Chapter 1
Repair of Electrical Apparatus Coil (Contactor Coil)
A contactor is an electrically-controlled switch used for switching an electrical power circuit.
it is typically controlled by a circuit which has a much lower power level than the switched
circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch. It is usually
designed for both normally closed and normally open applications. A contactor has three
components: this includes power contacts, auxiliary contacts, and contact springs.
The electromagnet (or "coil") provides a driving force to close or open the contacts.
When current passes through the electromagnet, a magnetic field is produced; this attracts the
moving core of the contactor. The moving contact is propelled by the moving core; the force
developed by the electromagnet holds the moving and fixed contacts together. When the
contactor coil is de-energized, a spring returns the electromagnet core to its initial position and
opens the contacts.
For repairing the apparatus coil the following equipment is required.
1. Winding Machine
2.
Press board
3. Soldering Equipment & Materials
4. Micrometer, Venire caliper, Screw drivers and Steel ruler
In order to rewind a coil, it is necessary to know the following data:
? Operating Voltage
? Number of turns
? Cross-section of wire ( or diameter of the wire) and type of operation ( ac or dc)
Usually the above data are taken from damaged coil itself. But sometimes a coil may be
completely burned out or damaged and difficult to get the necessary data. In such cases it is
necessary to know the winding data before winding the coil. The following method is used to
determine the necessary data quickly and with fair accuracy.
1. Number of turns
The number of turns in the coil depends upon the cross-section of a core on which it is
mounted. If the core cross-section is known, the number of turns per volt depending
upon coil duty, and this can be determined from graph in fig (1)
Core
Core cross-section C = a * b in cm2
a=core length in cm
b=core width in cm
For example, if the core cross-section of an apparatus coil to be designed for continuous
duty is 2.9 cm2 ,the number of turns per volt depending upon coil duty, can be
determined from graph in fig(1) becomes 12 turns per volt . Therefore, the total
number of turns is
N =operating voltage X number of turns per volt
For 380V operating voltage, N=380X12=4560 turns
For 220 V operating voltage, N = 220 *12 = 2640 turns
Fig.1: Graph for determining number of turns per volt for continuous duty apparatus coil.
X _axis – cross section area of core in cm2
Y_axis – turn per volt in T/V
@ 2.9 cm2 the number of turn per volt = 12 T/V
1. Cross-section of the wire
The cross-section area of wire or diameter of wire depends on the opening area of the
core and the total number of turns.
Window area (opening area of core) qo = L * h
For example if qo is 680 mm2, the corresponding fill factor (Kf) from the graph fig (2) is
0.21 or 21%, which means the winding conductor occupy only 21% of the core opening
area.
Fig 2: Graph for determining fill factor (Kf)
Y_axis – Fill factor (Kf)
@ 680 mm2opening area of core, fill factor is 0.21
Therefore, the total cross-section of the entire conductor can be determined by
multiplying the core opening area with its corresponding fill factor. Hence,
q = qo * Kf = 680 mm2 * 0.21 = 143 mm2
In order to determine the diameter of wire, first you have to determine the total number
of turns per mm2:
w = N/q = 4560 turn/143 mm2 = 31.88 = 32turn/mm2
Or 2640 turn/143 mm2 = 18.5 = 19 turn/mm2
Where, w is the number of turns per cross-section area of the window. The conductor diameter is
determined from the graph given in fig (3).
For our case, W=4560 turn/143 mm2=32 turns/mm2, hence from the graph the diameter of the
conductor without insulation dwi becomes 0.17mm. If the diameter obtained from the graph is
not standard, take the nearest standard diameter.
Fig. 3 Graph for determining the diameter of enamel insulation wire
Y axis – number of turn per 1mm2 cross-section area of window
@32 turn/mm2 the diameter of wire without insulation is approximately 0.17 mm
3. Prepare a former
For preparing a former, first it is necessary to know the dimension of the press board. For such
case determine the value of a, b, C (2*h), and ‘f’ from a coil or from the core itself.
A wood is inserted inside the former for better uniformity of the winding. Before inserting, make
a hole at a center of the wood.
Step 1:
Step 2:
Step 3: preparing former by combining the above
Step 4: Winding a coil using winding machine:
4.1 Before winding a coil on the former using winding machine, connect and solder the
beginning of the coil to be winding to the insulated wire (or terminal wire) and insulate it by
insulating tape.
4.2: Wind the coil on the former using the winding machine. Note that the coil being wound has
no bare spots.
4.3. When the winding is completed, connect and solder the end of the coil being wound to the
insulated wire, and insulate it by insulating tape.
4.4. Finally rolled up the coil by the cotton tape then impregnate the coil by varnish and dry it.
Step 5: Test the rewinding coil