The document discusses the operation of magnetic contactors and auxiliary contactors used in an automatic transfer switch (ATS) system for backup power. It describes: 1. How magnetic contactors use electromagnetic coils to switch between open and closed positions to allow or block power flow. Auxiliary contactors are used to control the coils. 2. The different operating conditions of the ATS system depending on whether primary or backup power sources are available. Auxiliary contacts ensure only one contactor is energized at a time to avoid overloading generators. 3. Solutions to prevent generators from being damaged if contactors switch over too quickly, such as using timers to delay transitions and ensure contactors fully open before the next closes.

1. Emran Hossain
FSE, R3-Bogra
+8801678702640
Emran Hossain 15/18/2016

2. Magnetic Contactor
Gives Contact by Magnetic action
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3. Upper Part
Lower Part
Separator spring
Coil
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5. Coil Power 220V AC
Single phase Input three Phase LineOutput three Phase Line
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6. Line InLine Out
Path is open and we
cannot get any output
from here.
If we give 220V in this coil then it will convert
the lower part to a electro magnet. When it
become a magnet, it will attract the upper
part. Upper part will come down and the
circuit will be closed
Now what to do… ?
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7. Line InLine Out Line InLine Out
Coil Not Energized Coil Energized
Before After
Open Path
Closed Path
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8. Basic is over
Time to learn something extraordinary….
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9. CP (MC-1) (MC-2) Genset
PDB DG
ATS
Condition-1: PDB on, DG off =MC-1 will
pass PDB.
Condition-2: PDB off, DG on =MC-2 will
pass DG power.
Condition-3: PDB off, DG on and in the
mean time PDB become active = Both MC
will be on and….
Coil Coil
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CP sensing for DG. Can be given from anywhere like PIB, MDB or Input of CP MC

10. Auxiliary Contactor
Without Auxiliary contactor
With Auxiliary contactor
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11. Below condition is for NC (normally closed)
Auxiliary….
When MC not energized When MC energized
Closed Path Open Path
Auxiliary also have a
mechanical switch which is
attached with the upper part
of the MC. When the upper
path of the MC attracted by
the lower path, status of the
auxiliary also changed to NO
from NC and vice versa.
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12. CP (MC-1) (MC-2) Genset
PDB DGDG
CP
DG= DG controller port. Gives AC phase voltage
CP= DG controller port. Gives DG voltage
A1 A2
A2= Auxiliary 2 (NC)
A1= Auxiliary 1 (NC)
Condition 1-PDB : CP will provide CP phase voltage, A2
auxiliary is NC so voltage directly sent to the coil of MC-1
and PDB will pass. In this condition If DG started by any
cause then A1 is NO in this moment so the coil of MC-2
cannot get 220V. So when MC-1 is on at that time MC-2 is
locked.
Condition 2-DG : DG will provide Genset voltage, A1
auxiliary is NC so voltage directly sent to the coil of MC-2
and DG voltage will pass. In this condition If CP comes then
A2 is NO in this moment so the coil of MC-1 cannot get
220V. So when MC-2 is on at that time MC-1 is locked.
Coil Coil
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CP sensing for DG. Can be given from anywhere like PIB, MDB or Input of CP MC

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ATS-1
ATS-2
In maximum site there is no dual port VDTC. Normally we used to
keep one DG locked.
ATS-2 will take output of ATS-1 as CP
Condition-1 PDB: Auxiliary A2 at ATS-1 is NC so MC-1 will turned on
and pass CP to ATS-2. Again A2 at ATS-2 is NC so MC-3 will pass power.
In this condition MC-2 & MC-4 will be locked this is because A1 in
both ATS will be turned to NO.
Condition-2 DG-1: Auxiliary A1 @ ATS-1 is NC so MC-2 will turned on
and pass CP to ATS-2. Again A1@ ATS-2 is NC so MC-4 will pass power.
In this condition MC-1 & MC-3 will be locked this is because A2 in
both ATS will be turned to NO.
Condition-3 DG-2: As CP unavailable and DG-1 locked so ATS-2 will get
no input from ATS-1. DG-2 started. A1 auxiliary is NC so it will pass DG
voltage to the coil of MC-4 & MC-4 will pass the DG voltage.

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Problem…?
If any of the MC become faulty then total
automation system will fail…
ATS-1
ATS-2

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CP (MC-1) (MC-2) Genset
PDB DG-1DG
CP
A1
A3
DG-2DG
CP
(MC-3) Genset
A5
A6A4A2
Solution

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Condition-1 (DG-1 & 2 locked) PDB: Auxiliary
A5 & A4 is NC MC-1 will turn on and pass CP.
Then A1 & A2 will be NO, by this MC-2 & MC-
3 will be locked.
Condition-2 (DG-2 & PDB locked) DG-1:
Auxiliary A1 & A6 is NC MC-2 will turn on.
Then A3 & A4 will be NO, by this MC-1 & MC-
3 will be locked.
Condition-3 (DG-1 & PDB locked) DG-2:
Auxiliary A3 & A2 is NC MC-3 will turn on.
Then A5 & A6 will be NO, by this MC-1 & MC-
2 will be locked.
CP sensing for DG

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Problem…?
At the time of switching DG MC off and in the same time PDB MC on, very
often it has been found that the DG alternator burns. This is because
sometimes for a very few second both MC keep on and CP voltage pass to the
DG alternator. This phenomena happens for a very few second or mili second
for some internal delay of MC but this is enough to burn to alternator. So
Solution is Ahead…..

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AC Timer
Top View
Side View
It is a relay with two switch. It used to
operate with time. Time can be set by
regulating the top nobes. Time could
be sec, minute and hour.
Its source port is 2&7. Normally its 1+4
& 8+5 is closed. When it energies in
source port with (220V AC) then 1+3 &
8+6 become closed.

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CP (MC-1) DG (MC-2)
DG Auto
port
(Regular
controller)
Timer-1 (PDB) 30 Sec Timer-2 (Auto) 5 Sec Timer-3 (DG) 30 Sec
6 5 4 3 6 5 4 3 6 5 4 3
7 8 1 2 7 8 1 2 7 8 1 2
Battery (-)
A1 A2
VDTC

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Condition-1 PDB: When PDB come to the lower part of MC-1
then timer-1&2 turned on. Now 3 things will happen. 1: After 5s
Timer-2 will turn off Timer-3 and MC-2 will release & auxiliary A2
will be NC. 2: After 5s of CP arrival (-12v) will be supplied to DG
auto port and DG will be shuted down. 3: After 30s Timer-1 will
pass 220v to the MC-1 coil by auxiliary A2 (Now A2 is NC). Until
CP fail DG will not start or if DG manually started then it power
will not pass, because Timer-3 is off so coil of MC-2 will get no
power.
Considering all the lower part as input and DG will start when there is no (-)V in auto port
Condition-2 DG: When CP fails MC-1 will release, A1 will be NC,
Timer-1 & Timer-2 will turn off. Then In Timer-2 8+5 & 1+4 will be
closed. DG auto port will not get (-)v and it will start if VDTC allow
(will allow when site battery voltage is low). After starting Timer-3
will get 220v in its source port and turn on. After 30s it will pass
220v to the coil of MC-2 by A1 (Now A1 is NC). MC-2 will pass DG
power and MC-1 will be locked until DG stops.
Every Timer have some delay to deliver voltage. In this delay time MC’s will perform ON/OFF and our problem resolved.

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CP (MC-1) DG (MC-2)
DG Auto port
(GCS409)
Timer-1 (PDB) 30 Sec Timer-2 (Auto) 5 Sec Timer-3 (DG) 30 Sec
6 5 4 3 6 5 4 3 6 5 4 3
7 8 1 2 7 8 1 2 7 8 1 2
Battery (-)
A1 A2
Start Stop
Battery (+)
VDTC
Battery (+)

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GCS409 controller have two auto port “Remote Start” & “Remote Stop”. Both works with (+12v).
DG will start with (+12V) in start point and will stop with
(+12v) at stop point. But we cannot keep (+v) at stop point
continuously. We have to cut off immediately.
When CP will fail +12v will come to VDTC from Timer-1.
When site battery voltage will be low VDTC will send +12v
to start point. And DG will start.
Timer-4 is a DC Timer. It works with DC voltage. Positive
part of the DC voltage comes directly from DG battery.
And -12v comes from Timer-2. This voltage available only
when CP is available. Whenever CP comes Timer-4 turns
on. And it will supply +12v to stop port until the delay time
(10s) is over.

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