Troubleshooting, Maintenance and Protection of AC Electrical Motors and Drives

Troubleshooting, Maintenance &
Protection of AC Electrical
Motors and Drives
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Industrial Electricity
Industrial electricity is all about Single-phase
and Three-phase transformers, AC and DC
machines - their construction, design, testing,
operation and maintenance.
For troubleshooting Electrical equipment and
control circuits, it is important to know the
basic principals on which the electrical
equipment works.

Principle of a transformer
A transformer works on the principle of
electromagnetic induction - an e.m.f. is
induced in a coil if it links a changing flux.
Transformers are designed to change the
voltage of electrical power supply from one
value to another. They are widely used in
power systems.

e.m.f. Induced
f = fmax sinwt
fmax –the maximum value of the flux in webers,
w = 2pf
f – frequency in cycles per secs.
The e.m.f. induced in the winding is given by Faraday’s Law and
the direction of induced e.m.f. is given by Lenz’s Law.

Faraday’s Law
When the flux linking with the circuit changes, an e.m.f. is induced in
the circuit proportional to the rate of change of flux. The instantaneous
value of the e.m.f. induced in the primary winding is given by :
e1 = – N1 df
dt
The maximum e.m.f. is given by :
E1max = – 2 p fmax f N1

Lenz’s Law
The polarity of the induced e.m.f. is given by the Lenz’s Law.
The direction of the induced e.m.f. is such that the current induced by it
tends to stop the change producing it.

Ideal Transformer
• Winding resistances are negligible
• All flux produced is confined to core of transformer and links fully both
windings
• The permeability of the core is high enough so magnetizing current
required to produce flux and establish it in the core is negligible.
• Eddy current and hysteresis losses are negligible

V = -E1
E2
E1
I0
I1
V = -E 1 1
I (Negligible) 0
E = V I 2 2 2
E1
(a) No Load (b) On Load

I1
2
(N /N ) 1 2
I2 I2
I1 I1
Z2
N1 N1 N2 N2 Z2
Z2
V1 V1
V1
2
(N /N ) 1 2
(A) (B) ( C )

Testing Transformers
• Measurement of winding resistance
• Measurement of Voltage ratio
• Test Phasor voltage relationship
• Measurement of impedance voltage, short circuit
• impedance and load loss.
• Measurement of no load loss and no load current.
• Measurement of insulation resistance.
• Dielectric test
• Temperature rise

Electromechanical Energy Conversion
• The electromechanical energy conversion device is a link
between electrical and mechanical systems.
• When the mechanical system delivers energy through the
device to the electrical system, the device is called a generator.
• When electrical system delivers energy through device to
the mechanical system, the device is called a motor.

For a Generator
Tm = Te + Tf
and, e and i are in same direction.
where,
Tm – is Mechanical Torque
Te – is Electrical Torque
Tf – is Torque lost due to friction

Right-hand Rule
Hold the conductor in right hand with fingers closed
around conductor and thumb pointing towards
the direction of the current. The fingers will point towards the
direction of the magnetic lines of the flux produced around the
conductor.

Cork Screw Rule
The direction and the travel in which it has to be
rotated are related to each other the same way as the
direction of the current in the conductor and the
direction of the field that would be produced due to the
current. The magnetic field exists in the plane
perpendicular to the conductor.

Flux produced by Current
Carrying Coil
Flux can be produced by causing the current to flow through a
coil instead of a conductor.
The direction of the magnetic flux in the coil is given by the
Right-hand rule.

E.m.f induced due to Rotation of a Conductor in a
Magnetic Field
The e.m.f. induced due to rotation of a conductor or a coil
in a magnetic field is given as:
e.m.f. (e) = B L v
where:
B – is flux density of magnetic field (wb/m2)
L – length of the conductor perpendicular to magnetic
field (meters)
v – velocity of conductor (m/sec)

Fleming’s Right Hand Rule for Determining the Direction
of e.m.f. Induced
Fleming’s right hand rule states that if the forefinger of the
right hand points in the direction of the field and the thumb
towards the motion, then the middle finger points towards the
direction of e.m.f. induced.

Fleming’s Left Hand Rule
Gives the relationship between the direction of the current, the
direction of field and the direction of the motion. If the
forefinger of the left hand points towards the field, the middle
finger points towards the direction of the current and the
thumb points towards the direction of motion.

Force on a Conductor in a Magnetic Field
If a conductor carries current in a magnetic field, then a
mechanical force is exerted on it. The force exerted on the
conductor is given as :
F = B L I

Torque on a Conductor in a Magnetic Field
If the conductor on a rotor has a radius of r, then the torque produced on
the conductor is given as :
T = F r
or
T = B L I r
For a coil with two sides, the electromagnetic torque will be double.
The Power is given as :
Power = T wr

A Generator or a Motor Action
e.m.f. is induced enough to overcome the drop of volts in
resistance of the winding and supply the load at the required
voltage.
The electromagnetic torque produced by current-carrying
conductors is counter torque, opposing the rotation.

Motor Armature
The rotational or speed e.m.f is produced in opposition to the
applied voltage. This is known as counter or back e.m.f.
Mechanical torque is produced as required by the load driven
by the motor. For more torque and mechanical power output,
there must be more input to the motor from the mains. The
motor draws current according to the requirement of the load.

Basic Characteristics of Electrical Machines
• The voltages are induced in the windings, the load currents
and the terminals voltages under different loading conditions.
• The speed at which the machine works under different
loading conditions, frequency.
• The power input or the output received from the machine.
• The torque produced under different loading conditions.

Single Phase Power System
W= V I cosf
here,
W– is the power (watts)
V – is R.M.S. Voltage
I – is R.M.S. Current
cosf – is Power Factor [ =W/ V I ]

Three-Phase Power Systems
For Delta-connected system :
Line Voltage = Phase Voltage
Line Current = 1.732 * Phase Current
For Star-connected system :
Line Voltage = 1.732 * Phase Voltage
Line Current = Phase Current

Clamp-on Meter
When an AC circuit is to be tested, a Clip-around or clamp-on
meter is quite useful. The clamp-on meter has a probe in the
form of iron or ferrite core which can be opened to allow it to
be clamped-on or clipped over the current carrying conductor.
When the clip is closed, it forms the magnetic core of a
transformer.

Megohmmeter
High resistance measurements of the order of megaohms are
required in electrical equipment testing and troubleshooting.
Megohmmeter based on megaohm Bridge method is used for
high resistance measurements.

Harmonics
• Fundamental frequency of AC power distribution system
is 50Hz or 60 Hz
• Harmonic frequency - Any sinusoidal frequency, which is
a multiple of the fundamental frequency
• Can be even or odd multiples of the sinusoidal
fundamental frequency

Linear and Non Linear Loads
• Linear electric load - Draws purely sinusoidal current
when connected to sinusoidal voltage source –
Resistors, capacitors, inductors
• Non Linear electric load – Draws non-sinusoidal
current when connected to sinusoidal voltage source –
diode bridge, thyristor bridge, variable speed drives,
rectifiers

Fundamental wave with 3rd Harmonic

Harmonic Order
• The Multiple, that the harmonic frequency is of the
fundamental frequency
Harmonic frequencies of 50 Hz fundamental are:
Even Harmonics Odd Harmonics
2nd harmonic -100 Hz 3rd harmonic - 150 Hz
4th harmonic - 200 Hz 5th harmonic - 250 Hz
6th harmonic -300 Hz 7th harmonic - 350 Hz

DO YOU WANT TO KNOW MORE?
If you are interested in further training or information,
please visit:
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Troubleshooting, Maintenance and Protection of AC Electrical Motors and Drives

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Troubleshooting, Maintenance and Protection of AC Electrical Motors and Drives