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SMJE 2103
Induction Motor
Induction Motor
Scope of discussion
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Structure
Basic concept
Equivalent Circuit
Power and Torque
Speed Control
Testing
Induction Motor
- Structure -
Squirrel Cage – no winding and slip ring
Wound Rotor – the winding connected to slip rings
Induction Motor
- Basic Concept Induced Torque:
- Speed of magnetic field
- Induced Voltage
- Induced Torque
Induction Motor
- Basic Concept Rotor Slip:
Induction Motor
- Basic Concept Rotor Frequency
Example 1
A 208V, 10hp, 4 ploe, 50 Hz, Y-connected induction
motor has a full-load slip of 5%.
a) What is the synchronous speed of this motor?
b) What is the rotor speed of this motor at the
rated load?
c) What is the rotor frequency of this motor at the
rated load?
d) What is the shaft torque of this motor at the
rated load?
Example 2
A 220-V, three-phase, two-pole, 50-Hz
induction motor is running at a slip of 5
percent. Find:
(a) The speed of the magnetic fields in
revolutions per minute
(b) The speed of the rotor in revolutions per
minute
(c) The slip speed of the rotor
(d) The rotor frequency in hertz
Example 3
A 480-V, three-phase, four-pole, 60-Hz
induction motor running at a slip of 0.035. Find;
(a) The speed of the magnetic fields in
revolutions per minute
(b) The speed of the rotor in revolutions per
minute
(c) The slip speed of the rotor
(d) The rotor frequency in hertz
Example 4
A three-phase, 60-Hz induction motor runs at
890 r/min at no load and at 840 r/min at full
load.
(a) How many poles does this motor have?
(b) What is the slip at rated load?
(c) What is the speed at one-quarter of the rated
load?
(d) What is the rotor’s electrical frequency at
one-quarter of the rated load?
Induction Motor
- Equivalent Circuit -
Rotor voltage – Lockedrotor voltage
Rotor frequency – Frequency of the
induced voltage
Induction Motor – Equi. Circuit
RR is constant but XR
depend on S
Rotor current
flow
Overall rotor impedance
considered S
Referring to stator side,
considered turn ratio – new
Rotor voltage
Induction Motor – Basic Concept
Rotor
impedance
Rotor
current
If,
Final perphase equi
circuit
Example 5/Tutorial
A 460V, 25hp, 50Hz, 4 pole, Y-connected induction
motor has the following impedances in ohms per
phase referred to stator circuit:
The total rotational losses are 1100W and are
assumed to be constant. The core loss is lumped in
with the rotational losses. For a rotor slip of 2.2% at
rated voltage and rated frequency, find the motor’s
a) speed
d) Pconv and Pout
b) stator current
e) ind and load
c) power factor
f ) efficiency
Example 6
A 208-V, two-pole, 60-Hz Y-connected wound-rotor induction
motor is rated at 15 hp. Its equivalent circuit components are
R1 = 0.200 Ω
R2 = 0.120 Ω
XM = 15.0 Ω
X1= 0.410 Ω X2 = 0.410 Ω
Pmech = 250 W
Pmisc ≈ 0
Pcore = 180 W
For a slip of 0.05, find
(a) The line current IL
(b) The stator copper losses PSCL
(c) The air-gap power PAG
(d) The power converted from electrical to mechanical form
Pconv
(e) The induced torque τind
(f) The load torque load τload
(g) The overall machine efficiency
(h) The motor speed in revolutions per minute and radians per
second
Induction Motor
- Power and Torque -
Induction Motor
- Power and Torque • No output voltage instead of mechanical
force.
• Higher speed, higher friction, windage and
stray losses.
• But component losses change inverse.
Example 7/Tutorial
A 480V, 60Hz, 50hp, 3 phase induction motor is
drawing 60A at 0.85 Pf lagging. The stator copper
losses are 2 kW, and the rotor losses are 700W. The
friction and windage losses re 600W, the core losses
are 180W, and the stray losses are negligible. Find:
a) The air gap power PAG
b) The power converted Pconv
c) The output power Pout
d) The efficiency of the motor
Induction Motor
- Power and Torque -
Induction Motor
- Power and Torque -
Induction Motor
- Power and Torque -
Induction Motor
- Power and Torque -
Induction Motor
- Power and Torque -
Induction Motor
- Power and Torque Air gap needs R2/S and rotor copper loss needs R2
Induction Motor
- Speed Control • Two methods
a) Varying stator and rotor magnetic field speed
- electrical frequency or changing the
number of poles
b) Varying slip
- varying rotor resistance or terminal
voltage
Example 8
A 50-kW, 440-V, 50-Hz, six-pole induction motor has a slip
of 6 percent when operating at full-load conditions. At
full-load conditions, the friction and windage losses are
300 W, and the core losses are 600 W. Find the following
values for full-load conditions:
(a) The shaft speed nm
(b) The output power in watts
(c) The load torque τload in newton-meters
(d) The induced torque τind in newton-meters
(e) The rotor frequency in hertz
Example 9
A three-phase, 60-Hz, four-pole induction motor
runs at a no-load speed of 1790 r/min and a fullload speed of 1720 r/min. Calculate the slip and
the electrical frequency of the rotor at no-load
and full-load conditions. What is the speed
regulation of this motor?
Induction Motor
- Speed Control (pole changing)Created two poles, N and S
Created an extra poles by
changing current polarity
(only 2 speeds)
Induction Motor
- Speed Control (Frequency) With or without adjustment to the terminal voltage:
a) Vary frequency, stator voltage adjusted –
generally vary speed and maintain operating
torque.
b) Vary frequency, stator voltage maintained – able
to achieve higher speeds but a reduction of
torque.
Could be combined together, easy way by
introducing power electronic fields.
Induction Motor
- Speed Control (terminal voltage) -
Induction Motor
- Speed Control (rotor resistance) -
Induction Motor
- Testing There are basically 3 types of test in modeling
induction motor parameters:
1. No-load test
2. DC test
3. Locked rotor test or blocked rotor test
Equivalent circuit element – R1, R2, X1, X2 and XM
Induction Motor
- Testing (no load) -
Measuring rotational losses and providing
info about its magnetizing current (X1 + XM)
Due to the
rotor
speed
close to
sync
speed, S
small and
higher
rotational
losses
Induction Motor
- Testing (no load) -
Induction Motor
- Testing (DC test) -
To find R1 (stator resistance).
a) Dc voltage is applied and adjusted to rated condition.
b) Voltage and current flow are recorded.
Induction Motor
- Testing (Locked rotor)-
Steps:
1- locked
2- AC voltage is applied and currents flow is
adjusted to full load condition
3- Measure voltage, current and power flow
Induction Motor
- Testing (Locked rotor)-
Induction Motor
- Testing (Locked rotor)-
Example 10/Tutorial
The following test data were taken on a 7.5 hp, 4-pole, 208V, 50Hz, design delta-star
connected induction motor having a rated current of 28A
(a) Sketch the per-phase equivalent circuit for this motor
(b) Find the slip at the pullout torque, and find the value of the pullout torque