Electrical Machinery (UEE413)
Presentation 7
Three-point starter, Four-point starter and Series
Motor Starter
Course Instructor- Dr. Prasenjit Basak
Associate Professor, Electrical and Instrumentation Engineering Department
Three-Point Starter
● The connection diagram of a three-point shunt motor starter is
shown in Fig. The starter terminals to be connected to the motor
are A (armature), F (field), and L (line). The starting resistance
is arranged in steps between conducting raised studs. As the
starting handle is rotated about its fulcrum, it moves from one
stud to the next; one resistance step is cut out and added to the
field circuit. Each stud has a short time to wait for the motor to
build up speed. This arrangement ensures a high average
starting torque.
● At the start, the handle is brought to stud one. The line voltage
gets applied to the armature with full starting resistance in series
with the armature and to the field with NVC (No-volt coil)/ HC
(Holding coil) in series. Thus, the motor starts with maximum
torque. As it picks up speed, the handle is moved from stud OFF
to stud to the ‘ON’ position shown in Fig. The starting
resistance has been fully cut out and is now included in the field
circuit; being small makes little difference in the field current.
The resistance of NVC is small and forms part of the field
resistance. The voltage across the armature is the line voltage.
The handle is held in this position by the electromagnet excited
by the field current flowing through HC/NVC.
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Three-Point Starter
Two protections are incorporated in the starter.
● NVC (no volt coil): In case of failure of field current (due to accidental or otherwise open circuiting), this coil releases the
handle (held electromagnetically), which goes back to the OFF position under the spring action.
● OL (overload) release: The contact of this relay at armature current above a certain value (overload/ short circuit) closes the
NVC ends, again bringing the handle to OFF position. In the three-point starter if the field regulator is used to reduce the field
current to low values for high motor speed NVC may release the handle causing the motor to shut down where such variation
of field current is desired a four-point starter is used.
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Four-Point Starter
To overcome the problem caused when the field current is
low, NVC/HC is connected across the two lines, one line
connected to the F terminal through the starter and the other
directly to the second line from another L terminal of the
starter. A protective resistance R is connected in series to
limit the NVC current. The starter diagram is drawn in Fig. It
now has four terminals = A F L1 L2. The rest operation
remains the same.
The modern practice is to use a push-button type automatic
starter in industries. Automatic starters carry out essentially
the same functions as the manual ones with electromagnetic
relays that short out sections of the robust metallic starting 1 2
resistors in a predetermined time sequence or when the
armature current has dropped to a preset value.
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Series Motor Starter
1 2
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References
[1] Bimbhra PS, Electrical Machinery, Khanna Publishers 2008
[2] Nagrath IJ and Kothari DP, Electrical Machines, Tata Mcgraw Hills 2004.
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THANK YOU
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