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Chapter 4
Three-Phase Motors
Three-Phase Motor Construction •
Operating Principles • Motor Load
and Torque • Motor Power • Motor
Efficiency
Chapter 4 — Three-Phase Motors
The stator consists of
a core and windings
and is enclosed
within a housing.
Chapter 4 — Three-Phase Motors
Many individual laminated sheets are pressed
together into the housing, becoming the stator frame.
Chapter 4 — Three-Phase Motors
A 3-phase stator is
wound with coils that
are connected to
produce the three
separate phases, A,
B, and C.
Chapter 4 — Three-Phase Motors
During motor manufacture, an insulating material
called slot paper is first laid in the slot to provide
protection and electrical insulation.
Chapter 4 — Three-Phase Motors
Varnish is sanded
from the bore to allow
for a minimal air gap
between the stator
and rotor.
Chapter 4 — Three-Phase Motors
Feet are attached to the
housing to provide a
method of mounting the
motor to a base.
Chapter 4 — Three-Phase Motors
The rotor core consists
of many thin iron sheets
laminated together.
Chapter 4 — Three-Phase Motors
The end of the shaft is
machined with a keyway
to contain a bar-type key.
Chapter 4 — Three-Phase Motors
It takes 720 electrical
degrees, or two electrical
cycles, to complete one
revolution in a 4-pole
motor.
Chapter 4 — Three-Phase Motors
Inductive reactance increases with increasing
frequency and decreases with decreasing frequency.
Chapter 4 — Three-Phase Motors
The sine curves at 0
degrees show –5 A for
phase A, +10 A for
phase B, and –5 A for
phase C. The + and –
signs indicate the
direction of the current
and the numbers
represent the
magnitude of the
current.
Chapter 4 — Three-Phase Motors
The motor nameplate
typically has a wiring
diagram depicting the
proper wiring connections for the desired
operation.
Chapter 4 — Three-Phase Motors
In a wye-connected,
3-phase motor, one
end of each of the
three phase windings
is internally connected
to the other phase
windings. The
remaining end of each
phase is then brought
out externally to form
T1, T2, and T3.
Chapter 4 — Three-Phase Motors
As the current
changes, the stator
poles move to follow
the strongest current.
Chapter 4 — Three-Phase Motors
In a delta-connected,
3-phase motor, each
phase is wired end-toend to form a
completely closed
circuit. At each point
where the phases are
connected, leads are
brought out externally
to form T1, T2, and T3.
Chapter 4 — Three-Phase Motors
As the current
changes, the stator
poles move to follow
the strongest current.
Chapter 4 — Three-Phase Motors
Each phase coil (A, B, and C)
is divided into two equal parts
and the coils are connected in
a standard wye connection.
Chapter 4 — Three-Phase Motors
Each phase coil (A, B, and C)
is divided into two equal parts
and the coils are connected in
a standard delta connection.
Chapter 4 — Three-Phase Motors
Manufacturers of dualvoltage, 3-phase motors
sometimes do not make
the internal connections.
The internally unconnected motors have 12
leads coming out of the
motor box labeled T10,
T11, and T12. The
connections are made
externally by the installer.
Chapter 4 — Three-Phase Motors
The direction of
rotation of 3-phase
motors can be
reversed by
interchanging any two
of the 3-phase power
lines to the motor.
Chapter 4 — Three-Phase Motors
Motor power is rated in
horsepower or watts.
Chapter 4 — Three-Phase Motors
The four most common
types of torque related to
motors are locked-rotor
torque, full-load torque,
pull-up torque, and
breakdown torque.
Chapter 4 — Three-Phase Motors
The torque-speed
characteristic of a
motor must match
the load the motor is
to drive.
Chapter 4 — Three-Phase Motors
Constant-horsepower
motors are used to drive
loads that require the
same horsepower output
at different speeds.
Chapter 4 — Three-Phase Motors
Constant-torque motors
are used to drive loads
that require a constant
torque output at different
speeds.
Chapter 4 — Three-Phase Motors
Variable-torque, multiplespeed motors are used to
drive fans, pumps, and
blowers that require an
increase in both torque
and horsepower when
speed is increased.
Chapter 4 — Three-Phase Motors
True power can be produced only when current
and voltage are both are
positive or both negative.
Chapter 4 — Three-Phase Motors
For circuits with mixed
inductive and resistive
components, the current
lags the voltage by a value
between 0° and 90°.
Chapter 4 — Three-Phase Motors
Power factor correction
capacitors can be
placed ahead of an
electric motor drive in
the AC supply lines but
not between the drive
and motor.
Chapter 4 — Three-Phase Motors
The five major
components of motor
energy losses are
resistance losses,
core losses, bearing
losses, windage
losses, and sound
losses. These losses
add up to the total
loss of a motor.