A typical low-voltage generator is built with multi turn stator coils, ranging from one to 16 turns per coil. These coils can either be form wound (where the wire is square or rectangular and the turns are systematically arranged) or random wound (where the wire is round and the arrangement between the turns is not defi nite).With units less than 1,500 kW, the size of the stator and the minimum wire thickness usually do not allow form-wound coils. However, in some cases, either form-wound or random-wound coils can be used. Generators with random-wound coils can be made at a reduced cost, and their capability to withstand severe environmental conditions can be enhanced through the use of vacuum-pressure impregnation (VPI). Still, performance, capability and endurance to the environment make units with form-wound coils superior.
FORM WOUND COILS:
A form winding uses square or rectangular magnet wire. The wire
insulation is designed to handle operating turnto-turn voltages as well as
maximum surge or impulse voltages The coil winding process begins with skeining
(looping) of the magnet wire. Because of the difficulty to form wire of high
width-to-thickness ratio, several wires in parallel may make one turn. Individual
turns are arranged in precise location with respect to each other. That is, turn one is always
next to turn two is always next to turn three, and so on.ADVANTAGES:
The main
advantage is as there is uniform turn-to-turn voltage stress due to uniform
arrangement of coils.Form windings have
uniform temperature distribution as well as resin build up, which minimize the
chance of localized hot spots. Insulation systems are categorized by
temperature limits of the materials. For example, a class F system is designed
to withstand an overall temperature of 155° C. This temperature is made up of
ambient air plus average temperature of the windings with allowance for hot
spots.
With form
wound machines the hot spot is always in a predictable location at the slot
portion of the laminations. The hot spot temperature can be measured and
duplicated from unit to unit. However, for random windings, because of uneven
resin build up, the hot spot location can vary in duplicate units. In
continuous duty applications, premature winding failures of identical units
having random coils is a prime example of localized excessive hot spot.
The
end windings of form coils have large openings for air circulation, which
prevent contamination build up. Therefore, the likelihood of tracking-type
failures is minimized. In addition, the insulating tape on the coils provides
additional environmental protection. The inherent rigidness of the rectangular
wire coupled with the lacing of the end turns and VPI ensures a rigid
insulation system. A rigid coil structure is very important to minimize
movement at the coil-lamination interface when there are high surge currents,
such as motor starting loads or short circuit faults. These can exert extreme
forces between the coils.
DISADVANTAGES:
DISADVANTAGES:
Magnet wire is rectangular or square with
double dacron glass cover or mica turn tape over 200° C heavy film. The wire is
more costly and inventory costs are increased because many different sized
wires are used. Round wire with 200° C heavy fi lm is used. Fewer sizes need to
be kept on hand, and the wire is more economically priced.
RANDOM WOUND COILS:
RANDOM WOUND COILS:
When
the arrangement between turns is not definite during coil skeining or
insertion, the windings are called “random.” That is, for example, turn one can
be touching turn four. Random windings are used on lower kW machines where it
is impractical to use form coils. Random windings are made from a lower cost magnet
wire that is fi lm coated and round. With random windings, the mechanization of
manufacturing is also increased to add to a lower cost.ADVANTAGES:
The primary advantage for manufacturing generators with random windings
is economics: lower cost wire and mechanized construction. However, without
vacuum-pressure impregnation (VPI), the life expectancy of random windings is
drastically reduced under severe environmental conditions or with applications
involving nonlinear loads, such as silicon-controlled-rectifi ed (SCR)-type
loads. SCR type loads induce high turn-to-turn surge voltages into the
windings, as high as twice the normal operating voltage.
DISADVANTAGES:
DISADVANTAGES:
In the case of random windings,
the ability to withstand operating turn-to-turn voltages, which can be marginal
for normal operation, will cause failures within a few hours under SCR type
loads. For example, a four-pole generator rated 600 kVA and 600 volts may have
four turns per coil and peak turn-to-turn voltage of 38 volts. For a
random-wound design if turn one and turn four are touching, this voltage can be
as high as 114 volts (38 volts x a three-turn difference), and can be amplifi
ed by SCR loads. That is why random coils may prematurely fail. A similar
form-coil design would have a peak voltage of only 38 volts because turns one
and four would never be next to each other.
