Three-Phase Testing Procedure
Electric motors are notoriously difficult to troubleshoot. When a motor fails to start, runs hot, continually trips or sputters, there are many possible causes. Some businesses may respond to the problem by simply replacing the motor entirely. However, this is not a cost-effective solution — most electric motor problems are entirely fixable with solutions that cost significantly less than a new motor. But how do you determine how to fix a motor cost-effectively?
While electric motors may be complex, diagnosing them doesn’t have to be. Understanding the basics of electric motors can help you understand where the problem may be coming from, while proper diagnostic tools can help you identify and clarify the issue. In this article, we’ll specifically discuss three-phase systems and how to diagnose them when problems arise.
Table of Contents
About 3-Phase Systems
Phase systems are alternating current power supplies that are defined by the number of phases in the power supply. Single-phase power supplies one phase at 120 volts, while dual-phase or split-phase power consists of two alternating currents supplied by two wires. Three-phase power is a type of power circuit that is characterized by three single-phase AC supplies. The system shares a return path by separating each phase by 120 degrees, resulting in constant power over each cycle and more power overall. Compared to single-phase power, three-phase power arrangements supply 1.732 times more power on the same current, resulting in a more economical system overall.
Three-phase systems are designed differently to fit different needs. For example, a wye configuration may be used in cases where a power supply needs to feed both single-phase and three-phase loads, such as lights and heaters, respectively. The amount of power may also vary. Most commercial buildings use 208 Y / 120 V setups to improve flexibility to feed both high-power and low-power loads, while industrial plants use the 480 Y / 277 V setup to maximize the amount of power available for high-powered machinery.
Types of Testing For 3-Phase Motors
If a three-phase motor is exhibiting problems, such as failure to start, overheating and inconsistent supply, there are several diagnostic tools and methods at your disposal. These tools and methods are discussed below. Before testing, however, be sure to take appropriate safety precautions. These include:
- Wearing protective safety gear: This safety gear may include grounding straps, gloves, and any other relevant protective gear for the environment.
- Having all tools readily available: Some common diagnostic tools include ubiquitous multimeters, clamp-on ammeters, temperature sensors and oscilloscopes. Having these tools available will help you avoid leaving the motor unattended.
- Disconnecting the motor from power: When you are ready, move the transformer’s motor disconnect switch to take it off of power. Be careful to make sure that the power is actually turned off — on some motors, the disconnect switch is the same as the on/off switch, so switching the disconnect switch to the on position will power the motor. Also, be sure to disconnect all equipment and wiring that is not going to be included in the testing process.
- Discharge before and after testing: Before beginning testing and after each electrical test, be sure to discharge the motor, as it has an inherent amount of capacitance. This can be accomplished by shunting conductors to ground and each other before reconnecting.
- Check the nameplate: The nameplate or motor specifications provide valuable information about the motor, such as the intended amperage of the motor. This information can be used to assess the health of the motor compared to its intended design.
At this point, prepare your multimeter for testing. This includes setting the multimeter to detect AC voltage and setting the voltage range to a reasonable level based on the specifications of the box. The next several tests primarily use this tool, so we will explain how to check a three-phase motor with a multimeter.
1. General Inspections
The most basic inspection is a visual inspection. Once the motor is removed from power and you are prepared to begin inspection, remove the cover of the motor. Once this is removed, you can start to check the motor for visual indications of damage. Some things to look for during this process include:
- General Damage: General damage tends to be easy to spot. This may appear in the form of burn marks or indentations. Check all over the motor for any indications of heat or environmental damage.
- Shaft Condition: Manually rotate the motor shaft to assess its condition. This should be easy unless the motor is especially large. The shaft should rotate smoothly, with no catches or loose parts. Newer motors may have some difficulty turning due to tight tolerances, disuse or ambient moisture, which will need to be addressed with oiling and further inspection. However, older motors may have more serious obstructions that need repair or replacing.
- Connection Quality: Look at all the connections within the motor for indications of wear or damage, and assess any wires outside of the motor for potential breaks. Any broken wires should be handled and replaced with care.
Once the motor has undergone a general inspection, double-check your inspection tools and start troubleshooting the electrical properties of the motor.
2. Continuity Tests
Continuity testing tests the resistance between two points. If there is low resistance, the two points are electrically connected. If there is higher resistance, the circuit is open. The earth continuity test determines whether the motor is connected to ground.
To complete the earth continuity test, set the multimeter to continuity mode. Once this is done, place one point on the frame of the motor and the other point on a known connection to the earth, preferably an area close to the motor installation. A good motor should result in a reading that is less than 0.5 ohms. If the value is greater than 0.5 ohms, however, this indicates that the motor’s insulation is failing and has the potential to cause an electric shock. Determining the causes of this failure may require further testing.
3. Power Supply Test
The next test that should be completed is the power supply test. This checks that the incoming power supply is as expected and meets the design specifications of the motor. The power supply test can be done by checking the voltage being applied to the motor with a multimeter. Compare this with the specifications as noted on the nameplate. If the applied voltage is significantly lower or higher than specified, this may be one source of your problems.
In addition to this test, check that the terminal for the power supply is in good condition. Damage and poor connections may also be at fault for any variances or performance issues.
4. AC Motor Winding Continuity Test
Next, examine the inside of the motor and the wires involved in the three-phase current. Set and calibrate your multimeter for voltage and find the six wires of the three-phase motor.
When looking at the box, you should see six wires, three on each side. There should be terminals on each side of the box that these wires are connected to. One side will have terminals labeled L1, L2 and L3 or Line 1, Line 2 and Line 3. The other side will have terminals labeled T1, T2 and T3 or Load 1, Load 2 and Load 3. L terminals indicate line wires with incoming current, while T terminals indicate outgoing wires. The exception is European motors, which will have the designations U, V and W. These wires should be tested to determine the health of the motor’s power supply. This can be tested using the following methods:
- No Power Incoming Test: To test the incoming voltage, place the multimeter probes on different permutations of the L terminals while the box is powered down. Take a reading for the L1 to L2 connection, the L1 to L3 connection and the L2 to L3 connection. These readings should be the same if the motor is working properly. For a 230/400V system, the expected voltage should be 400V between each of the three-phase supply lines.
- Line to Neutral Test: If there is an available neutral terminal, place one multimeter probe on that and the other on each of the line terminals. The voltage reading should be half of whatever voltage reading was received during the previous test.
- No Power Outgoing Test: This test is similar to the test above, but tests the outgoing voltage. While the box is still off, take a reading between the T1 and T2 leads, the T1 and T3 leads and T2 and T3 leads. In this case, the voltage reading should be zero for each test.
- Power Outgoing Test: Carefully power up the box and repeat the same tests as above, testing each permutation of the T leads. There should be little to no variation between each lead combination.
If the readings differ from the expected results and the power supply test showed no issues, this may indicate problems with the health of the three-phase AC motor. Most often, this indicates that the motor is burned out.
5. Insulation Resistance Test
The insulation resistance test is the next test that must be done to determine the overall health of the motor. This is done by comparing the resistance between each pair of motor phases and between each motor phase and the frame. This can be done with an insulation tester or megger. The tests must be completed as follows:
- Phase Resistance: Take the insulation tester and set it to 500V. Take each end and place it on different permutations of L1, L2 and L3 and record each reading.
- Phase to Earth Resistance: Take the insulation tester, using the same setting, and check each lead from phase to the frame of the motor. The minimum value of insulation resistance should be 1 megohm. If the value is less than 0.2 megohms, replace the motor.
Any errors during this round of testing could indicate problems with insulation, which is a problem when it comes to the safety and functionality of the motor.
6. Running Amps Test
This final test determines how much energy is drawn to drive the motor. More powerful motors will draw more current, measured in amps. Before testing, it is important to check the amperage draw your motor requires — this will usually be on the nameplate.
When you are ready, use the following steps, which guide you through how to measure three-phase current:
- Prepare for Test: Set your multimeter to measure amperes and set it to the correct ampere range for your motor per the specifications listed on the nameplate. You should also be sure to wear rubber gloves during the test to protect yourself from electric shock.
- Turn On the Motor: Turn on the motor and locate the terminals. The positive terminal will be labeled with a plus sign and have a red wire connected to it. The negative terminal will be labeled with a minus sign and have a connected black wire.
- Place the Sensors: Place the negative sensor of the multimeter to the negative terminal of the motor, then place the positive sensor to the positive terminal. Be sure to keep your hands clear of moving parts at all times to avoid injury.
When the sensors are connected, take the ampere reading and turn off the motor. The ampere reading should be within range if it is operating correctly. The ampere reading will not exceed the manufacturer’s specification but should be at or slightly below the given amperage draw. If the ampere reading is well below specification or out of range, this may indicate problems with the motor.
What to Do Next
If you complete your tests and find one or more problems with your motor, there are a few things that you can do depending on the problem at hand. Some issues, like faulty wiring or a damaged shaft, may require replacing the problematic parts. However, more serious problems like insulation issues may require a new motor entirely. If you’re not quite sure what to do or where the problem is coming from, however, it may be worthwhile to call in an electronics repair service to help assess the motor. Global Electronic Services can help.
Contact Global Electronic Services Repair for 3-Phase Testing
At Global Electronic Services, we specialize in industrial electronics repair. We’ve worked with over 60,000 of the largest and most advanced manufacturers and distributors in the world spanning a wide range of industries. No matter if your problem is with an electric motor, servo motor, hydraulic system or pneumatic system, we can help you find a solution.
When you choose Global, you choose high-quality customer service and 24/7 support. Our factory-trained and certified technicians maintain excellent turnaround times of one to five days, and we even offer a two-day rush service. We also provide a 10 percent repair price guarantee.
If you’re interested in having Global work with your three-phase motor, contact us today by phone or use our easy online form to request a quote.