Heat and Noise
In addition to unwanted electrical
effects, transformers may also exhibit undesirable physical effects, the
most notable being the production of heat and noise. Noise is primarily
a nuisance effect, but heat is a potentially serious problem because
winding insulation will be damaged if allowed to overheat. Heating may
be minimized by good design, ensuring that the core does not approach
saturation levels, that eddy currents are minimized, and that the
windings are not overloaded or operated too close to maximum ampacity.
Large power transformers have their core
and windings submerged in an oil bath to transfer heat and muffle noise,
and also to displace moisture which would otherwise compromise the
integrity of the winding insulation. Heat-dissipating "radiator" tubes
on the outside of the transformer case provide a convective oil flow
path to transfer heat from the transformer's core to ambient air:
Oil-less, or "dry," transformers are
often rated in terms of maximum operating temperature "rise"
(temperature increase beyond ambient) according to a letter-class
system: A, B, F, or H. These letter codes are arranged in order of
lowest heat tolerance to highest:
- Class A:
No more than 55o Celsius winding temperature rise, at 40o
Celsius (maximum) ambient air temperature.
- Class B:
No more than 80o Celsius winding temperature rise, at 40o
Celsius (maximum)ambient air temperature.
- Class F:
No more than 115o Celsius winding temperature rise, at 40o
Celsius (maximum)ambient air temperature.
- Class H:
No more than 150o Celsius winding temperature rise, at 40o
Celsius (maximum)ambient air temperature.
Audible noise is an effect primarily
originating from the phenomenon of magnetostriction: the slight
change of length exhibited by a ferromagnetic object when magnetized.
The familiar "hum" heard around large power transformers is the sound of
the iron core expanding and contracting at 120 Hz (twice the system
frequency, which is 60 Hz in the United States) -- one cycle of core
contraction and expansion for every peak of the magnetic flux waveform
-- plus noise created by mechanical forces between primary and secondary
windings. Again, maintaining low magnetic flux levels in the core is the
key to minimizing this effect, which explains why ferroresonant
transformers -- which must operate in saturation for a large portion of
the current waveform -- operate both hot and noisy.
Another noise-producing phenomenon in
power transformers is the physical reaction force between primary and
secondary windings when heavily loaded. If the secondary winding is
open-circuited, there will be no current through it, and consequently no
magneto-motive force (mmf) produced by it. However, when the secondary
is "loaded" (current supplied to a load), the winding generates an mmf,
which becomes counteracted by a "reflected" mmf in the primary winding
to prevent core flux levels from changing. These opposing mmf's
generated between primary and secondary windings as a result of
secondary (load) current produce a repulsive, physical force between the
windings which will tend to make them vibrate. Transformer designers
have to consider these physical forces in the construction of the
winding coils, to ensure there is adequate mechanical support to handle
the stresses. Under heavy load conditions, though, these stresses may be
great enough to cause audible noise to emanate from the transformer.
- REVIEW:
- Power transformers are limited in the
amount of power they can transfer from primary to secondary winding(s).
Large units are typically rated in VA (volt-amps) or kVA (kilo
volt-amps).
- Resistance in transformer windings
contributes to inefficiency, as current will dissipate heat, wasting
energy.
- Magnetic effects in a transformer's
iron core also contribute to inefficiency. Among the effects are
eddy currents (circulating induction currents in the iron core)
and hysteresis (power lost due to overcoming the tendency of
iron to magnetize in a particular direction).
- Increased frequency results in
increased power losses within a power transformer. The presence of
harmonics in a power system is a source of frequencies significantly
higher than normal, which may cause overheating in large transformers.
- Both transformers and inductors harbor
certain unavoidable amounts of capacitance due to wire insulation
(dielectric) separating winding turns from the iron core and from each
other. This capacitance can be significant enough to give the
transformer a natural resonant frequency, which can be
problematic in signal applications.
- Leakage inductance
is caused by magnetic flux not being 100% coupled between windings in
a transformer. Any flux not involved with transferring energy
from one winding to another will store and release energy, which is
how (self-) inductance works. Leakage inductance tends to worsen a
transformer's voltage regulation (secondary voltage "sags" more for a
given amount of load current).
- Magnetic saturation of a
transformer core may be caused by excessive primary voltage, operation
at too low of a frequency, and/or by the presence of a DC current in
any of the windings. Saturation may be minimized or avoided by
conservative design, which provides an adequate margin of safety
between peak magnetic flux density values and the saturation limits of
the core.
- Transformers often experience
significant inrush currents when initially connected to an AC
voltage source. Inrush current is most severe when connection to the
AC source is made at the moment instantaneous source voltage is zero.
- Noise is a common phenomenon exhibited
by transformers -- especially power transformers -- and is primarily
caused by magnetostriction of the core. Physical forces causing
winding vibration may also generate noise under conditions of heavy
(high current) secondary winding load.
Contributors
Contributors to this chapter are listed
in chronological order of their contributions, from most recent to
first.
Jason Starck
(June 2000): HTML document formatting, which led to a much
better-looking second edition.
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