Compressor Terminal Fault Interruption - Terminal Venting

 COMPRESSOR TERMINAL FAULT

INTERRUPTION METHOD AND APPARATUS

FIELD OF THE INVENTION

[0001] The present invention relates generally to an over

current protection method and circuit, and more particularly

to a method and circuit for disconnecting poWer to a motor

for a hermetic compressor upon detecting excess current.

BACKGROUND OF THE INVENTION

[0002] Refrigeration systems, such as residential refrig

erators, use electric motor poWered hermetic compressors

Which compress the system refrigerant according to prin

ciples Well knoWn in the art. Under certain conditions, the

compressor motor in a system can enter a fault mode

Wherein the poWer lines to the compressor input terminals

carry excessively high current. This high current condition

may result in a phenomenon commonly referred to as

“terminal venting”.

[0003] Terminal venting is generally characteriZed by a

separation of the metallic compressor input terminal pins

from the surrounding insulating material in Which the pins

are mounted. This can occur if excessively high current is

supplied to the terminals for sufficient time to destroy the

glass insulating seal. The problem is exacerbated by the

different thermal expansion coefficients of the pins and the

insulating material thereby causing destructive tensile

stresses in the glass. The end result of such a failure is

damage to the hermetic seal of the compressor terminal and,

in some situations, the uncontrolled release of refrigerant

gas.

[0004] Many compressor manufacturers incorporate

mechanical safeguards into their compressor designs to

reduce the likelihood and/or the effects of terminal venting.

Some conventional compressors employ robust insulating

materials With high temperature breakdown characteristics.

Other compressors include covers Which enclose the com

pressor terminals.

[0005] Conventional fuse-based interrupt circuits for simi

lar applications do not adequately prevent terminal venting

because such circuits are typically triggered by a prolonged

presence of current levels substantially loWer than the cur

rent levels associated With terminal venting. For example,

When the compressor rotor becomes locked, the compressor

motor draWs high current (commonly referred to as “locked

rotor current”) such as 20 amps, for example, but not nearly

as high as the current associated With terminal venting,

Which is typically in excess of 50 amps. Conventional

interrupt circuits interrupt poWer to the compressor to pro

tect the motor coils When the current draW of the compressor

motor is in the range of locked rotor currents, and is

sustained for a sufficiently long period of time. While the

exceptionally high current associated With terminal venting

Would typically trigger a conventional interrupt circuit, the

relatively sloW response time of such circuits requires a

prolonged application of this high current. Thus, damage to

the compressor terminals may occur long before a conven

tional interrupt circuit is triggered.

SUMMARY OF THE INVENTION

[0006] It has been determined that if the temperature

differential betWeen the pin and glass exceeds a given

Aug. 8, 2002

threshold for a particular terminal, the resulting tensile

stresses in the glass Will cause failure of the pin-to-glass seal

and result in terminal venting. In accordance With the

method of the present invention and the particular exem

plary circuit implementation shoWn, the current ?oWing

through the terminal is detected. If the detected current

exceeds a threshold level that, unless substantially immedi

ately terminated, Will cause the pin/glass temperature dif

ferential to rapidly exceed a threshold level resulting in glass

stresses that Will cause the pin-to-glass failure and terminal

venting, poWer through the terminal is immediately termi

nated. The threshold current level is much higher than

locked rotor current for the compressor motor, preferably at

least tWo times the locked rotor current. It has been found

that once the pin current exceeds a given threshold for a

particular terminal, that even if the current rise is no higher,

the pin and glass temperatures continue to rise and the

pin/glass temperature differential Where failure of the pin

to-glass seal occurs Will rapidly be reached. Therefore, the

threshold current selected for a particular terminal must be

loWer than that Which correlates to simultaneous pin and

glass temperatures at the failure level.

[0007] The present invention can be implemented by an

exemplary protection circuit connected in series betWeen the

poWer lines and terminal of the compressor Which detects

the presence of a motor fault or other over-current condition

and disconnects poWer to the terminal to prevent terminal

venting due to this condition. The circuit generally includes

a line-connected poWer supply for poWering the circuit, a

current sensor for sensing the current draWn by the com

pressor motor, and a control circuit for disconnecting poWer

to the motor When a fault is detected. The circuit may

include an audible or visual alarm to indicate the presence of

a fault. Additionally, since the present protection circuit is

connected in-line With the poWer connections to the com

pressor and external of the compressor housing, existing

compressors may readily be retro?tted to obtain the protec

tion against terminal venting provided by the present inven

tion.

[0008] The method and circuit of the present invention

protect the compressor terminals, as opposed to the motor

coils, by quickly disconnecting poWer to the compressor, but

only upon detection of exceptionally high current levels.

This high threshold permits simultaneous operation of con

ventional interrupt circuits and prevents “nuisance trigger

ing” as a result of the large current demands at motor start-up

or current noise spikes that may occur during operation.

While the current threshold of the present protection circuit

is quite high relative to the locked rotor current, damage to

the compressor terminals is nonetheless prevented because

the response time of the circuit is substantially faster than

conventional interrupt circuits. For example, current is ter

minated Within 20 milliseconds of detecting the preset

current threshold. Thus, the exceptionally high current is

removed from the compressor terminals before the tempera

ture of the terminal pin causes damage to the pin-to-glass

seal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other features of the present invention

Will become more apparent and the invention Will be better

understood upon consideration of the folloWing description

of the accompanying draWings Wherein:

US 2002/0106945 A1

[0010] FIG. 1 is a block diagram of a portion of a

refrigeration system With an over-current protection circuit

according to the present invention.

[0011] FIG. 2 is a cross-sectional vieW of a compressor

shoWing the compressor input terminals.

[0012] FIG. 3 is a schematic diagram of an over-current

protection circuit according to an exemplary embodiment of

the present invention.

[0013] FIG. 4 is a graphical representation of the tem

peratures of the pin and glass When high current is applied

across the pin.

[0014] FIG. 5 is a graphical representation of the differ

ential temperature of the pin and glass of the hermetic

terminal for different currents.

[0015] FIG. 6 is a further graphical representation of the

differential temperature of the pin and glass of the hermetic

terminal When different currents are applied.

[0016] FIG. 7 is a graphical representation of the maxi

mum principal tensile stress in the hermetic terminal When

different currents are applied for seven seconds.

DESCRIPTION OF EMBODIMENTS OF THE

INVENTION

[0017] The embodiment of the invention described herein

is not intended to be exhaustive or to limit the invention to

the precise forms disclosed. Rather, the embodiment

selected for description has been chosen to enable one

skilled in the art to practice the invention.

[0018] Referring noW to FIG. 1, in a typical refrigeration

system 10, poWer is supplied from a poWer source 12, such

as a Wall outlet, to a compressor motor 14 Which drives a

compressor 16. The present invention may be applied to any

hermetic compressor used, for example, in air conditioning

and refrigeration applications such as the numerous models

of compressors commercially available from the assignee of

the present application, Tecumseh Products Company. For

example, compressor 16 could be of the type disclosed in

US. Pat. No. 5,199,898 Which is assigned to the assignee of

the present invention and is expressly incorporated herein by

reference. According to the present invention, a protection

circuit 100 is connected betWeen poWer source 12 and

terminal assembly 18 for the compressor 16 and motor 14 to

detect an excessive current draW and disconnect poWer in

response thereto. The poWer connections shoWn in FIG. 1

include a high line, a common line, and a ground line. As

Will be explained further beloW, While protection circuit 100

Will be described as disconnecting the common line to

compressor motor 14 upon detecting an over-current con

dition, protection circuit 100 could readily be adapted to

disconnect either the high signal or the common signal

provided to motor 14.

[0019] Referring noW to FIG. 2, the con?guration of the

compressor input terminals is shoWn. Compressor 16 gen

erally includes a hermetic housing 17 and a terminal assem

bly 18 in Which are mounted three terminals (only tWo

shoWn). Terminal 20 carries the poWer high line from poWer

source 12 to compressor motor 14 through Wire 22, connec

tor 24, and pin 26. Similarly, terminal 28 carries the common

line from poWer source 12 through Wire 30, connector 32,

and pin 34. Terminal pins 26 and 34 are mounted Within

Aug. 8, 2002

housing 18 and glass insulating material 36 according to

principles Well knoWn in the art. A terminal venting condi

tion is characteriZed by separation betWeen any of terminal

pins 26, 34, or the neutral terminal pin (not shoWn) from

glass insulating material 36, potentially resulting in an

uncontrolled release of refrigerant from compressor 16. The

excessive current draWn Which may lead to such a failure is

prevented from reaching compressor 16 by over-protection

current 100 as described beloW.

[0020] FIG. 3 shoWs a protection circuit 100 Which can be

used to implement the method and apparatus of the present

invention. Circuit 100 includes a regulator circuit 102 to

establish a ?xed DC voltage for comparing to a voltage

representing the current draWn by compressor motor 14, a

current sensing circuit 104 for deriving this representative

voltage, and a control circuit 106 for disconnecting poWer to

compressor 16 as Will be further described beloW. Regulator

circuit 102 includes a transformer T1, shoWn as a 36 volt

device, the primary side of Which is connected to 117 VAC

poWer from poWer source 12 (FIG. 1). The output signal

from the secondary side of transformer T1 is recti?ed by

diodes D1, D2 to produce a 24 VDC signal. This 24 VDC

signal is used to energiZe relay RYI as Will be further

described beloW. The 24 VDC signal is ?ltered by capacitor

C1 and passed through a 5 volt regulator U1 to produce a 5

VDC signal at the output of regulator circuit 102. This signal

is further ?ltered by capacitor C2, and passed through a

voltage divider netWork in control circuit 106 including

resistors R2, R3.

[0021] The output node 108 of voltage divider R2, R3 is

the reference voltage used to set a maximum threshold for

the acceptable current provided through terminal 18 to

compressor motor 14. As indicated above, this threshold

reference voltage is set such that the increased current draW

associated With motor start-up or other typical operating

conditions does not result in activation of circuit 100.

Moreover, the reference voltage is set such that triggering of

circuit 100 occurs only upon detection of motor 14 current

substantially higher than locked rotor current. The threshold

current level causing activation of the disconnect circuit is

that current Which, unless substantially immediately termi

nated, Will cause the pin/glass temperature differential to

rapidly exceed a threshold level resulting in glass stresses

that Will cause pin-to-glass failure and terminal venting. This

threshold current is much higher than locked rotor current

for the compressor motor, for example, at least tWo times the

locked rotor current. For a typical terminal, such as a No.

40387 terminal provided on a TP or TW series compressor

having a 200-300 Watt motor available from Tecumseh

Products Company, the threshold current at 115 volts is 52

amps.

[0022] As indicated earlier, it has been found that once the

pin current exceeds a given threshold for a particular ter

minal, even if the current rises no higher or is terminated, the

pin and glass temperatures continue to rise and the pin/glass

temperature differential Where failure of the pin-to-glass seal

occurs Will rapidly be reached. Accordingly, the threshold

current selected for a particular terminal must be loWer than

that Which correlates to simultaneous pin and glass tempera

tures at the failure level.

[0023] The 117 VAC high line is passed through current

sensor CS1 of current sensing circuit 104 to compressor

US 2002/0106945 A1

motor 14 (compressor terminal 20). Current sensor CS1 is a

conventional torroidal current sensor, and is connected to

resistor R4 and recti?er D3, D4. Since small voltage changes

are produced by current sensor CS1 in response to current

changes on the 117 VAC poWer line, Schottky diodes are

used for recti?er D3, D4 to minimiZe the forWard voltage

drop incurred by the output voltage of current sensor CS1.

As current through current sensor CS1 increases, the voltage

at the output of recti?er D3, D4 also increases. This signal

is passed through resistor R5 and ?ltered by resistor R6 and

capacitor C3. The ?ltered signal is connected to the positive

input of comparator U2A of control circuit 106. A diode D5

is connected betWeen the positive input of comparator U2A

and ground to protect comparator U2A in the event a large

voltage is generated by current sensor CS1. Speci?cally, if

the voltage at the positive input of comparator U2A exceeds

the 6.2 voltage breakdoWn voltage of diode D5, diode D5

Will reverse bias and conduct to ground, thereby protecting

the remainder of circuit 100.

[0024] The negative input to comparator U2A is connected

to the reference voltage at node 108 of voltage divider R2,

R3. The output of comparator U2A is connected to pull up

resistor R7 Which is connected to the 5 VDC output poWer

from regulator circuit 102. The output of comparator U2A is

also connected to Schottky diode D6 Which isolates com

parator U2A from an AND gate U3. Both inputs of AND

gate U3 are connected together and connected to the ?lter

including resistor R8 and capacitor C4. A hysteresis resistor

R9 is connected from the output of AND gate U3 to the

inputs. The output of AND gate U3 is also connected to the

negative input of comparator U2B, the positive input of

Which is connected to the reference voltage at node 108 of

voltage divider R2, R3. As Will be further explained beloW,

comparator U2B functions as an inverter.

[0025] The output of comparator U2B is pulled up by

resistor R10 and connected to the gate of transistor Q1. The

drain of transistor Q1 is connected to ground and the source

is connected to the loW side of the solenoid coil of relay

RY1. The high side of the solenoid coil is connected to the

24 VDC signal from recti?er D1, D2 of regulator circuit 102.

Relay RY1 is shoWn in its energiZed con?guration Wherein

the common line from poWer source 12 (FIG. 1) is passed

through the sWitch of RY1, terminal 28 of compressor 16, to

compressor motor 14.

[0026] In operation, When excess current is draWn by

motor 14 through the 117 VAC high line, current sensor CS1

produces an output voltage Which is recti?ed by diodes D3,

D4 and provided to the positive input of comparator U2A

after ?ltering by resistor R6 and capacitor C3. If the voltage

exceeds the reference voltage (from note 108 of voltage

divider R2, R3) at the negative input to comparator U2A,

comparator U2A outputs a positive logic signal. Accord

ingly, a positive logic signal is present at both inputs to AND

gate U3, causing a positive output. The combination of

Schottky diode D6 and hysteresis resistor R9 latch the

output of AND gate U3 in the logic high state. A logic high

state is therefore present at the negative input to comparator

U2B. Control circuit 106 is designed such that this signal

exceeds the reference voltage at the positive input to com

parator U2B. Accordingly, comparator U2B outputs a logic

loW signal disabling transistor Q1. The path to ground for the

Aug. 8, 2002

solenoid coil of relay RY1 is thereby removed, de-energiZ

ing relay RY1 such that relay RY1 sWitches to an open

position.

[0027] When relay RY1 opens, poWer is disconnected to

compressor motor 14, and the current passing through

current sensor CS1 quickly goes to Zero. This rapid discon

nect prevents the excessive current at terminals 20, 28 (and

the third terminal, not shoWn) from heating terminals 20, 28

to a temperature resulting in terminal venting. As should be

apparent to one skilled in the art, a relationship exists

betWeen the reference voltage and the speed at Which circuit

100 disconnects poWer to compressor motor 14 (i.e., the

response time). Since circuit 100 is designed to prevent

damaging temperature levels at terminals 20, 28, the higher

the reference voltage is set, the faster the required response

time. As a corollary, a sloWer response time may be used

(requiring a longer duration high current condition) if a

loWer reference voltage is set. For the particular example

described above, the time betWeen detection of the threshold

current and the energiZing relay RY1 is 22 milliseconds.

[0028] When poWer is disconnected to compressor motor

14 and current sensor CS1 goes to Zero, the positive input to

comparator U2A falls beloW the negative input (the refer

ence voltage from voltage divider R2, R3), causing com

parator U2A to output a logic loW signal. As mentioned

above, hoWever, the output of AND gate U3 remains in a

logic high state since Schottky diode D6 isolates the output

of comparator U2A from the inputs to AND gate U3, and

hysteresis R9 feeds back the logic high output of AND gate

U3 to its inputs. Accordingly, once the reference voltage is

exceeded by the voltage representing the current sensed by

current sensor CS1, circuit 100 disables relay RY1 and

maintains relay RY1 in a disabled state, thereby disconnect

ing poWer from compressor motor 14, until poWer is

removed from circuit 100 and re-applied. Thus, When circuit

1000 disables compressor motor 14, compressor motor 14

remains disabled until it is properly serviced.

[0029] Referring noW to FIG. 4, there is provided a

graphical representation of the pin and glass temperatures as

a function of time When different currents are applied to

terminal pin 26 or 34. As can be seen, the higher the pin

current the more rapid the rise in pin temperature and

concomitantly the temperature differential betWeen the pin

and glass. FIG. 5 illustrates this rise in pin/ glass temperature

differential, and particularly for high current levels, such as

120 amps, the temperature differential curve rises very

sharply after only one second folloWing initiation of the high

current condition. FIG. 6 is a similar representation but

includes additional current levels in a mathematical simu

lation.

[0030] FIG. 7 shoWs graphically the rapid rise in maxi

mum principal stresses in the glass as the temperature

differential betWeen the glass and pin increases. As is quite

evident, the curve is substantially exponential thereby indi

cating that unless current is terminated at a very early time

When the threshold current is detected, rapid heating and

failure of the pin-to-glass seal Will occur.

[0031] UtiliZing the data from FIG. 5, the folloWing

mathematical model equation describing the process to

prevent terminal venting Was obtained:

US 2002/0106945 A1

[0032] In this equation, T is the differential temperature

betWeen the pin and the glass in degrees Celsius, i is current

through the pin in amperes and t is the amount of time in

seconds current has been applied. Once the maximum tem

perature differential betWeen the pin and glass is determined

for a particular terminal, the equation can be solved for

current in order to set the threshold level in circuit 100.

[0033] The experimental data to generate and validate the

curves discussed above Was obtained by applying different

levels of current through the terminal and measuring the

temperature of the glass and pin. The constants a1, a2, a3, b1,

b2, b3 are derived from the curves and are used for the

particular terminal construction tested. For the aforemen

tioned terminal, the constants are as folloWs:

[0034] a1=1.079><10_4

[0035] a2=2.420><10_3

[0036] a3=1.4447><10_2

[0037] b1=1.8875

[003s] b2=1.8000

[0039] b3=1.7335

[0040]