Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~L~974(~,3
Electrical Control for a Refrigeration
System
This invention relates in general to truck
refrigeration systems and more particularly to truck re-
frigeration systems in which the compressor and fan motors are
powered by an alternating electrical potential.
Most conventional truck electric powered
refrigeration systems use a direct current electrical power
system. The truck engine is used to mechanically drive an
alternating current generator or alternator, and the output
of the alternator is rectified to power direct current com-
pressor and fan motors in the refrigeration system. In
addition to the high cost of the rectifiers, which necessarily
must be sized to handle all of the system power requirements
direct current refrigeration systems require more maintenance
and thus are out of service for longer periods of time. The
brushes on the direct current motors are particularly subject
to wear and must be frequently replaced.
It is, therefore, desirable to provide a truck
2Q refrigeration system in which only the alternator field
winding and control elements require a direct current potential.
It is desirable for the compressor and fan motors to operate
directly from the alternator output, thus eliminating motor
brush maintenance and substantially reducing the size and cost
of the rectifier elements. To operate the alternating potential
motors at peak efficiency and constant line current, the
electrical output of the alternator should desirably maintain
~ a constant voltage to frequency ratio. To further reduce
i system cost and maintenance, the system should function with-
out current transformer or ield rectifier and consideration
should be given to energy saving.
A previous patent (See U.S.P. 3,283,525), which
has had limited commercial success taught a system in which
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the truck battery only assists in supplying excitation potential
to the alternator field winding when the direct current
potential provided by field rectifiers falls below a predetermined
magnitude.
One embodiment of the invention described in U.~.
Patent 3,283,525 maintains the field excitation current pro-
portional to the alternating current load, and maintains a
nearly constant voltage to frequency ratio, and achieves this
with only two current transformers instead of the conventional
three. The primary windings of the current transformers
are connected in series with two of the load conductors, and
the secondary windings each have one end connected in common
to one of the input terminals of a three phase full wave
field rectifier, and the remaining ends of the secondary wind-
ings are each connected to a different input terminal of the
three phase field rectifier. Another embodiment of the
invention described in U.S. P. 3,2133,525 maintains the field
excitation current proportional to the load and maintains an
alternator output having a nearly constant voltage to frequency
ratio and achieves this with only one current transformer.
The primary winding of the single current transformer is
connected in series with one of the load conductors, and the
secondary winding of the current transformer is connected to
two of the alternating current input terminals of the three
phase, full wave field rectifier. Still another embodiment
of the invention described in U.S. P. No. 3,283,525 completely
eliminates the use of current transformers and still maintains
an alternator output having a nearly constant voltage to
frequency ratio, by connecting the field rectifier in the
neutral of the alternator armature windings and using the
- truck battery to provide excitation at light and no loads
conditions.
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In all the above mentioned methods of field
excitation, the current supplied to the field winding is
directly proportional to the load current. Due to the motorized
nature of the load being driven, (Compressor and fan motors)
under normal running conditions the capacity of the refrigeration
system and the alternator voltage vary in direct proportion
to the truck engine speed while the current to the field
winding remains fairly constant.
During motor start-up, in aforementioned
embodiments, current supplied to the~field winding is increased
inherently to maintain the alternator output voltage at a
constant level. During motor start-up, the above mentioned
technique is desirable when dealing with commercial network
power distribution where fluctuation in voltage due to motor
start-up load might affect other equipment such as lighting,
electronic equipment, etc. connected on the same network.
But it is a known fact in the indu~;try that this method of
motor starting, called "across the line starting", is more
energy demanding and places ~ore stress on the motors and
drive members.
Accordingly, it is an object of this invention
to provide a new and improved refrigeration system.
Another object of the invention is to provide
a new and improved refrigeration system in which the compressor
and fan motors operate directly from the output of the
~`~ alternating current generator without requiring conversion to
a direct current potential.
A further object of the invention is to provide
a new and improved refrigeration system wherein an alternating
potential having a constant voltage-to-frequency ratio for
operation of alternating current motors is produced.
Another object of the invention is to provide a
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new and improved refrigeration system wherein reduced starting
voltage for the motors forming part of the system is produced
automatically.
A further object of the invention is to provide
a new and improved refrigeration system in which an associated
battery and a low voltage alternator are the sole sources of
power for supplying excitation potential to the alternator
winding thus assuring a constant ratio of voltage-to-frequency
that varies with the load.
Another object of the invention is to provide a
new and improved refrigeration system that is powered by an
alternator that is capable of supplying full load current with
a field excitation voltage less than an associated DC system for
example, 12 or 24 volts DC available in a vehicle.
To simplify this description, controls that are
known to the trade such as Cooling-Defrost and heating controls
under and over voltage protection, over current protection,
standby power etc., will not be dealt with.
A preferred embodiment of the invention will now
be described with reference to the accompanying drawing which
shows a circuit for an alternator-motor combination.
One embodiment of this invention maintains a
`~ constant predetermined field excitation current to the power
alternator. The above mentioned current is supplied by a
secondary 12 volts or 24 volts alternator which may or may
not form part of the vehicle DC system, through a standard
vehicle type rectifier and voltage regulator control unit and
a predetermined field resistor.
Another embodiment of this invention provides
automatic reduced starting voltage for motors, forming part
of the refrigeration system, by maintaining constant field
excitation current instead of a field excitation current which
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is proportional to the load. Another embodiment of this in-
vention provides a predetermined limit in the voltage drop
upon motor starting by~momentarily increasing the field
excitation upon motor start-up.
Another embodiment of this invention completely
eliminates the use of a current transformer and field rectifier
but which maintains an alternator output having a fairly and
quite acceptable voltage to frequency ratio. Still another
el~odiment of this invention is the use of a power alternator
that can be driven to full load with a field excitation voltage
less than the vehicle DC system, for example + 8 volts
excitation current for a 12 volts DC vehicle system.
Since the refrigeration system capacity and, by the
same token, the demand on the output of the power alternator is
proportional to the truck engine speed when alternating current
compressor and fan motors are used, if a constant excitation
current is applied to the field it can be found that the current
at the output of the alternator will remain constant for a given
` condition and that both the frequency and voltage will vary in
direct proportion. As shown hereinafter, when not considering
the motor start-up condition and by careful examination of
different duty cycles it is found that only a few conditions
of load variation exist on a refrigeration system which can be
-` dealt with separately.
A) Cycle #1 Cooling: Compressor motor running and with two
fractional horse-power fan motors runningO Full
load condition.
B) Cycle ~2 No Cooling: Only one or two fractional horse
power fan motors running. Light load condition.
C~ Cycle ~3 Hot Gas Defrost: Compressor motor running and
one fractional horse-power fan running. Full
load condition.
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D) Cycle #4 Heating: Electric heating elements of approximately
the same capacity as the compressor electrical load
in operation and one fractional horse-power fan
running. Full load condition.
It may be seen from the above that the cycles
No. 1, 3 and 4 exhibit approximately the same heavy loading
condition and cycle No. 2 exhibits a light loading condition.
By the use of a multi-tap field resistor and an auxiliary
contact on the compressor and heating element contactors,
which form part of a normal refrigeration system, the field
current can be predetermined to satisfy all of the above
conditions.
Further objects and advantages of the invention
will become apparent as the following description proceeds
`- and features of novelty which characterize the invention will
be pointed out in particularity in the claims annexed to and
forming a part of the speciEicatiol~.
A preferred embodiment of the invention will now
be described with reference to the accompanying drawing which
~` 20 shows a circuit diagx~m of a vehicle having a refrigeration
alternator.
Referring to the drawing, there is illustrated a
schematic diagram of a vehicle refrigeration electrical power
system descriptive of the invention. More specifically, an
alternator lO having armature phase windings 12, 14 and 16,
and an excitation fi~ld winding 18, is connected to supply
'~ electrical energy to electrical conductors, 22, 24 and 26.
The alternator lO may be mechanically driven by a suitable drive
20, such as being belted to the engine of a vehicle. The
drive 20 may be of fixed ratio or of a variable speed type.
In order to provide a direct current potential
~or the field excitation winding 18 as well`as for the various
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control functions, an auxiliary alternator 28 which may or may
not form part of a vehicle DC system, having armature phase
windings 42, 44, 46 and an excitation field winding 40, is
connected to supply alternating current electrical energy via
electrical conductors 30, 32, 34 to a standard type rectifier
and voltage regulator control unit 48 which provides direct
current potential, via conductor 36, for the field winding 40
of the auxiliary al~ernator 28 and also supplies electrical
energy to the vehicle battery 52 through conductor 50.
Auxiliary alternator 28 may be mechanically
driven by a suitable drive such as being belted to the truck
engine. Auxiliary alternator 28 and the rectifier and/or
voltage regulator control unit 48 may be integral within the
same unit.
Direct current potent:ial for the field
. excitation winding 18 is tapped to -the regulator's 48 output
`i through co.nductor 54, field resistor 56 and depending on load
conditions, through conductor 58 and auxiliary contact 72
upon motor start up, through conductor 60 and compressor
` 20 auxiliary contact 74 or heating elements auxiliary contact
~:` 76 for full load condition and through conductor 62 on light
load condition and then through conductor 64 then back to .:
the regulator through ground conductors 68 and 70. It is to be
~; understood that the field resistor 56 may comprise other
voltage-dropping means, for example, a solid state device or
an electrolyte.
It should be understood that the number and
; configuration of the auxiliary contacts may vary depending on
the components utilized in the refrigeration system.
An alternative method of varying the field current
in the field winding 18 could be by providing the field winding
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18 with a plurality of taps, Switching means similar to 72, 74
and 76 could then ~e used for selecting taps appropriate to the
desired field current.
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