Practical Motor Protection, Control and Maintenance Technologies

Practical Motor
Protection,
Control and
Maintenance
Technologies
Technology www.idc-online.com/slideshare Technology TTrraaiinniinngg tthhaatt Wwoorrkkss

Fundamentals of motor technology and construction
• Basic principles of rotating electric machines
• Fundamental principles of speed control
• Efficiency, torque, inertia, horsepower/power factor
• Torque-speed curves
• Induction/synchronous/wound rotor types
• Basic construction and physical configuration, windings
• Principles of operation and performance

Motoring Action
1. As voltage is applied to the stationary conductors, magnetic field is produced.
2. This magnetic field in turn induces voltage in the rotor conductors, in case of
some motors (induction motors) or voltage is externally applied to the rotor
conductors.
3. This voltage also produces magnetic field.
4. The magnetic field of the stator and rotor, together, put the rotor in running
condition.

Depiction of a motoring action

Electric machine concepts
• Electrical machines (either a motor or a generator) can be broadly classified as DC
Machines and AC Machines – depending on the nature of supply given to them.
• An electrical machine can be interchangeably associated with either a motor or a
generator.
• The fundamentals for a particular category (like DC or Synchronous) are identical
for both the generator and motor.

Simple depiction of a motor

A simple DC machine
• A simple DC machine has a current carrying coil supported in between two
permanent magnets
• These magnets create the necessary flux (opposite poles facing each other)
• Poles of a magnet, North & South, are always together
• Accordingly Poles of a motor are expressed in pairs
• The coil – armature – can rotate freely inside

Electromagnetic circuit of a DC machine
Armature: The coil ends are connected to a DC source
- Current flows through the circuit
- Coils behave like a bar magnet
- Results in Magnetic flux
Field: Produces flux lines – across the Poles
Rotation: As a result of Forces of attraction / repulsion between two
fields.

Electromagnetic circuit of a DC machine

Electromechanical features of DC machines
•Yoke: Motor housing
•Stator: Houses Field winding Poles & Interpoles
•Rotor: Armature carrying the commutator apart from the rotating
shaft
•Main pole: Houses field winding
•Commutator: Rotating, current transferring agent. Helps in
developing a stationary field by the rotating armature
•Brushes: Stationary, current carrying conductors making contact
with the commutator
•Pole Shoe: Helps in maximizing the flux distribution

Electromechanical features of DC machines

Commutator of a DC machine
•Used only in DC Machines
•Extends supply to the Armature
•Any electrical machine will have two magnetic
fields developed by its:
•Stator and
•Rotor
•Both the fields of a DC motor are Stationary
•Acts as a bridge between stationary supply wires
and rotating Armature

Simple cross sectional view of a DC machine

Speed characteristics of various types of DC
Motors
Shunt: Most commonly used –
Speed drops slowly as the motor is loaded.
Series: Speed drops heavily as the motor is loaded
Develops very high speeds at low load
Used mainly for traction
Compound:
Cumulative – in between Shunt and Series
Differential – speed rises with the load applied

Speed characteristics of various types of DC Motors

Representation of apparent power and real power

Torque characteristics of various types of Motors
•A curve drawn between the TORQUE developed by a Motor against
the speed / loading (current)
•Useful in the selection of appropriate motors for a particular Load
requirement
•A proper match – gives the best performance
•A mismatch – havoc for running the Plant
•Helpful in estimating the acceleration time

Torque characteristics of various types of DC Motors

Torque characteristics of AC Induction Motors

Torque characteristic of a Synchronous Motor

Squirrel Cage Induction Motor (SCIM)
•It is the most common type of AC motor.
•Known as the workhorse of Modern Industry
•It is the most cost effective motor
•Can be designed for any kind of environment.
•Construction of the rotor gives this name.
•The rotor consists of a series of conducting bars laid into slots
carved in the face of the rotor and shorted at either end by large
shorting rings.

Simple view of squirrel cage induction rotor

Slip Ring Induction Motor
•The motor starts with a full resistance bank
•As speed of the motor increases, the resistances are shorted, one by
one
•As the motor reaches full speed, the whole bank of resistance is
shorted out and the motor now runs alike a squirrel cage induction
motor
•Very much useful to develop high starting torque
•Reduced starting current puts less burden on the power system

Simple view of slip ring induction rotor

Synchronous motor
•Synchronous motor is a constant speed motor
•Can be used to correct the power factor of the 3-phase system.
•Like the Induction motor in terms of the stator, the synchronous
machine has either a permanent magnet arrangement or an
electromagnet (with current supplied via slip rings) rotor.
•In simple terms, the rotor will keep locking with the rotating magnetic
field in the stator.
•In many synchronous machines, a squirrel cage is in incorporated into
the rotor for starting.

Single phase induction motor
•This motor is used mostly in small sizes, where polyphase current is not
available
•Characteristics are not as good as the polyphase motor and for size
larger that 10 HP, the line disturbance is likely to be objectionable
•These motors are commonly used for light starting and for running
loads up to 1/3 HP Capacitor and repulsion types provide greater torque
and are built in sizes up to 10 HP.
•Mostly finds application in domestic sector

Shaded pole single phase induction motor

Assembly details of a typical AC induction motor

Stator and rotor laminations
• The magnetic path of a motor comprises a set
of slotted steel laminations
• These silicon steel laminations are pressed
into the cylindrical space inside the outer
frame
• The magnetic path is laminated to reduce
eddy currents.... lower losses and lower
heating
• A set of insulated electrical windings, which
are placed inside the slots of the laminated
magnetic path
• Rotor laminations are varied based on the
type of torque characteristic to be realized

Speed-Torque curves of Motor Vs Load
• The machine and the load are the two components of an electro-mechanical
energy conversion system.
• The machine characteristics play a very important role in the operating
behavior of the entire system.
• The steady operating point is the point where the speed torque characteristics
of the load and the motor intersect each other.
• The drive system will settle to a speed corresponding to the point of
intersection of these two curves

Speed-Torque curves of Motor Vs Load

Drive systems engineering
• At the time of engineering the drive systems, following shall be considered
for both the Motor as well as the Load:
– the speed / torque characteristics
– the speed / power characteristics
• Load characteristics can be classified as
– Constant Torque
– Constant Horsepower
– Squared-Exponential Loads - torque varies directly as the speed, and power as the
square of speed
– Cubed-Exponential Loads - torque varies as the square of speed, and power as the
cube of speed

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Practical Motor Protection, Control and Maintenance Technologies

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