Three Phase Ac Motor Faults, Remedies

3 Phase AC Induction Motor

The 3-phase AC Induction Motor is known for its efficiency and reliability. It operates on the principle of electromagnetic induction. where a rotating magnetic field is generated by the stator.

The stationary part of the motor induces a current in the rotor. This interaction creates torque, causing the rotor to turn and drive mechanical systems.

The stator contains three-phase windings and is connected to the AC power supply. One of the key advantages of this motor is its self-starting capability. Its robust construction ensures minimal maintenance.

Here are parts of the three-phase AC motor

Stator
Rotor
Stator Windings
Rotating Magnetic Field
Synchronous Speed

3-phase AC Induction Motor Stator

The 3-phase induction motor stator consists of a steel frame that houses a hollow, cylindrical core made from stacked silicon steel laminations. The purpose of silicon steel laminations is to minimize losses due to hysteresis and eddy currents.

The stator contains numerous slots that hold the three-phase winding circuit, which is connected to the three-phase AC power source. When power is applied, these windings generate a rotating magnetic field that interacts with the rotor to produce torque.

The design of the stator windings is such that they are offset by an electrical angle of 120°.

3 Phase AC Induction Motor stator

Rotor of 3-phase induction motor

Rotor consists of a cylindrical laminated core, with parallel slots that house conductors, typically made of heavy copper or aluminium bars. These conductors are short-circuited by end rings, and the entire assembly is designed to rotate within the magnetic field.

The unique construction of the rotor, with its skewed slots, not only minimizes magnetic humming noise but also prevents the motor from stalling.

When the stator's rotating magnetic field interacts with the static rotor conductors, an electromotive force (EMF) is induced. The difference in speed between the stator's magnetic field and the rotor's rotation is referred to as slip.

3 Phase AC Induction Motor rotor

Stator Windings

when AC power is applied to the stationary part of the motor, creates a rotating magnetic field. Typically, the windings are made of copper wire and are insulated to prevent electrical shorts.

The common configurations are the star or delta connections. In a star configuration, one end of each winding is connected to a common central point, while the other ends are connected to the power supply.

In contrast, a delta configuration has the ends of the windings connected to form a closed loop, with each junction point connected to the power supply.

This setup is essential for the motor's operation as it generates the rotating magnetic field necessary to induce current in the rotor, thereby creating torque and allowing the motor to turn.

3 Phase AC Induction Motor stator winding

Rotating Magnetic Field

The rotating magnetic field (RMF) is a fundamental concept in the operation of three-phase induction motors. When a three-phase electrical supply is connected to the stator windings of an induction motor, it generates an RMF that moves around the stator.

The RMF's speed, known as synchronous speed, is determined by the supply frequency and the number of poles in the motor, following the formula

NS = 120f/P RPM,

where

f is the frequency and
P is the number of poles.

Rotating magnetic field

Synchronous Speed

The speed at which the rotating magnetic field of the stator rotates is known as the synchronous speed. It depends on the frequency of the AC power supply and the number of poles in the motor.

Synchronous speed

Induction Motor Operation

An induction motor, also known as an asynchronous motor, is a type of alternating current (AC) electric motor that operates on the principle of electromagnetic induction.

When AC supply is given to the stator, the stationary part of the motor, it generates a rotating magnetic field. This field induces an electric current in the rotor, the moving part of the motor, which in turn produces torque due to the interaction between the magnetic fields of the stator and rotor.

Induction motors are widely used because they are robust, reliable, and require minimal maintenance. They come in various types, primarily single-phase and three-phase motors.

The most common type of three-phase AC motor is the induction motor. The rotating magnetic field in the stator induces currents in the rotor, creating a torque that drives the motor.

The speed at which the rotor rotates is slightly lower than the synchronous speed, creating a "slip" that allows the motor to generate torque.

Types of 3-phase AC Motor

Squirrel Cage Induction Motor
Wound Rotor Induction Motor
Synchronous Motor

Squirrel Cage Induction Motor

The squirrel cage induction motor is a type of induction motor in which rotor windings are made with laminated copper or aluminium slots. the ending point of slots is short with rings. this is the most used induction motor. induction motor is always running below than synchronous speed.

Squirrel cage induction motor

Wound Rotor Induction Motor

A Wound Rotor Induction Motor is a type of three-phase AC induction motor designed to provide high starting torque by connecting an external resistance to the rotor circuit.

This motor rotor is wound with insulated windings, which are connected to slip rings. These slip rings are linked to brushes.

The Wound Rotor Induction Motor allows for a controlled start and speed adjustment by varying the external resistance.

Synchronous Motor

A synchronous motor is a type of alternating current (AC) motor where the rotation of the shaft is synchronized with the frequency of the supply current.

In these motors, the rotor speed is equal to the speed of the rotating magnetic field produced by the stator. Synchronous motors are highly efficient.

Synchronous motors are widely used in industrial and commercial applications, particularly where low-speed, high-torque drives are required.

Faults In AC Motors

AC motors are an integral part of domestic appliances to industrial equipment. These faults can be broadly categorized into

Mechanical Faults
Electrical Faults

Mechanical Faults

AC motors can suffer from bearing failures, misalignment, and imbalance. Bearings are critical for the smooth operation of the motor, and their failure can lead to noise and vibration.

Mechanical faults are typically diagnosed using vibration analysis tools, which can detect imbalances, misalignment, and bearing wear. Laser shaft alignment tools are also used to ensure precise alignment between the motor and connected equipment.

Mechanical faults are categorized as including

Improper Bearing operation
Absence of oil in bearings
Bent rotor shaft
Wrong fittings of different Mechanical parts
Broken side covers of tightening bolts
Closed cooling ducts
loose Rotor laminations
Unbalance Rotor
Incorrect installation or Foundation of motor
Breaking of motor fans
Incorrect Alignment of the rotor
Incorrect air gap Between stator and rotor
Breaking the connection box
Breaking of foundation bolts
Misalignment of load motor coupling
Broken Brush Holders Connected With Sliprings
Broken squirrel cage motor bar

Electrical Faults

Electrical faults in AC motors primarily involve the motor windings, power supply, and connections. Stator winding faults are a common electrical issue, which can arise due to insulation failure. it leads to short circuits or open circuits.

These faults can cause overheating, reduced efficiency, and even complete motor failure if not addressed promptly.

To diagnose electrical faults, technicians use multimeters to measure resistance and insulation testers. Infrared thermography can also detect hot spots indicative of electrical issues.

Electrical faults are categorized as including

Open Circuit
Short Circuit in Complete Windings
Short Circuited Group
Punctured insulations
Wrong connection for specific voltage
Phase to phase to faults
Phase to Earth Faults
Wrong connection in coils
Loss of contacts in current Carrying parts
Burning of Slipring
Open Rotor bars
Reversed coil
Reversed pole-phase group
Reversed-phase
wrong Grouping
Short Circuit in core laminations
Incorrect Size of Windings wire
Single Phasing
Unbalanced Phasing
Unbalanced supply voltage

Faults, Cause and Their Remedies of 3-Phase AC Squirrel Cage Induction Motor

Fault: The motor Fails to start

Causes

supply is off
the fuse was melted
The supply voltage is not as per the rated voltage
open circuit faults appeared in the circuit
open circuit in a starter
the load is heavy at the motor starting
The main switch connection is not in the right position
Bearings are under fault condition
The motor shaft is skewed
The overload Relay is tripped.
The connection of the power circuit was wrong.
The Motor terminal connection is loose.
Short circuit or open circuit in the stator winding.
The motor winding is grounded.

Remedies

Check the supply voltage with a multimeter.
check the fuse and replace it with a higher value.
check the voltage between the two phases.

Fault

The Motor Refuses to start although the motor issues humming sound

Causes

the motor is running in the two-phase because the third phase is unavailable.
the open circuit in the winding.
the connection is loose.
the air gap is unbalanced in the stator and rotor.
The motor rotor is not rotated frequently
The motor belt is tight.
open circuit in the stator

Remedies

Check the supply of three-phase with the help of a Test lamp.
Check the continuity in the motor terminals with the help of a mager.
if you find any connection is loose, tighten the connection.
Check the Motor shaft bearing if it feels loose, change it with a new one.
check the motor cover if it's tight lose it to rotate the rotor.
Check the belt condition
Check the motor stator winding to open the motor, and connect the open wire connection.

Fault

shock is felt by touching the motor

Causes

a ground fault occurs in the motor
The earth wire is broken.

Remedies

Check the motor terminal with the help of a series test lamp if the lamp is on its means wire is connected to the motor frame, disconnect and insulate the connection.
Check the earth wire continuity with the help of Mager.

Fault

The motor does not bear its load

Causes

The load is heavy
The motor load belt is tight.
The short circuit in one of the motor coils.
The motor winding insulation is weak.

Remedies

Regulate the load value.
Lose the load belt.
Check the short circuit in the coil. open the coils and check their resistance, the minimum resistance coil is short-circuited.
varnish the motor winding correctly using any insulating material.

Fault

Motor is overheated

Causes

the connection of the motor is incorrect
the motor load is heavy
The supply voltage is low compared to the standard voltage
the bearing is loose
the one coil of winding is short

Remedies

Check the motor connection and correct the connection.
Regulate the motor load as per needs.
Check the supply voltage with the help of a voltmeter.
Open the side cover and check the bearing position if feels tight replace it with the new one.
Implement the short circuit test in the coils. the minimum resistance coil is short.

Fault

The motor starts up sluggishly

Causes

The motor stator connection in Star is but they need it in the delta
One terminal of the motor is connected to the neutral but they need with phase load

Remedies

Open the terminal box and accurate their connection.
Check the motor connection and supply connection

Fault

Motor Issues a scratching sound

Causes

The belt of the motor is loose
The Motor is overload
The bearing needs lubricant

Remedies

Tight the motor load belt as needed.
regulate the motor load
and lubricate the bearing.

Fault

Short circuit in a motor circuit

Causes

The two wires of the motor connected to each other
Any wire is connected to the main switch

Remedies

Check the short circuit test in the coils.
open the main switch and insulate the non-insulated wire.

Fault

Bearing too hot to touch

Causes

Bearing is dry
Bearing is tight
Bearing is loose
The shaft is skew

Remedies

lubricate the bearings
lose the bearings
tight the bearings
checked the shaft position.

Fault

Every Third Group In a Three-phase motor is hotter

Causes

The Resistance of hotted wire is Enough
The phase is earth

Remedies

Due to a short circuit, the wire is hotted, implement the short circuit test and remove the fault or jump to the temporary open coil.
implement the earth test, remove the earth connection, insulate, and connect again.

Fault

Motor runs hot and explosions accompanied by fire occur in winding.
Causes
Due to humidity, the motor faces a temporary short circuit or temporary earth.

Remedies

Dry the motor winding or insulate it correctly.

Conclusion

In summary, common faults in AC motors range from electrical issues like insulation breakdown and phase imbalance to mechanical problems like bearing faults and rotor misalignment.

Regular maintenance, including proper testing and diagnostics, is essential to identify these issues, ensuring AC motors' reliable operation in various applications.