3 Phase alternating current Motor Troubleshooting
This is a BRIEF COURSE INTENDED TO ACQUAINT YOU WITH BASIC electric MOTOR TROUBLESHOOTING AND testing
If you have not been trained in how to work safely near live electrical circuits, do not attempt to measure line voltages. Find someone who has been trained in electrical safety and let him or her take voltage readings. Great care is needed to eliminate the possibility of DEATH or serious injury.
ALWAYS disconnect the power and verify all parts are dead before touching or handling any parts of electrical equipment. Lock out and tag out all electrical circuits. Test for voltage before touching any components. Check for and eliminate the danger of “stored energy” caused by raised or spring-loaded equipment.
The basic test equipment you will need to troubleshoot AC motors includes:
AC clamp-on ammeter
Voltage is the term used to describe the magnitude of the Electro-Motive Force, or in other words, the pressure at which electrons are being forced through a circuit.
It’s current that kills, but it’s voltage that really establishes the level of danger involved in working with electricity. Knowing the voltage you are working with enables you to take appropriate steps to safeguard yourself and those working near you from electrocution.
Typical Delta/Wye Transformer Connections
Motors run while connected to the Secondary windings of a transformer bank. The transformers design and interconnection determines what voltage will be applied to your motors, as well as what voltage will be present from each line conductor to earth ground. (see a,b,c, neutral above)
In Industrial plants today, the predominant voltage is 380 volts, Three-phase, fifty-cycles. Most motors are rated at 400 volts.
The voltage applied to your motors should not vary more than ten percent (plus or minus) from the motors rated voltage. That means a motor rated for 400 volts should have voltage applied that is between 360 and 440 volts. While motors will operate to their rated capacity at the lower end of the voltage tolerance, their performance and overload capacity will be much better at the higher end of the range. Higher voltage is generally better for performance and less troublesome than lower voltage.
Effects of Voltage Unbalance
If the applied voltages are unbalanced, the motor in question may need to be de-rated. Voltage imbalance that is more than five percent of the line-to-line voltage will greatly reduce a motor’s mechanical output and dramatically increase its internal heating.
The graph above shows how bad things start to happen when the line-to-line voltages are unbalanced beyond 3 to 5 percent.
Basic voltage tests to identify applied voltage (motor is not running)
In the process of checking for the presence, and balance, of all three-phase voltages, you may, by process of elimination find a blown fuse. The line that always reads “low volts” is the one with the blown fuse.
Voltage tests to verify “Line to ground” potentials and to isolate a blown fuse.
The blown fuse should read only a few “milli-volts” to earth ground. The good fuses should read normal line to ground potentials.
Continuity test to confirm blown fuse
Be aware that in the event of a heavy fault current, “carbon tracking” can occur within the blown fuse and produce a volt reading that can confuse a very sensitive voltmeter and you. So a final “Continuity Test” should be performed. Be certain to pull the disconnect to its OFF position before doing your continuity test. Be sure to repeat your first series of tests on the TOP END of all three fuses to verify that the power is off.
Any blown fuse will read a high resistance.
Ground Fault Tests
AC motor windings are NOT to be grounded.
There are to be no electrical connections from electrical windings to earth ground.
(Exception: alternators, some transformer windings)
The unit of measurement for electrical resistance is the ohm (Ω)
Electrical Resistance is a numeric value assigned to the relative inability of materials to transfer electrons from one molecule to the next. One Ohm is the amount of resistance that lets 1 Volt make 1 Amp of current to flow in a conductor.
One Meg-Ohm equals 1,000,000 ohms (high resistance)
One Milli-Ohm equals 1/1000 ohm (low resistance)
All windings, whether connected to earth ground or not have “Ground Wall Insulation”.
Ground Wall Insulation keeps the electricity from getting to earth ground in the wrong place. If electricity gets to earth ground too soon, it doesn’t do the work we want it to do.
Your “Megger Testing” is to verify that no damage has been done to the “Ground Wall Insulation”. (Ref: Ground Wall Insulation is the Blue insulation in the figure above)
A Meg-Ohm meter will use a High Voltage Potential (usually 500 or 1000 Volts) to “Push” or “Stress” the limits of electrical insulation. The high voltage is required in order to give you a meaningful measurement of the High Resistance. (Meg-Ohms, Millions of Ohms) that should exist across the “Ground Wall”. A Meg-Ohm Meter is used to find “failures” in electrical insulation.
When using a Meg-Ohm Meter you connect one lead to the winding, and the other lead to the frame of the unit under test. When you activate the Meg-Ohm Meter you are impressing 500, or 1000 volts of pressure against the “Ground Wall Insulation”. You are trying to force electrons to get through the Ground Wall Insulation.
Megger Testing an installed motor
If your motor is connected to an “electronic drive”, disconnect the wiring from the drive terminals before doing your megger testing.
A winding can burn off, or “open” when a large fault occurs. Be sure to check all three lines to the motor before saying the motor and wiring is OK.
You can use an Ohm Meter to find what wires are connected to specific circuits. In the process you can determine the resistance of the circuit in “Ohms” and make comparisons of equivalent circuits.
In the example above the Ohm Meter is being used to measure the resistance on a single coil group.
An Ohmmeter uses a Low Voltage Potential, (Usually 1 to 3 volts) to measure electrical resistance or check “continuity”.
Every motor has distinct coil groups that are connected internally in the motor to comprise the phase windings. In troubleshooting a motor you may need to verify that the motor lead numbers are correct, and that there have been no electrical faults that create “short circuits” between the different phases.
Every good electrician knows the lead numbering sequence of three phase motors, or he has diagrams available for ready reference.
Continuity Testing of Motor Windings
The ohmmeter should show continuity when connected to #1 and #4 because they are the opposite ends of a circuit in the motor. Your ohmmeter will give you a reading.
In this example the ohmmeter is connected to different sections of the winding, where no connection should exist. If the winding is OK, in this instance, the ohmmeter should indicate a high resistance because there is no circuit.
Any defects in the winding indicate that the motor will need to be removed from service and evaluated for repair or replacement.
TESTING MOTORS WITH A PREVIOUS HISTORY OF SUCCESSFUL OPERATION
If the motor has been operated successfully, problems such as incorrect hook-up or internal misconnection can be ruled out immediately.
Before proceeding, Read and record pertinent motor nameplate data.
HP, RPM, Rated voltage, Rated current, Frame size, Enclosure
Look over the installation and inspect the motor for any obvious defects that would prevent safe operation and testing. Look for:
As evidenced by smoke deposits or copper particles in J-box
Loose connections in J-box (melted wire nuts, burned insulation, arcing to cover or box)
Broken or missing parts (Pulleys, belts, covers, etc.)
PROBLEM: MOTOR WILL NOT START
Check to make sure all three phases are present at the control unit. (Use AC volt meter)
Three phase motors will not start on single phase current.
If the main fuse is blown, DO NOT apply power to the motor until you have replaced defective fuses and checked for any ground faults in the motor and its wiring.
Check for ground faults:
Disconnect the motor’s power source. (Open the disconnect switch and verify with your voltmeter that the power has been disconnected “downstream” of the switch)
Use the megohmmeter to measure the insulation resistance of all windings to earth ground.
Take care to isolate the motor from any “electronic controls” such as soft starters and frequency drives before using the meg-ohm meter. You may have to undo the motor leads at the controls terminals before testing. The voltages from a Meg-ohm meter could possibly damage the controls.
Any “grounded” conditions must be corrected before power is applied to the motor.
Check to see that the motor will turn over by hand. Remove any obstructions or fee up the jammed machine if that condition exists. Find out now if the motor bearings are rough or wiped out.
Inspect Motor Connections
Inspect electrical connections to the motor in the control and in the motor’s J-box.
Correct any loose or broken connections.
Check for signs of heating or “resistive connections”
If the main fuses are OK, all ground faults have been removed, and the machine will rotate by hand, prepare to attempt a restart.
Attempt a restart:
Position yourself away from rotating equipment, with the motor remaining in your sight. If necessary, get help initiating the start signal, so you can observe the motor during start-up. Instruct your helper so he is prepared to quickly shut down the motor at your signal if a problem develops.
Set your (digital) clamp-on ammeter to its highest range and attach it to one of the lines feeding the motor. Be aware of what the motors full load amp rating is.
(Be careful if you are using an analog ammeter. High inrush currents could damage the meter movement)
Close the disconnect switch, and start the motor.
Watch for rotation to begin, being careful to immediately disconnect the motor if it fails to rotate.
If the motor fails to rotate when the power is applied, disconnect the power and resume testing to determine the problem.
As the motor accelerates, observe rotation, and listen to the sound of the motor. Remain prepared to quickly shut the motor off if does not continue to accelerate smoothly to full speed. Be careful to notice if the motor “hangs” at a fixed speed and fails to finish its acceleration. If the acceleration to full speed does not occur smoothly, immediately shut down the motor and proceed with other testing.
While the motor is accelerating, check your ammeter so you can observe the starting currents diminish as it reaches full speed.
When the amps fall off to normal operating levels, quickly move your ammeter to each Line in order to check all three phases. Verify that the motor currents are “even”, and that they do NOT exceed the motors rated amperage. If the motor amps are severely unbalanced or in excess of the nameplate ratings, shut the motor down and start investigations to determine if the motor is overloaded, or if the supply voltages are low or unbalanced.
In the event of unbalanced currents, check the applied voltage as near to the fully loaded motor as is safe, to verify that the applied voltages are even. Motor voltage unbalance should not exceed 5% of line voltage. For a 460 volt motor, that is 23 volts variance line to line. If you cannot read the voltage close to the motor, consider the length of the run and size of wire to get a grip on actual voltage drop at the motor.
Any voltage unbalance will significantly reduce the output capacity of a motor. Current imbalance over the 5% range dictates that the motor’s load be reduced to compensate for the lost power.
If the line voltages are even and the current imbalance still exceeds 10%, the winding is probably shorted and the motor should be repaired.
In the event of a motor running overcurrent, disconnect the load and restart the motor. With the motor running unloaded, verify that the “No Load” currents are within the following guidelines.
900 – 1200 rpm motors Approximately 50 to 70% Full Load amps(Some may be higher)
1800 rpm motors Approximately 30% Full Load amps
3600 rpm motors Approximately 20 to 30% Full Load amps
If the No-Load currents are reasonably balanced, and within the suggested limits, the motor is probably being overloaded. Reduce the load or install a larger motor.
If the uncoupled motor’s No-Load currents significantly exceed the above guidelines, or the currents are grossly uneven, it is safe to assume that the windings are shorted and the motor is in need of repair. A shorted winding will also produce a “labored”, “whining” sound that is quickly identifiable to the experienced ear. Of course, watch for smoke…..
Test and inspect controller (Soft start, Adjustable Frequency Drive)
If no output is read from the controller, determine if the problem is in the control circuit and correct it.
Is the controller tripped? Modern Variable Frequency Drives have some pretty sophisticated troubleshooting aids.
If the VFD has “faulted”, proceed to determine the cause and correct the problem.
Over current (Excessive load over a period of time)
Over voltage (Overhauling type of load)
Over Heat (High ambient temperatures-Overloading)
Are the thermostats in the motor tripped (N/C contacts)?
Attempt a reset.
Make sure the controller is getting a start signal (N/O contacts)
Make sure there is not a STOP signal (N/C contacts)
If the controller isn’t functioning by this point, it’s pretty safe to say that the controller is defective.
PROBLEM: OVERLOAD RELAY TRIPS; OR FUSES BLOW WHEN MOTOR STARTS
A starting current that is too high, or lasts too long, will causes tripping of the overload relay or blow fuses. Motor starting currents that don’t diminish quickly will be too high to be sustained by normal overload protection. The motor and its associated load must accelerate quickly. If acceleration is delayed due to increased load nuisance, tripping can be the result.
Test all windings for ground failure using the megohmmeter. Any grounded windings must be repaired before power is applied to the motor
Mechanical problems with the motor or driven equipment.
Mechanical problems such as worn bearings or other problems with the motor or machine could cause a mechanical overload.
Determine if the problem is in the motor itself or in the driven equipment. Uncouple the motor and turn the rotor by hand. Check for bad bearings or other mechanical binding.
If the rotor turns freely, attempt a restart as outlined earlier, and check the no load currents in comparison to the amperage guidelines stated earlier. If the motor starts and runs within those limits, the problem is most likely in the driven equipment and not in the motor.
PROBLEM: MOTOR RUNS AT LOWER THAN RATED RPM
AC squirrel cage motors run at a continuous speed, unless they are a special multi-speed design, or if they are connected to a Variable Frequency Drive.
If you have a normal motor installation, and the speed of the load varies, check your motor currents to see that the motor isn’t being overloaded.
In most cases you will find the motor is running as it should, but slipping belts or other mechanical problems are letting the load vary in speed.
There is however an instance where the motor currents don’t seem excessive, the belts are tight enough, but the motor doesn’t seem able to pull the load. These are RARE instances, but if these are the facts, then you can suspect a bad rotor.
Broken rotor bars will greatly reduce a motor’s torque and still allow the currents to remain “reasonable” , if the motor is not severely overloaded. The testing described here requires thorough preparation and assistance from another mechanic or electrician.
The motor winding can be “single-phased” to test the rotor. That is to say that you will disconnect one phase of the motor winding, and energize the remaining two phases. Under these conditions, motors with broken rotor bars will exhibit a “cogging” effect while the shaft is being rotated by hand. The current being applied to the stator winding will also fluctuate correspondingly to the rotor’s “cogging”.
This test will produce potentially damaging currents, so it must be conducted quickly and with great care for your personal safety.
This test should NOT be conducted using line voltages on motors greater than 100hp.
SINGLE PHASE ROTOR TEST
1. With the power disconnected, open one phase at either the motor starter, or the motor’s J-box.
2. Disconnect the motor from its load. (Remove belts/open coupling)
3. Attach an AC Ammeter to one of the connected motor leads.
4. Set the ammeter to a scale that is 200 to 300 percent of the motor’s full load current.
5. Close the Disconnect Switch.
6. At your direction, have your assistant apply power to the motor. Immediately rotate the motor shaft by hand while feeling for a pronounced “cogging” effect. While doing so, you or your assistant should observe the ammeter for deflections in its reading.
7. Shut off the power. The entire test sequence should be accomplished in less than ten seconds to avoid blowing fuses or damaging the motors windings.
If the shaft turns freely, with very little movement of the ammeter, you can conclude that the rotor is OK.
If you find cogging and variable motor currents, the rotor has open bars requiring the motor to be repaired or replaced.
NEWLY INSTALLED MOTORS
The troubleshooting procedures outlined previously all apply to motors that develop problems after having been in operation for sometime. Now we will discuss troubleshooting motors that give problems during or shortly after installation.
PROBLEM: NEWLY INSTALLED MOTOR DOES NOT START AFTER INSTALLATION
If a new motor or newly repaired motor malfunctions the first time it is put in service:
Check the control unit’s input and output connections.
Are the incoming line connections made at the correct points in the controller?
Are the output connections made at the correct points?
Are all the connections tight and secure?
Make sure that all three phases are present at the input of the motor controller.
Measure the Line voltages to verify that they are present and evenly balanced.
Make sure the supply voltages are correct.
Check the motor nameplate to verify that the line voltages agree with the nameplate rating of the motor.
Check the motor lead connections to be sure they are correct and tight.
Inspect the line and motor lead connections in the motors J-box.
Are the connections tight and well insulated?
Are the line connections made to the appropriate motor leads?
Are all of the motor leads securely and properly connected?
Make sure the controller is functioning properly.
In the case of an electro-mechanical motor starter, does the contactor close securely?
In the case of a Variable Frequency Drive, does output result on the initiation of a start signal?
Determine if the overcurrent devices are properly sized and properly adjusted.
Is the overload tripped?
Is the overload correctly sized for the motor?
In the case of an Adjustable Frequency Drive, Is the drive faulted?
PROBLEM: NEWLY INSTALLED MOTOR RUNS IN REVERSE DIRECTION
To reverse the rotation of a three-phase motor, switch any two incoming lines.
Swapping line connections is the simplest option, but in the case of large motors where the incoming lines are too large and difficult to move easily, careful study may be needed to decide how to rearrange the motor leads inside the controller. Special reduced voltage starting arrangements complicate reconnection.
If you have more than three motor lead conductors connected to your motor starter, call your friends at Electrical Equipment Company for assistance. We’ll be glad to help!