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INTRODUCTION
The induction unit is the basic unit in all lAC
relays. Fig. 1 shows the induction unit mounted in
the cradle. These units are of the induction -disk
construction type. The disk is actuated by a cur
rent operating coil on a laminated U-magnet. The
disk shaft carries the moving contact which com
pletes the alarm or trip circuit when it touches the
stationary contact or contacts. The disk shaft is
1
restrained by a s p i r a
contact -closing current and its motion is retarded
by permanent magnets acting on the disk to give the
correct time delay.
There is a seal-in element mounted on the front
to the left of the shaft. This element has its coil in
series and its contacts in parallel with the main
contacts such that when the main contacts close the
seal-in element picks up and seals in. When the
seal-in element picks up, it raises a target into view
which latches up and remains exposed until released.
the cover.
APPLICATION
The induction unit is· :the main unit in all lAC
r elays, supplying the inverse time delay character
istics of the relay and sounding an alarm or tripping
the breakers for overload currents which cause it
to close its contacts.
OPERATING
CHARACTERISTICS
The induction unit may have one or two circuit
closing contacts which close as the current increases
to the pick -up value as set on the tap block. The
time delay in closing the contacts is determined by
the setting of the time dial (Fig. 1). The time-cur
rent characteristics are shown in Fig. 2.
RATINGS
The induction element is designed to use any one
of
th r e e operating coils, each having a different
combination of taps as follows: 4,
16 amperes; 1.5, 2.0, 2.5, 3.0, 4.0, 5,0 and 6.0 am
peres; 0,5, 0,6, 0.8, 1.0, 1.2, 1.5, and 2.0 amperes.
The current -closing rating of the contacts is
30 amperes for voltages not exceeding 250 volts.
The current -carrying ratings are affected by the
selection of the tap on the target and seal-in coil
as indicated in the following table:
Function
INDUCTION UNIT
spring to give the proper
5,
6, 8, 10, 12, and
0.2 Amp Tap
30
5
4
0.8
Time Overcurrent Felays
The 2-ampere tap has a d-e resistance of 0.13
ohms and a 60 cycle impedance of 0,53 ohms while
the 0.2-ampere tap has a 7 ohm d-e resistance and
a 52 ohm 60 cycle impedance. The tap setting used
on the seal-in element is determined by the current
drawn by the trip coil.
The 0.2-ampere tap is for use. wi th trip coils
that operate on currents ranging from 0.2 up to 2.0
amperes at the minimum control voltage. If this
tap is used with trip coils requiring more than 2
amperes, there is a possibility that the the 7-ohm
resistance will reduce the current to so low a value
that the breaker will not be tripped.
The 2 -ampere tap should be used with trip coils
that take 2 amperes or more at minimum control
ceed 3u amperes at the maximum control voltage.
If
the tripping current exceeds 30 amperes an aux
iliary relay should be used, the connections being
such that the tripping current does not pass through
the contacts or the target and seal-in coils of the
protective relay.
BURDENS
Burdens for the standard coils are given in the
following table. These a r e calculated burdens at
five amperes based on burden of minimum tap.
Volt-ampere burdens for the lowest tap on any
of the three coils can be determined for any value
of current, up to 20 times tap setting, from Fig.
Coil
Freq.
Tap
4-16
60
4.0
4.0
50
4.0
25
1.5-6.0
1.5
60
50
1.5
25
1.5
0,5-2.0
60
0.5
530.0
50
0.5
470.0
25
0.5
430.0
Type lAC
3.
Volt-
Imp.
Amps
Ohms
PF
0.35
0.29
8.8
8.0
0.32
0.31
7.5
0.30
0.36
59.0
2.36
0.26
0.2 8
52.0
2.08
1.92
0.34
48.0
21.2
0.26
0.28
18.8
17.2
5
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