation or repair.
Full-size motors, such as those for industrial applications, generally have much lower resistance readings due to the increased thickness of the windings. A 50 kW motor operating at 690V would have resistance values in the range of 0.1 to 0.3 ohms per winding. For such motors, high-accuracy measurement of resistance is imperative because even small imbalances promote uneven heating, which reduces efficiency and can lead to early failure. It is desirable to use a four-wire (Kelvin) method of resistance measurement for such motors to avoid errors due to contact resistance or lead resistance in the multimeter itself.
On smaller motors, like a 2.2 kW unit rated at 230V, the resistance values are higher because the windings are thinner. You could read 4 to 6 ohms between each set of terminals. Comparating these readings makes sure the motor windings are balanced. For example, if one winding reads 5 ohms and the others read 4.8 ohms, the small deviation is not a problem. On the other hand, one winding reading of 10 ohms may indicate a fault such as corrosion or internal damage and has the potential for overheating or failure when under load.
Open Circuit Check
The check on the open circuit is one important diagnostic way to ensure that 3-phase motor windings are connected perfectly. You can perform this by setting a multimeter in mode continuities, or in any other function of resistance measurement that provides you with the identification of breaks in the windings. As an example, for a 5.5 kW, 400V designed motor, there would be infinite resistance reading using any meter across open circuit between the terminals like U1 and U2, V1 and V2, W1 and W2. For an assurance that no winding is open-circuited, all three windings will be showing continuity with resistance values that are normally balanced, such as about 1.5 ohms.
Due to the amount of current and voltage handled—especially on larger industrial motors, like a 75 kW motor rated for 690V—open circuit checks become of great importance. In such motors, windings usually have lower resistance values, generally in the range of 0.2 to 0.5 ohms. An open circuit in one winding could cause an imbalance in phases, with high vibration, heating, and low performance being the consequences. For instance, when performing routine maintenance—if one winding reads infinite resistance and the others read normal, there is a break that should be repaired immediately. The early detection of these conditions will prevent major damage to the motor and the equipment connected to it.
Open circuit checks are also important for determining problems with older motors or those that see much use. For example, a 15-year-old 22 kW motor operating in a corrosive environment can develop an open circuit due to either damaged insulation or broken wire connections. In a particular case, infinite resistance was measured across one phase while the remaining two phases had 0.8 ohms of resistance each. A corroded terminal connection was then found and fixed, thus restoring functionality. This shows that open circuit checks are important for maintaining reliability, especially under harsh conditions.
Insulation Test
The insulation test is one of the most significant procedures to guarantee the assurance of electrical integrity for the 3-phase motors and also to prevent possible breakdowns. This test is taken out by a megohmmeter, a device used in measuring resistance between the windings and the frame of the motor. For a standard 7.5 kW motor operating at 400V, its insulation resistance should be above 1MΩ. If the reading is below this threshold, then the insulation has degraded and may subsequently cause a short circuit or leakage current. In new motors, readings are usually considerably higher, sometimes well over 100 MΩ, which also confirms that the windings are in very good condition.
Insulation tests are even more important with industrial motors, such as those rated for 75 kW and 690V, due to the increased electrical stress. That said, for such motors, it is usually higher and recommended to be at least 10 MΩ per kilovolt of operating voltage. For example, a motor rated at 690V should be at least 6.9 MΩ. The lower values—as low as 1 MΩ—predict wear caused by moisture, vibration, or even chemical exposure. Insulation testing on a regular basis will find early signs of deterioration and help avoid sudden catastrophic failures while in service.
Older motors, especially those over 15 years of operational service, are most susceptible to insulation failures. For instance, a 22 kW motor at 415V may exhibit an insulation resistance reading as low as 0.5 MΩ after years of heat and humidity exposure; that is well below the safety threshold and needs urgent service. In such cases, the motor may need to undergo a drying process or rewinding in order to fully restore its insulation integrity. Insulation tests not only confirm the current status of the motor but may also help determine the effectivity of preventive maintenance actions.
Star or Delta Configuration Check
Determining whether a 3-phase motor is connected in star or delta is essential to its proper operation and efficiency. In a star configuration, a common neutral point is used to connect one end of each of the three windings, while leaving the other ends free to connect with the power supply. This means that on a 7.5 kW rated motor, designed for operation at 400V in star, each winding will see only the phase-to-neutral voltage, i.e., 230V. This reduces the current and makes it suitable for higher voltage systems. A connection that is usually denoted in the terminal box by a connection bar between three terminals, which may be labeled U2, V2, and W2.
The delta configuration, on the other hand, is obtained by connecting the end of one winding to the start of another, essentially creating a closed-loop triangle. Assuming the same 7.5 kW motor operating in a delta configuration, each winding sees the full line-to-line voltage, 400V. This naturally pulls more current and is typically used for applications requiring lower voltages. In the terminal box, a delta connection can be recognized by having three pairs of terminals connected together, such as U1 to W2, V1 to U2, and W1 to V2. Being able to identify these physical wiring patterns assures that the motor will match both voltage and load requirements.
For larger motors, for example, a 50 kW unit designed for 690V, star is the default connection since this provides the lower current in higher voltages. These motors often have diagrams on the nameplate or inside the terminal box to assist in wiring. The connections can be specified to start in star and transition to delta when the motor picks up enough speed to support the load applied, for instance. Such a configuration is applied in practice to reduce inrush currents while starting, which could otherwise go as high as eight times the rated current of the motor.
