How to Troubleshoot 3 Phase AC Motors.

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Three phase induction motors are one of the most popular electric motors commonly found in processing plants or any manufacturing concern. They are used in situations where large power is required. The squirrel cage brand is the most popular and they perform various tasks wherever they are applied.

Because of the critical roles these motors play in any plant, a failure of the motor, inability to start, noisy operation and sundry other problems need to be remedied as soon as possible to avoid costly production downtime. The table below gives the commonly encountered problems in 3 phase squirrel cage induction motors, cause of problems and what remedy to apply to bring the motor back to production. This troubleshooting guide can also be applied to other types of three phase induction motors:

Motor Problem Cause Remedy
Motor fails to start Blown fuses Replace fuse with proper type and rating
Overload Trips Check and reset overload in starter
Improper power supply Check to see that power supplied agrees with nameplate specifications  and load factor
Improper line connections Check connections with wiring diagram supplied with motor
Open circuit in winding or control switch This is normally indicated by a humming sound when switch is closed. Check for loose wiring connections. Confirm that all control contacts are closing.
Mechanical failure Check to see that motor and drive turns freely. Check bearings and lubrication
Short circuited stator Indicated by blown fuses.  Motor must be rewound
Poor stator coil connections Remove end belts. Locate poor connections with test lamp.
Rotor defective Check for broken bars or end rings
Motor may be overloaded Reduce motor load
Motor stalls One phase may be open Check supply lines for open phase
Wrong application Change type or size. Consult motor manufacturer
Overload Reduce load
Low voltage Check that nameplate voltage is maintained. Check connection.
Open circuit Fuses blown. Check overload relay, stator and push buttons
Motor runs and then dies down Power failure Check for loose connections to line, to fuses and to control
Motor does not come up to speed Motor is applied for the wrong application Consult manufacturer for right application of motor
Voltage too low at motor terminals because of line drop Use higher voltage on transformer terminals or reduce load. Check connections. Check conductors for proper size.
Starting load too high Check load motor is supposed to carry at start.
Broken rotor bars or loose rotor Look for cracks near the rings. A new rotor may be required as repairs are usually temporary not permanent
Open primary circuit Locate fault with testing device and repair.
Motor takes too long to accelerate and/or draws high current (Amps) Excessive load Reduce load
Low voltage during start up Check for high resistance. Adequate wire size.
Defective squirrel cage rotor Replace with new rotor
Applied voltage too low Improve voltage at terminals of transformer by tap changing.
Wrong rotation Wrong sequence of phases Reverse connections at motor or at switchboard.
Motor overheats while running under load Overload Reduce load
Frame or bracket vents may be clogged with dirt and prevent proper ventilation of motor. Open vent holes and check for a continuous stream of air from the motor.
Motor may have one phase open Check to make sure that all leads are well connected.
Grounded coil Locate and repair
Unbalanced terminal voltage Check for faulty leads, connections and transformers.
Motor vibrates Motor  misaligned Realign
Weak support Strengthen base
Coupling out of balance Balance coupling
Driven equipment unbalanced Re-balance driven equipment
Defective bearings Replace bearing
Bearings not in line Line bearings up properly
Balancing weights shifted Re-balance motor
Poly-phase motor running single phase Check for open circuit
Excessive end play Adjust bearing
Unbalanced line current on poly-phase motors during normal operation Unequal terminal volts Check leads and connections
Single phase operation Check for open contacts
Unbalanced voltage Correct unbalanced power supply
Noisy Operation Airgap not uniform Check and correct bracket fits or bearing.
Rotor unbalance Rebalance
Hot bearings general Bent or sprung shaft Straighten or replace shaft
Excessive belt pull Decrease belt tension
Pulley too far away Move pulley closer to motor bearing
Pulley diameter too small Use larger pulleys
Misalignment Correct by realignment of drive
Hot bearings ball Insufficient grease Maintain proper quantity of grease in bearing
Deterioration of grease or lubricant contaminated Remove old grease, wash bearings thoroughly in kerosene and replace with new grease.
Excessive lubricant Reduce quantity of grease, bearing should not be more than 1/2 filled
Overloaded bearing Check alignment, side and end thrust.
Broken ball or rough races Replace bearing, first clean housing thoroughly

This troubleshooting guide is by no means exhaustive but can provide guidance for the resolution of commonly encountered problems as far as poly-phase induction motors are concerned.

How to Test a 3 Phase Motor Windings With an Ohmmeter

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Every 3 phase motor has six (6) terminals with the supply voltage connected to three (3) of those terminals. The most common configuration of a three-phase  motor is the Delta (∆) – Star (Wye) configuration with the Delta side connected to supply voltage. The terminal configuration of a 3 phase motor is shown below:

Terminals Configuration of a 3 Phase Motor
The W2U2V2 terminal set is the star side of the 3 phase motor while the U1VIW1 is the Delta side of the motor connected to the supply voltage.

The 3 phase motor is a rugged piece of equipment but as with everything man made, there comes a time when this beautiful piece of machinery fails either due to old age, misapplication, mal-operation or any other adverse cause.

The most common failure mode of a 3 phase AC motor is burnt winding or shorted winding leading to the damage of the motor. Often it is required to test the winding of the 3 phase windings with the aid of a multimeter or ohmmeter to determine whether the motor is still good or burnt or shorted.

How to Test the Winding of a 3 phase Motor

To determine whether a 3 phase motor is still good or has gone bad, a simple ohmmeter test across the windings of the motor will reveal its true state of health. As shown below, the indicated terminal matrix (blue lines) shows the way the windings of a 3 phase motor should be tested with an Ohmmeter:
How to test the windings of a 3 Phase Motor with an Ohmmeter

The first thing to do before testing the windings of the motor is to remove the links linking terminals W2U2V2 and the disconnect the motor from supply (L1, L2, L3). A multimeter terminals placed across this matrix of terminals will indicate the following readings for a good 3 phase motor:
(a) Terminals W1W2, U1U2, V1V2 will indicate continuity for a good motor
(b) Every other terminal combinations should indicate Open for a good motor
(c) Readings between any of the six (6) terminals and the motor frame signifying earth     
    (E) should indicate open for a good motor.

Ohmmeter Readings for a Bad 3 phase Motor

In the case of a burnt or bad 3 phase motor, this matrix of terminals should indicate  the opposite readings for a bad motor:
(a) If any of the terminal combinations W1W2, U1U2, V1V2 should indicate open then 
     the motor is bad.
(b) If any other terminal combinations should indicate continuity instead of open, then            
     the motor is bad.
(c) If the reading between any of the six (6) terminals and motor frame (E) should  
     indicate continuity, then the motor is dead.


Electric Motor Specifications for Hazardous Locations

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A hazardous environment or location is some combustible materials that could easily ignite and create a fire hazard. Electric motors operated in such an environment need to be carefully specified and selected in order not to pose an explosion threat to an installation.

Arcs, sparks and high temperatures on the surfaces of electric motors are common sources of ignition of hazardous substances. Therefore the right motors need to be specified and selected for use in this kind of environments otherwise the consequences of an explosion and eventual fire resulting from using the wrong kind of motor could range from minor injuries, production down time and destruction of valuable investment as well as death.

Information Required to Specify an Electric Motor for Use in Hazardous Locations

Four basic information are required to specify and select the right motor for use in a hazardous environment:
1. Class of materials in the hazardous environment
2. Division classification of the hazardous environment
3. Group classification of the hazardous environment
4.     The autoignition temperature (AIT) of the hazardous material in the environment
How to Select Electric Motors for Use in Hazardous Locations

Class of Materials in the Hazardous Environment.

There are three classes of hazardous locations according to the NEC and CEC:


Class Location Hazardous Materials Present
         I Contains flammable gases or vapors e.g Hydrocarbon gases
         II Contains dusts either electrically conductive or explosive in air
        III Contains filings and flyings that are easily ignitable e.g texile industry 

*NEC -  National Electric Code
*CEC -  Canadian Electrical Code

Division of the Hazardous Environment

Division describes the condition under which the hazardous material is present in the particular environment. There are two basic divisions: Division 1 and Division 2 locations

Division 1 Location
A Division 1 location is one where an explosive or ignitable material is present under normal operating condition. Division 1 locations include environment where explosive materials are routinely exposed to the environment during regular operation and scheduled maintenance.


Division 2 Location
A Division 2 location is one where hazardous substances are stored or handled under abnormal conditions.


Group Classification of Hazardous Environment.

Class I and Class II are divided into groups according to the behaviour of the hazardous material after it has been ignited. These groupings are shown below:
Class Location Groups 
                        I A, B, C, D
                       II E, F, G

Autoignition Temperature of Hazardous Materials

A key information required to accurately specify an electric motor in a hazardous environment is the autoignition temperature of the hazardous material, AIT for short.

The AIT refers to the minimum temperature at which there is sufficient energy for a hazardous material (chemical) to ignite spontaneously in the absence of a spark, flame or other source of ignition. The AITs of various Class I and Class II hazardous materials are shown in the table below:

*Class Group Hazardous Material
Autoignition Temperature
°C°F
     I        A  Acetylene
 305
581
      B Butadiene
420
788
Ethlylene Oxide
570
1,058
Hydrogen
500
932
     C Acetaldehyde
175
347
Cyclopropane
498
928
Diethyl Ether
180
356
Ethylene
450
842
Isoprene
398
743
    D Acetone
465
869
Amonia
651
1,204
Benzene
498
928
Butane
287
550
Ethane
472
882
Ethanol
363
685
Gasoline
246 - 280
475 - 536
Methane
537
999
Propane
450
842
Styrene
490
914
   II    E Aluminium
650
1,202
Bronze
370
698
Chromium
580
1,078
Magnesium
620
1,148
Titanium
330
626
Zinc
630
1,166
   F Coal
610
1,130
   G Corn
400
752
Nylon
500
932
Polyethylene
450
842
Sugar
350
662
Wheat
480
896
Wheat Flower
380
716

*Source : American Institute of Chemical Engineers (AIChE) - www.aiche.org

Characteristics of Motors in Class I, Division 1 & 2 Hazardous Locations

To be able to apply an electric motor successfully in a hazardous environment, these motors must possess certain critical characteristics that make them suitable to operate in these environment without creating problems.
  1. These motors must be built and labelled as explosion-proof
  2. An explosion-proof motor must contain an internal explosion without rupturing
  3. An explosion-proof motor must have flame paths for exhausting hazardous gases during an explosion and for cooling the hazardous material as they leave the motor to prevent further explosion.
  4. The more severe the explosion hazards, the stronger the enclosures of the motor and the longer the flame paths. For example, motors for use in group A environment which has acetylene gas require the highest enclosure strength and longest flame paths compared with those used in group D environment with only propane gas.
  5. Motors for use in hazardous environment are assigned a temperature code (T-Code), an identification number which describes the maximum temperature of surfaces subject to contact with hazardous materials. The temperature value defined by the T-code applies under all conditions of motor operation including burnt out, overload and locked rotor current.
  6. The T-code for a given motor must be less than the AIT of the hazardous gas or mixture in the environment where the motor operates. This is to ensure that the hazardous materials do not spontaneously ignite when it contacts the motor surfaces and enclosure during operation.

Temperature Codes of Motors Operating in Hazardous Locations According to NEC, CEC & IEC Designations:

The table below gives the temperature codes for motors operating in hazardous environment according to NEC/CEC  & IEC letter designations are given in the table below:


**NEC/CEC
Designation
T - Code
IEC
Designation
T - Code
Maximum Surface Temperature
°C
°F
T1
T1
450
842
T2
T2
300
572
T2A

280
536
T2B

260
500
T2C

230
446
T2D

215
419
T3

200
392
T3A
T3
180
356
T3B

165
329
T3C

160
320
T4
T4
135
275
T4A

120
248
T5
T5
100
212
T6
T6
85
185
   
 **Source : American Institute of Chemical Engineers (AIChE) - www.aiche.org

Class I, Division 2 Motors

The minimum requirements for motors for use in Class I, Division 2 locations are less stringent than those for use in Class I, Division 1 locations. Some basic characteristics and requirement for motors for use in Class I, Division 2 locations are enumerated below:
  1. An explosion-proof motor certified for use in a Class I, Division 1 location may also be operated in a Class I, Division 2 area provided the motor meets the group and T-Code requirements.
  2. TEFC (Totally Enclosed, Fan Cooled) motors and ODP (Open Drip -proof) motors can be used in Division 2 locations provided they do not have ignition sources such as arc-producing brushes or switching mechanisms.
  3. Three-phase induction motors with low surface temperatures and no sparking parts can be used in Division 2 locations.

Characteristics of Motors for use in Class II locations
  1. As class II locations contain ignitable dusts, electric motors operating in this kind of environment must be dust-ignition-proof.
  2. The enclosures of dust-ignition-proof motors are designed to exclude hazardous materials from accessing the internals of the motors unlike explosion-proof motors.
  3. The T-code of a dust-ignition-proof motor must correspond to a maximum surface temperature below the AIT of the hazardous dust materials.

Nameplate Requirements for Motors Operating in Hazardous Locations

Selecting the right kind of motor that meets the requirements for use in hazardous environment is not an easy task. However, manufacturers of electric motors for use in hazardous locations have standard nameplates that contain the following information:
(a) Type of Enclosure of Motor
(b) Class of hazardous materials for which the motor is suitable
(c) Group of the hazardous materials 
(d) T-code of the electric motor

The above information makes it less tedious in selecting and specifying a motor for use in a hazardous environment.