Troubleshooting techniques help keep ac induction motors running
When equipment in your plant malfunctions, you need to isolate the trouble quickly. Is the problem in the motor, or the starting circuit? In the August 2005 issue of PLANT ENGINEERING , “Take a step-by-step approach to analyze motor starting problems” looked at an organized process for troubleshooting electric motor starting problems. In the April 2006 issue, “Repair or replace? Make the right decision on motors” discussed the repair or replace decision. In this article, we take a look at common operational problems for three-phase ac induction motors, how to diagnose them and how to fix them.
No responsible discussion of electrical equipment maintenance would be complete without first covering the need to maintain a safe working environment. Be sure to take all necessary precautions to protect yourself or your employees from harm. Follow your company’s rules for proper employee protection. These rules may include personal protection equipment, electrical safety precautions, lock-out/ tag-out procedures and any other established safety requirements and procedures when working around electricity. Remember, when it comes to electricity, one misstep could be your last.
Basic diagnostic tools
Before you begin, assemble some basic diagnostic equipment for the job. Typical tools used for troubleshooting motor operation problems include an ac voltmeter, ac clamp-on ammeter, ohmmeter and meg-ohmmeter. These tools are used to measure the motor voltage, current and resistance.
One of the most common sources of motor operating problems is overheating. It’s no secret that motors produce heat as a byproduct of their operation. This heat is a result of winding resistance and other inefficiencies in the generation and induction of the magnetic flux used to produce torque at the motor shaft. The rule of thumb for motor life expectancy is “every 10 degrees C rise in motor temperature results in a 50% decrease of motor life.” It is important to minimize the adverse effects of overheating.
This is why motors are designed to dissipate heat during normal operation by use of their external surfaces and typically a cooling fan. Other motor configurations are available for improved heat reduction including totally enclosed non vent (TENV), water cooled and air-to-air heat exchangers.
Still, despite the best efforts of the manufacturers, overheating is a common operating problem. Symptoms for motor overheating problems include excess heat on the motor exterior, tripping of the motor overload or drive and motor winding failure.
Voltage imbalance is a common and damaging source of overheating. A rule of thumb for the effect of voltage imbalance is “the percent of motor temperature rise equals two times the square of the percentage of voltage imbalance.” For example, a 3% voltage imbalance can result in an 18% (calculated as: 2 X (3%)2 ) temperature rise in the motor.
To determine if a voltage imbalance exists, first check the supply voltage to the motor control device when the motor is not running. Set the ac voltmeter to the range for the three-phase voltage of the motor supply. At the line side of the motor control device (power supply side of a motor starter), check and record the voltage phase-to-phase and phase-to-ground for each combination.
The voltage measurements from phase-to-phase should be very close to the same readings. If a voltage imbalance is present on the supply side when the motor is not running, the problem is in the voltage supply. Check out your facility’s power system and resolve the supply problem. Problems could include an open fuse, incoming utility imbalance, transformer problems or feed-wire sizing. Voltage measurements from phase-to-ground may vary due to incoming voltage configurations and these measurements may be useful for further troubleshooting.
If the voltage supply is okay, next use the ac voltmeter to check the voltage on the load side of the motor control device when the motor is running (motor “T” leads T1 to T2, T2 to T3, T3 to T1, T1 to ground, T2 to ground, and T3 to ground). These measurements will check the voltage going to the motor and the wiring from the motor control device to the motor.
If the voltage imbalance is present only when the motor is running, the problem is in the motor or the wiring from the motor starting equipment to the motor. Remove the power from the motor control device and follow all plant lock-out/tag-out and safety requirements. Using the ohmmeter, check the resistance of the motor leads at the control equipment. This will check the motor and the wires to the motor. Check and record phase-to-phase and phase-to-ground resistances.
The resistance measurements phase-to-phase should be very close for each measurement and within the specifications for the motor as suggested by the manufacturer. Resistance values vary based on motor horsepower and voltage hookup. The resistance from phase-to-ground should be very high for each combination of measurements.
If the phase-to-phase resistance is high, there may be an open winding in the motor, open wire to the motor or a bad connection in the motor connection box. Open the motor connection box, disconnect the motor connections and check the motor windings using the ohmmeter. Determine if the problem is the motor or the feed wires. Replace the motor, feed wires or repair the motor connections at the connection box.
If the phase-to-ground resistance is low, disconnect the motor at the motor connection box and check the motor leads. If the resistance on the motor is low phase-to-ground, there is a short in the motor, and it must be replaced. If the motor resistance to ground is high on the motor leads, check the feed wires. Replace the bad wires or reconnect and insulate the motor connections in the motor connection box to eliminate the short circuit.
If there are no resistance problems found using the ohmmeter use the meg-ohmmeter to test the same series of test measurements as described with the ohmmeter. A meg-ohmmeter tests the wiring with a higher voltage to determine if an intermittent or breakdown in the insulation of the motor windings, motor wires, or motor connections exists.
Motors are designed to run during normal operation within the ratings on the motor nameplate. The nameplate has a full-load amp rating. This rating should not be exceeded during normal operation, however occasional and brief occurrences are typically not a problem. The motor is not designed to remove the heat that is generated at levels above the full load amp rating except for starting and intermittent load surges.
After verifying that the motor voltage and motor resistance are okay, use an ac ammeter to check the motor current. Set the ac ammeter to a level higher than the full load rating of the motor. Affix the ac ammeter clamp around one of the motor wires. Measure and record the amp reading for each phase (T1, T2 and T3).
The amp reading for each phase should be similar. If the amperage reading for each phase is larger than the full load nameplate reading, the motor is overloaded. Check the load for problems such as a jammed load, too much material or bearing failure. The motor may also be sized incorrectly for the load requirements. If the amp readings for each phase are very different, this may indicate a problem with the voltage or a failure in a motor winding or connection. Then check the motor voltage and resistance readings as described earlier.
Motors require cooling to remove heat generated during operation. If the motor is overheating, check the area around the motor for high ambient temperature conditions. Heat can be transferred to the motor from the environment by radiation, convection and conduction. If the motor is mounted near an oven, burner or other heat source, move the motor to an area away from the heat if possible. Heat can be transferred to the motor through the mechanical connection to a hot load.
Is the motor located in a dirty or dusty environment? Accumulation of dust and dirt on a motor insulates the motor. This will decrease the ability of the motor fan to remove the heat produced by the motor.
Other sources of overheating
Another possibility is that the motor may be running too slow. Motors are designed for running near their nameplate RPM and may run above or below this rating depending on the manufacturer’s specifications. When running a motor with a variable frequency drive, motors have a speed range rating at which it effectively removes the heat at the rated load. Motors should offer a constant-torque load speed range for conveyors, augers, extruders, etc., and for variable-torque loads such as centrifugal pumps and blowers. The ratings refer to the speed range capability of the motor below the nameplate RPM rating.
When operating a motor on a VFD above the nameplate RPM, the motor output torque capability is reduced. Typical motor torque curves indicate a constant motor horsepower output and a variable torque operation above base speed. The higher the motor speed above base RPM, the less torque available from the motor.
High cycle operation of a motor also adds heat to the motor. When a motor starts, higher current than the full load rating is typically required to begin motor rotation and bring the motor up to speed. If the motor is started and stopped frequently, the motor may not be running long enough at speed for the fan to remove the heat.
Alternatives for removing the heat from a motor include a constant velocity blower fan, ducting outside air to the motor and the use of a heat exchanger.
Low speed or lack of torque
A motor in proper working order should rapidly reach near-nameplate speed when started across the line. If the motor takes a long time to reach the nameplate RPM or has little torque check for the following:
Low voltage — A basic cause of a poor performing motor is low voltage. A low voltage situation does not provide the necessary power to allow the motor to achieve the expected torque rating. If your motor is not generating the required torque for your operation, first check motor voltages as described previously.
Single phase — Single-phase voltage, like low voltage, does not provide the necessary power to develop the torque rating of the motor. Typical motor characteristics may include a motor hum or a motor with very little torque when turning. Again, check motor voltages as described previously.
Excessive vibration — Excessive vibration is as much a symptom as it is a cause of motor operating problems. Excessive motor vibration is usually a sign of either motor or load problems and can lead to premature failure of the motor and the load. The following steps should assist you in discovering and fixing vibration problems:
Misalignment and unbalanced loads — Misalignment between the motor shaft and the load shaft causes unnecessary vibration. Premature bearing failure in the motor and/or the load can result from misalignment. The motor shaft must be centered with the load shaft to optimize operating efficiency. Various tools for motor and load alignment are available such as laser alignment kits. Motors and loads must also be firmly mounted to a base to maintain alignment and minimize vibration from loose mounting hardware.
Load imbalance — Load imbalance is an additional cause of motor vibration. Check the load for unbalanced conditions such as excess material on the outside of the drum, broken fan blades, etc. Clear the material or repair the load and run the equipment again to check for imbalance problems.
Pump cavitation — Pump cavitation is a common cause of excessive pump vibration which may in turn damage the motor. Cavitation is present when the pump is running outside its capabilities. This could include a head pressure that’s too low, an impeller that’s too large, a pump running too fast or a discharge pressure that’s too low. Check with your pump manufacturer to ensure the pump is operating within the design capabilities.
Motor selection and maintenance
Generally, modern electric motors provide long trouble-free service if care is taken in the initial application and routine maintenance of the motor. Correct motor selection and application criteria include:
Mounting requirements (Foot mount, C-Face mount, etc.)
Enclosure type (TEFC, ODP, washdown, etc.)
Environmental concerns (temperature, moisture, dust, dirt, hazardous, etc.)
Load type (constant torque, variable torque, high inertia)
Starting method (across the line, VFD etc.)
Special considerations (regenerative load, positioning, etc.)
Proper motor maintenance includes regular bearing greasing (do not over-grease), vibration monitoring, cleaning and conditioning monitoring. Proper application and preventive maintenance practices can provide reliable production uptime.
The Bottom Line…
Motor operational problems are a common cause of downtime and maintenance headaches.
Good troubleshooting skills help to identify the root cause of motor problems and avoid their recurrence.
Take time when handling a motor failure situation to carefully diagnose the situation and gain a complete understanding of the source (or sources) of the problem.
Analyzing application issues and applying long-term corrective solutions will help to minimize your operational disruptions.