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ELECTRIC MOTORAn electric motor is an electromechanical device that converts electrical energy to
mechanical energy.
The mechanical energy can be used to perform work such as rotating a pump
impeller, fan, blower, driving a compressor, lifting materials etc.
BASIC WORKING PRINCIPLE
TYPES OF MOTOR LOADSMotor loads
Description Examples
Constant torque loads
Output power varies but torque is constant
Conveyors, rotary kilns, constant-displacement pumps
Variable torque loads
Torque varies with square of operation speed
Centrifugal pumps, fans
Constant power loads
Torque changes inversely with speed
Machine tools
CLASSIFICATION OF MOTORS
Electric Motors
Alternating Current (AC) Motors
Direct Current (DC) Motors
Synchronous Induction
Three-PhaseSingle-Phase
Self ExcitedSeparately Excited
Series ShuntCompound
TYPES OF AC MOTORS
* Electrical current reverses direction* Two parts: stator and rotor
Stator: stationary electrical component Rotor: rotates the motor shaft
* Speed difficult to control* Two types
Synchronous motor Induction motor
AC MOTOR: INDUCTION MOTOR Most common motors in industry
Advantages: Simple design Inexpensive High power to weight ratio Easy to maintain Direct connection to AC power source
COMPONENTS OF INDUCTION MOTOR
A 3-phase induction motor has two main parts:
• A stator – consisting of a steel frame that supports a hollow, cylindrical core of stacked laminations. Slots on
the internal circumference of the stator house the stator winding.
• A rotor – also composed of punched laminations, with rotor slots for the rotor winding.
COMPONENTS OF INDUCTION MOTOR contd…
There are two-types of rotor windings:
• Squirrel-cage windings, which produce a squirrel-cage induction motor (most common)
• Conventional 3-phase windings made of insulated wire, which produce a wound-rotor induction motor (special characteristics)
Induction Motor: Squirrel cage rotor Squirrel cage rotor consists of copper bars,
slightly longer than the rotor, which are pushed into the slots.
The ends are welded to copper end rings, so that all the bars are short circuited.
In small motors, the bars and end-rings are diecast in aluminium to form an integral block.
Induction Motor: Wound Rotor A wound rotor has a 3-phase winding, similar to
the stator winding. The rotor winding terminals are connected to
three slip rings which turn with the rotor. The slip rings/brushes allow external resistors to be connected in series with the winding.
The external resistors are mainly used during start-up –under normal running conditions the windings short circuited externally.
* Construction is on next slide.
Wound Rotor & its connections
Induction Motor: Operating Principle Operation of 3-phase induction motors is based upon the
application of Faraday’s Law and the Lorentz Force on a conductor.
Consider a series of conductors (length L) whose extremities are shorted by bars A and B. A permanent magnet moves at a speed v, so that its magnetic field sweeps across the conductors.
Operating Principle Contd… The following sequence of events takes place:1. A voltage E = BLv is induced in each conductor while it
is being cut by the flux (Faraday’s Law)2. The induced voltage produces currents which circulate
in a loop around the conductors (through the bars).3. Since the current-carrying conductors lie in a magnetic
field, they experience a mechanical force (Lorentz force).
4. The force always acts in a direction to drag the conductor along with the magnetic field.
Now close the ladder upon itself to form a squirrel cage, and place it in a rotating magnetic field – an induction motor is formed!
Induction Motor: Rotating Field Consider a simple stator with 6 salient poles -
windings AN, BN, CN. The windings are mechanically spaced at 120° from
each other. The windings are connected to a 3-phase source. AC currents Ia, Ib and Ic will flow in the windings, but
will be displaced in time by 120°. Each winding produces its own MMF,which creates a
flux across the hollow interior of the stator. The 3 fluxes combine to produce a magnetic field that
rotates at the same frequency as the supply.
Rotating Field Contd…
Induction Motor: Stator Winding
In practice, induction motors have internal diameters that are smooth, instead of having salient poles.
In this case, each pole covers 180° of the inner circumference of the rotor (pole pitch = 180°).
Also, instead of a single coil per pole, many coils are lodged in adjacent slots.
The staggered coils are connected in series to form a phase group.
Spreading the coil in this manner creates a sinusoidal flux distribution per pole, which improves performance and makes the motor less noisy.
Stator Winding Contd…
Number of Poles – Synchronous Speed The rotating speed of the revolving flux can be reduced by
increasing the number of poles (in multiples of two). In a four-pole stator, the phase groups span an angle of 90°.
In a six-pole stator, the phase groups span an angle of 60°. This leads to the definition of synchronous speed: Ns = 120 f / pWhere Ns = synchronous speed (rpm)f = frequency of the supply (Hz)p = number of poles
For 50Hz ,synchronousSpeeds (Ns) include 3000rpm,1500rpm, 1000 rpm, 750rpm…
INDUCTION MOTOR : SLIP The difference between the synchronous speed and
rotor speed can be expressed as a percentage of synchronous speed, known as the slip.
s = (Ns – N) Ns
Where s = slip, Ns = synchronous speed (rpm), N = rotor speed (rpm)
• At no-load, the slip is nearly zero (<0.1%).• At full load, the slip for large motors rarely exceeds
0.5%. For small motors at full load, it rarely exceeds 5%.
• The slip is 100% for locked rotor.
Induction Motor: Frequency induced in the rotor
The frequency induced in the rotor depends on the slip:
fR = s f
fR = frequency of voltage and current in the rotor
f = frequency of the supply and stator fields = slip
Induction Motor: Active Power Flow Efficiency – by definition, is the ratio of output /
input power: η = PL / Pe Rotor copper losses: PJr = s Pr
Mechanical power: Pm = ( 1-s)Pr
Motor torque: Tm = 30Pr
πNs
Where: Pe = active power to stator
Pr = active power supplied to rotor
PL = Shaft Power
Power Losses
Induction Motor: Relationship between Load, Speed and Torque
At full speed: torque and stator current are zero
At start: high current and low “pull-up” torque
At start: high current and low “pull-up” torque
At 80% of full speed: highest “pull-out” torque and current drops