Take a step-by-step approach to analyze motor starting problems
When troubleshooting a faulty electric motor, time (or in this case downtime) is money. This notion was drilled into my head in my early years and has proven itself over and over again. Isolating the problem in the shortest amount of time saves costly downtime (and maybe even your job). The other point that has been permanently burned into my consciousness involves a different type of self-pres...
When troubleshooting a faulty electric motor, time (or in this case downtime) is money. This notion was drilled into my head in my early years and has proven itself over and over again. Isolating the problem in the shortest amount of time saves costly downtime (and maybe even your job).
The other point that has been permanently burned into my consciousness involves a different type of self-preservation: always control the electrical circuit you are troubleshooting by following proper lock-out/tag-out procedures. Observing safe maintenance procedures must always be given proper attention.
For these reasons, it is imperative that the maintenance electrician, technician or engineer uses a troubleshooting methodology that enables them to isolate the problem in the shortest amount of time. Still, the logical process of elimination used to identify the root cause of an ac motor’s malfunctioning can differ. At times it is more effective to start at the motor and work back to the starter; in other instances it’s better to begin at the starter and move towards the motor. My general preference is to begin at the excitation source and work towards the motor.
For the purpose of this article, I’m addressing typical starting problems only. Running problems can be numerous and varied, and in many cases can be traced all the way back to improper motor specification for a particular application.
Starting problems with three-phase ac inductive motors can be grouped into three primary categories: voltage source problems; motor excitation problems, which include the motor starter and ac drive; and application or mechanical problems.
Troubleshooting voltage source problems
Use a process of elimination to isolate and diagnose voltage source problems. Refer to “Voltage source problems flowchart” on page 66, which illustrates the steps of the following troubleshooting procedure:
- Check the incoming power to the motor starter. Using a digital multimeter (DMM) set to voltage mode, verify that all three phases of electrical potential are present. If one or more of the phases is missing, disconnect the primary voltage source (incoming power disconnect switch) and replace the incoming power fuses or reset the motor-control circuit breaker.
- Prior to restoring the primary voltage source, set the DMM to continuity mode (ohms) and verify at the motor starter line contactors that there is not a phase-to-phase short or a phase-to-ground short. If there is a short or ground on the line side of the starter, replace the starter contactor and verify that the short or ground is now clear. Next, verify that the load side of the circuit does not have a phase-to-phase short or a phase-to-ground short. This final continuity check will also verify that the motor line does not have an open phase. If the motor starter line circuits are clear, the problem is on the load side of the starter. Follow the circuit to the load side of the starter.
- Try to reset each of the motor thermal overloads on the motor starter. If any one of the overloads have tripped you will be able to feel when the overload resets while pressing the reset pushbutton. If you completed the continuity check of the load side of the motor contactor described in Step 2 and the circuits are clear, restore the incoming power and start the motor.
- If the continuity check on the load side of the contactor is shorted phase-to-phase, phase-to-ground or open phase-to-phase, then follow the power circuit to the motor.
- Disconnect the incoming motor leads at the ac motor electrical conduit box. Check the motor starter leads for a phase-to-phase or a phase-to-ground short. If either of these two problems exists, check the motor lead wiring. If the motor starter leads are clear, move on to the motor.
- With the motor still disconnected, check the motor leads for a phase-to-phase or a phase-to-ground short. There is a high probability that if there were no problems found previously in the circuit, the motor has a phase-to-phase short or a short-to-ground fault. If you have verified a phase-to-phase or phase-to-ground fault in the motor, replace the motor and start the system.
- If there appears to be no problem with the ac motor or its power circuit, then test the motor insulation with an insulation tester. The insulation test determines if the motor windings are “breaking down” during running conditions.
- If the insulation test results are negative, a mechanical problem is probably causing the motor overload. Uncouple the motor from the mechanical load and start the motor. If the motor runs, the problem is in the mechanical drive train. If the motor does not run and the motor overloads trip again, the problem is in the motor bearings.
Troubleshooting control circuit problems
You can also use a process of elimination to isolate and diagnose control circuit problems. Refer to “Control circuit problems flowchart” on page 68, which illustrates the steps of the following troubleshooting procedure:
- If all three phases of electrical voltage were present when you performed Step 1 of “Troubleshooting voltage source problems,” then the motor starter overload and control circuits should be checked. Determine if all or any of the motor starter overloads are tripped by pressing the motor overload reset pushbutton. If the motor overload(s) were tripped, try to restart the motor. If the problem was isolated to the motor overload(s), the motor should run. If the motor overload(s) trips again, return to Step 5 of “Troubleshooting voltage source problems” to determine if the problem is a motor problem or load problem.
- If the motor overloads are not tripped, verify that the starter control circuit has the proper secondary voltage using a DMM set to voltage mode and the appropriate range. If the proper voltage is not present, disconnect the motor starter’s incoming power supply.
- Using the DMM in continuity mode (Ohms), verify the integrity of the control voltage transformer fuse. Replace the fuse if blown, and
- identify the motor starter control circuit problem. If the starter control transformer fuse is good, proceed to Step 8 of this troubleshooting procedure.
- Disconnect the motor starter contactor coil and check for a short, ground or open circuit. If a short or ground is found, replace the motor starter contactor coil and start the motor.
- If the motor start contactor coil is good, with the coil still disconnected, check the control circuit wiring to the contactor coil. If a short or ground is detected, disconnect the field wiring circuit to the motor starter to determine if the problem is internal to the motor contactor overload circuit(s) or in the field control wiring.
- If an electrical ground is located in the motor starter overloads circuit, replace the motor overload(s). Once the failed motor overloads have been replaced verify circuit continuity, reconnect the starter coil and control field wiring, restore incoming power and start the motor.
- If an electrical ground is not located in the motor starter overload circuit, the problem has been isolated to the control circuit field wiring. Clear the ground in the field wiring circuit. Then reconnect the motor starter coil, overload circuit and field wiring, restore incoming power and start the motor.
- If you determine that the starter control voltage is intact and present, suspect a potential open circuit in the motor overloads or control circuit. To check for an open circuit, first disconnect the incoming power supply, and using the DMM in continuity mode (Ohms), verify the circuit integrity of the motor overloads control circuit (secondary contacts). If the motor overloads control circuit is good (all NC contacts have continuity), then the field control circuit to the motor starter is the problem.
- To troubleshoot the field control circuit to the motor starter, check continuity across the device that supplies the start signal to the motor starter, which could be a PLC contact closure, relay contact closure or start/stop pushbutton circuit. After the control circuit problem has been located and corrected, restore the incoming power and start the motor.
AC drive excitation — drive faults
When a drive fault occurs in an ac drive and motor application, determine if the fault is a motor, drive or application problem.
The diagnostic technology offered in modern ac drives can help you troubleshoot many drive problems. Manufacturers often include internal diagnostic capabilities in the drive’s microprocessor control. By using these drive fault diagnostics, you can easily determine and remedy the fault.
Some of the most common problems related to integrating an ac drive with an ac motor are listed in the table “Typical ac drive faults.” However, there are may other possible drive faults associated with motor/drive tuning, application requirements, communication errors, external device errors and initial programming errors. If you are still unable to locate the source of the trouble after ruling out the causes listed in the table, contact the equipment manufacturer or your local distributor for further diagnostic assistance.
The majority of ac motor and drive problems can be solved by using the drive manufacturer’s manual. Each drive fault that is monitored and annunciated by the drive’s microprocessor is listed in the troubleshooting section of the manual. If you cannot locate your manual, contact your local manufacturer’s representative or refer to the manufacturer’s website to download a copy.
AC motor and drive technology
The application of ac motor and ac drive technology is becoming much more prevalent in applications that were previously done with dc technology (variable speed applications) or across-the-line starters (historically constant speed applications used on fans and pumps). With the development of different ac drive technologies, such as soft start, V/Hz, open loop vector and closed loop vector drives, this trend will likely continue — and if anything, increase.
The ability to apply the right ac motor technology in the right application ensures that the motor/excitation technology used will provide trouble-free operation for many years. The ability to develop and execute a logical troubleshooting methodology when equipment failure does occur will assure that your process achieves the maximum amount of uptime.
Typical ac drive faults
|DC bus undervoltage||Apparent low supply voltage condition||%%POINT%% Incoming voltage is lower than the drive’s rated incoming voltage requirement (usually|
|%%POINT%% Drive acceleration is too short|
|DC bus overvoltage||Apparent high supply voltage condition||%%POINT%% Incoming voltage is higher than the drive’s rated incoming voltage requirement (usually|
|%%POINT%% Drive deceleration is too short|
|%%POINT%% Power factor correction capacitors on the incoming power line|
|Overcurrent||Drive output current that exceeds the maximum drive current output rating||%%POINT%% Phase-to-phase short|
|%%POINT%% Locked rotor|
|%%POINT%% Acceleration too short|
|%%POINT%% Load too large|
|Motor overload||Drive output current exceeds the current rating of the motor||%%POINT%% Load too large|
|%%POINT%% Acceleration/deceleration too short|
|%%POINT%% Incorrect V/F pattern|
|%%POINT%% Motor-rated current incorrectly loaded into drive|
|Drive overload||Drive output current exceeds the current rating of the drive||%%POINT%% Load too large|
|%%POINT%% Acceleration/deceleration too short|
|%%POINT%% Incorrect V/F pattern|
|%%POINT%% Drive is sized too small for motor application|
Patrick E. Owens is a Technology Manager at Kaman Industrial Technologies. If you have questions about motor troubleshooting and maintenance you may contact Mr. Owens directly at firstname.lastname@example.org . Article edited by Jack Smith, Senior Editor, Plant Engineering magazine, (630) 288-8783, email@example.com .
Typical motor failures
Typical failures that occur with ac motors include:
Motor bearing failure — Motor bearing failure is the #1 cause of motor failures and can be caused by lubrication problems, contamination, bearing current, shaft misalignment or side loading
Single-phase motor winding failure — One phase is open in the motor circuit, which can be caused by a blown fuse, open contactor, open motor lead or bad electrical connections
Phase-to-phase or turn-to-turn motor winding fault — The motor winding is shorted phase-to-phase or turn-to-turn because of motor insulation failure, which can be caused by contaminants, abrasion, vibration or voltage surge
Winding grounded fault —The motor winding is shorted from the motor winding to the grounded motor frame because of motor insulation failure, which can be caused by contaminants, abrasion, vibration or voltage surge
Motor phase damage —Motor phase damage typically occurs from unequal phase voltages, which can be caused by an unbalanced load at the power source, poor connections or high resistance in a single leg of the motor circuit
Motor winding damage —Motor winding damage (all windings) is caused by the thermal deterioration of the insulation in all phases, which is caused by motor overload or over/under voltage situations. Switching power circuit, capacitor discharges, or solid-state power devices can also cause this type of motor winding damage