Note: Descriptions are shown in the official language in which they were submitted.
~V~72~ case 2492
This invention relates to a telemetry
system, and in particular it relates to a ~ystem for
providing at a remote point a sig~al representing
a temperature on the rotor of a dynamoelectric machine,
In dynamoelectric machines it is desirable to
know the temperature at certain spot on the rotor.
This is particularly so in the larger rotating machines
where, for example, a starting load or locked rotor
may result in a rapid rise in temperature. N~ only
can temperature information be used to actuate al~rms
or protective devices, but it can be used to operate
a machine more efficiently.
In the past, indications of temperature of
the rotor of a dynamoelectric machine have been
obtained indirectly and directly. One indirect
scheme for obtaining temperature used an induction
type relay and determined the stator inrush current
during starting. This current drops off markedly
as operating speed is approached. By selecting an
induction type overcurrent relay with an inverse
characteristic it is often possible to obtain a
protective ~ystem that will permit a normal start
but will trip if the motor stalls. This system
approximates the heating of the rotor and trips when
an indirectly determined temperature should have been
reached. It takes no account of stored heat in the
rotor from previous starts or from previous r~ning
periods.
Another indirect scheme for approximating
rotor temperature u~es a speed switch and an ass-
ociated relaying system. If a predetermined ~peed
is achieved in a predetermined time during starting,
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the motor continue~ to run. If the predetermined
speed is not achieved in this predetermined time,
it is assumed the rotor temperature i~ excessive and
the motor i8 shut down.
Other indirect systems have been developed
for approximating rotor temperature, but they have
not been ~ufficiently accurate and consequently they
did not provide adequate protection or they were
inefficient.
There were in the pa~t, two main schemes
for obtaining a direct indication of the temperature
of the rotor of a dynamoelectric machine. One of
these schemes made use of a temperature detector on
the rotor with leads brought to rotor slip rings.
Brushes which engaged the slip rings, and associated
conductors, made the temperature signal available
externally of the rotor. The temperature signal was
thus conducted directly from the temperature detector
to an external point where it could be amplified.
This scheme was not only direct but it was simple.
Unfortunately the temperature signal is very small
and the ~ignal frequently became inaccurate because
of the variability of the slip ring to brush contact.
In spite of its simplicity the scheme wa~ not popular,
largely due to the poor quality and accuracy of the
; temperature signal.
In order to overcome the inaccuracies of
the 31ip ring and brush system, a high frequency
scheme using a rotary transformer was developed.
This arrangement provided a temperature responsive
device on the rotor, normally at a hot spot on the
rotor, together with an amplifier and an oscillator.
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The o~cillator was connected to a winding on the
ahaft of a rotor and thi~ winding wa~ inductively
coupled to an adjacent ~tator winding to form ~
rotary transformer. The oscillator frequency varied
with temperature as detected by the temperature
responsive device and consequently 3 signal repre-
senting temperature wa~ available at the stationary
winding of the rotary transformer. The temperature
transmitting circuitry, that is the temperature
responsive device, the amplifier and the oscillator,
all required electrical power. This was provided
by using another rotary transformer. The stator
winding of thi~ second rotary transformer was
connected to an oscillator (oscillating at a frequency
different from that of the signal oscillator). The
rotor winding was connected to a rectifier on the
rotor which supplied power to the temperature tran-
smitting circuitry. This arrangement provided
adequate accuracy but the re]iability was considered
to be less than that of the direct sy~tem using 81ip
rings and brushes. Other direct arrangements were
relatively complex and also were considered to be
less reliable than a system using slip rings and
brushes,
It is therefore a feature of thi~ invention
to provide an improved direct telemetry system for
determining the temperature at a spot on the rotor
and using slip rings and brushes to transmit the
temperature signal from the rotor with satisfactory
ac~uracy.
Very briefly, the telemetry system of this
invention uses a resistance temperature sensor mounted
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at an appropriate spot on the rotor. A constant
current source supplies current to the temperature
sensor, and an amplifier amplifies the signal from the
temperature sensor representing the temperature.
In Canadian patent No. 962,088 - Boothman and Nutt,
granted February 4, 1975 to the Canadian General
Electric Company Limited, there is described an
apparatus for measuring temperature on a stator winding
using a resistance temperature sensor, a constant
current source, and an amplifier. The present
invention uses similar circuitry on the rotor to
provide a signal representing temperature, and this
signal is applied to a voltage-tofrequency converter.
The converter gives a signal whose frequency varies with
temperature and this is transmitted over a pair of
slip rings to a frequency~to-voltage converter and
then to a voltage-to-current converter. The output
from the voltage-to-current converter is a current
whose value represents temperature. Because the
signal transmitted over the lsip rings has been
converted to a frequency variable signal and is not
amplitude dependent, any changes in contact resistance
or any stray voltages at the slip rings, will not
affect the signal. In order to provide electrical
power for the constant current source, amplifier
and voltage-to-frequency converter on the rotor,
the same slip rings are used to transmit DC power.
In other words the power to the rotor circuitry and
the signal representing temperature from the rotor
are superimposed. In one form of power supply a DC
voltage is applied to the slip rings and is passed
through a filter when received by the rotor.
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1~7~ Ca~e 2492
It i~ then applied to a DC to DC converter which
provides the necessary DC voltages for the rotor
circuitry.
Thus, in accordance with the invention
there is provided a telemetry system for use with
a dynamoelectric machine h~ving a rotor ~nd a stator.
comprising temperature re.~ponsive means mounted on
said rotor to sen3e temperature at a particu~ar
location on said rotor and to provide a signal having
a frequency which varies with changes in temperature
at said location, slip rings mounted on said rotor
with respective brushes mounted on said stator, a
frequency responsive converter mean~ connected to said
brushes to receive the variable frequency signal and
to provide an output signal whose amplitude varies
with said changes in temperature, a DC power source
connected to said brushes to supply electrical power
thereto, and means on said rotor connected to said
slip rings to receive said electrical power and to
provide a regulated DC power for said temperature
respon~ive means, said power and said variable
frequency signal being superimposed on said slip
rings.
This invention will be described with
reference to the accompanying drawings, in which
: Figure 1 is a block schematic diagram of the
circuitry of the invention,
Figure 2 is a schematic diagram, partly
in block form, which gives more information on a
suitable circuit according to the invention, and
Figure 3 is a simplified schematic diagram
~howing part of Figure 2 in more detail.
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Case 2~92
~7277
Referring first to Figure 1, a telemetry
transmitter is represented by the block 10 formed
with a broken line, and a telemetry converter is
represented by the block 11 formed with a broken line.
The telemetry transmitter 10 comprises circuitry
mounted on the rotor of a dynamoelectric machine
(not shown) and is connected to slip rings 14 and 15
of the dynamoelectric machine. The telemetry converter
11 comprises circuitry external of the dynamoelectric
machine and is connected to brushes (not shown)
which engage the slip rings 14 and 15. A source
of AC power 12 is connected with the telemetry
converter 11.
For simplicity the circuit of Figure 1 is
shown with blocks connected by single lines. It will
be apparent to those skilled in the art that two
paths are required for most interconnections and one
path is frequently a common path such as a ground
connection.
The telemetry transmitter 10 includes a
temperature sensor 16 which is preferably a resistance
temperature sensor or resistance temperature detector
(RTD), that is a sensor having a resistance which
hcanges with changes in temperature. A constant current
; source 17 provides a constant current for resistance
temperature sensor 16. The temperature sensor 16
provides a signal where voltage changes with temperature
and this is amplified by amplifier 18 and applied to a
voltage-to-frequency converter 20. Voltage-to-frequency
converter 20 provides a signal whose frequency varies
with temperature and this signal is applied via capacitor
21 to slip ring 14.
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1~7277
In the telemetry converter 11, a capacitor
22 is electrically connected to slip ring 14 via a
brush (not shown). The capacitor 22 isolates a
frequency-to-voltage converter 23 from the DC on
the slip ring 14 and passes the signal whose frequency
varies with temperature to the frequency-to-voltage
converter 23. The converter 23 provides a signal
whose voltage varies with temperature and this is
applied to a voltage-to-current converter 24, and the
output from the converter 24, available at output 29,
which is a current signal representing temperature,
is used to control a display, alarm or protective
device.
The electrical power for both the telemetry
converter 11 and the telemetry transmitter 10 is
derived from the AC source 12. A regulated DC supply
25 is connected to AC source 12 and it comprises a
rectifier and regulator. It provides a regulated DC
to converters 23 and 24. A DC supply 26 is also
connected to AC source 12. The DC supply 26 provides
a DC voltage, which is ~iltered by filter 27 and
applied to slip ring 14 for the telemetry transmitter.
The DC supply is passed through a filter 28 and is
connected to a DC/DC converter 30 which provides a
controlled DC voltage for amplifier 18 and converter
20. It also provides power for the constant current
source 17.
It is believed the operation of the circuitry
of Figure 1 is clear. In the telemetry converter
11 there is a source of DC power 25 and 26. The supply
,, 25 provides power for the circuitry of the telemetry
; ~ converter 11 and supply 26 is connected via slip
~087~ Case 2492
rinys 14 and 15 to provide power for telemetry
transmitter 10 on the rotor. A resistance temperature
sensor 16, or resistanc~ temperature detector (RTD)
as it ha~ sometimes been called, provides a voltage
sign~ whose amplitude changes as temperature changes,
and this signal i~ amplified by amplifier 18 and
converted to a variable frequency signal by converter
20. The variable frequency signal is ~uperimposed
on the DC power and transmitted by the slip rings 14
and 15 to converters 23 and 24 which convert the
variable frequency signal into a respective voltage
varying signal and current varying signal. Thus,
there i8 at output 29 a signal whose current changes
as the temperature detected by the temperature sensor
16 changes.
Referring now to Figure 2, the slip rings
14 and 15, the temperature ~ensor 16, capacitors
21 and 22, and blocks 20, 23, 25 and 30 all bear the
8 ame designaltion numbers as in Figure 1. Cir~uitry
representecl by other blocks in Figure 1 is shown
more specifically in Figure 2.
In Figure 2 the resistance temperature
sensor 16 receives a constant current from constant
current source 17 which comprises an operational
amplifier 32, a transistor 33 and a zener diode 34.
The operation of such a constant current source is
well known. The noninverting input or + input of
amplifier 32 i~ connected to the junction of zener
diode 34 and resistance 35 to provide a reference
voltage. The output of amplifier 32 is connected
~' to the junction of zener diode 34 and resistance 35
to provide a reference voltage. The output of
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amplifier 32 is connected to the base of transistor
33. A resistance 36 connects the emitter of transistor
33 to a positive voltage source, for example + 15
volts. A feedback resictance 37 connects the emitter
of transistor 33 to the inverting or - input of
operational amplifier 32. The collector of transistor
33 is connected by series resi~ters 38 and 40 to the
re~istance temperature sensor (or RTD) 16, the other
side of which is connected to ground. The operational
amplifier 32, in response to a change in voltage at
its inverting input caused by a change in current
through resistance 36, controls transistor 33 to
maintain a constant current. Thus, a constant current
flows through resistances 36, 38, 40, and RTD 16
to ground.
The operation of resistance temperature
sensor 16 (or RTD 16) is described in the afore-
mentioned Canadian patent No. 962,088 however a
brief description will be given with reference to
Figure 3 to provide a complete description of the
present invention, For simplicity the circuit shown
in Figure 3 omits some capacities which are nor-
mally used to bypass any transients to ground.
Figure 3 shows the temperature sens~ 16
in the form of a broken line block 16. The actual
temperature sensitive element is shown as resistance
41. This temperature sensitive element 41 may be of
copper or platinum or other suitable metal or alloy.
For example, a copper element may have a resistance
of the order of 10 or 15 ohms and a platinum element
may have a resistance of the order of 100 ohms.
A copper temperature sensitive element 41 may have
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a resistance which changeq by perhaps 0.038 ohm~
per C, and consequently lead resistance could be
significant. The leads have resistances shown in
block 16 as RL. If it ia assumed a current I is
flowing from the constant current source through the
series path comprising resistancs 40, resistance RL
resistance element 41, resistance RL to ground. then
the following equations apply:
VA = I ( R40 + R41 + 2 RL) (l)
10 VB = I (R41 + 2 RL) ( 2)
VC ~ I ~ RL
where VA is the voltage at point A, Vg is the voltage
at point B and Vc is the voltage at point C.
The other elements in the circuit of Figure
3 include an operational amplifier 42 with a feedback
resistance 43 (i.e. R ), a resistance 47 connecting
point A to the inverting input of amplifier 42
(having a value of 2 ), a resistance 44 connecting the
lead from point C to the inverting input of amplifier
20 42 (resistance 44 has a value R), a resistance 45
connecting point B to the noninverting input of
amplifier 42 (resistance 45 has a value R), and a
resistance 46 connecting the noninverting input of
amplifier 42 to ground (resistance 46 is selected
to have a value equal to the resistance of 2R in
parallel with RF) .
The output of operational amplifier 42
may be given as:
VOUT = RF ( 2Vg ~ VA - 2Vc) ~4)
Substituting equations (1), (2) and (3~ in
equation 4:
VouT = RF .I. (2R41 + 4RL - R40 - R41 - 2RL - 2RL~
2R
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10~7~ Case 2492
= RF I (R41 - R40) (5)
2R
It will be seen that the lead resistance RL cancels
out and does not affect the output of the operational
amplifier 42. The current I is constant and the
amplification RF of amplifier 42 is constant.
Therefore the output VOUT of amplifier 42 varies as
the resistance of the temperature sensitive element
41.
The resistor 40 i~ included for convenience
in calibration. If resistor 40 is selected to be
equal to the resistance of temperature sensitive
element 41 when it is at a reference temperature,
for example zero C, then V0uT will be zero at th~t
reference temperature (i.e. at 0C~, This simplifies
calibration.
It i8 also convenient to have amplifier 42
provide an output which changes by an integral number
for a change in temperature of 100C. Table I shows,
by way of example, values used in one installation.
TEMP (C) R41 (ohms) VOUT (Volts)
. . . _
0 9.035 0
50 10.966 2
100 12.897 4
150 14.828 6
In the circuitry of the example referred
to in connection with Table I, resistance 40 was
selected to have a value of 9.035 ohms.
Referring now to Figure 2, a capacitance
49 is shown in parallel with feedback resistance 43.
Resistance 44 is shown in two portions to accommodate
a by-pasæ capacitor 48. By-pass capacitors 50 ~nd 51
are also provided to by-pass transient voltages.
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1087277
Because the voltage changes are relatively small it
is desirable to eliminate transient voltages which
would affect the output reading.
The output of operational amplifier 42 is
connected to voltage-to-frequency converter 20 which
may, for example, be a DATEL V/F converter, VFV-lOK.
This can be arranged to provide at its output 52
a series of negative voltage pulses whose frequency
is 1000 times the input voltage. The converter 20
has connections 53 and 54 to a supply of positive
and negative voltage, for example + 15 and -15 volts,
from converter 30.
The output 52 of converter 20 is connected
to the noninverting input of operational amplifier
55. Amplifier 55 is connected as a voltage follower
to reduce output impedance. The output of operational
amplifier 55 is connected through isolating capacitor
21 to slip ring 14, and thence through the associated
brush (not shown) and isolating capacitor 22 to the
inverting inE)ut of operational amplifier 56. A filter
element comprising capacitor 57 and resistor 58
in parallel is connected from the inverting input
of amplifier 56 to ground and conducts high frequencies
to ground. The noninverting input of amplifier 56
is connected to the midpoint of resistors 60 and 61
which are connected in series between a source of
positive voltage, say + 15 volts, and ground. Amplifier
56 acts as a biased comparator and reconstitutes the
filtered waveform at its input into a square wave.
The output from amplifier or biased comparator
56 is applied to frequency-to-voltage converter
12
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~7277 Case 2492
23~ This converter 23 may, for example, be a
PHILBP~ICK 4702 F~V converter. The converter 23 is
connected to supply 25 by conductors 62, 63 and 64
providing + 15,0 and -15 volts. A resi3tance network
comprising resi~tors 65, 66 and 67 in series is conn-
ected between conductors 62 and 64 and a variable
tap on resistor 66 is connected by 3 conductor 68
to a "SUM" input on converter 23. The variable
tap on resistor 66 sets the "zero" setting for
converter 23, The gain of converter 23 is cont-
rol]ed by a variable resistance 70 connected
between the output of converter 23 and the "SUM"
input. Capacitor 71 is in parallel with resistance
70, The gain of converter 23 is conveniently set
80 that 6000 hertz, representing a temperature of
150C, will give a 10 volt output. That iq, the
range of output is 0 to 10 volts for 0 to 150C.
The output of converter 23 is connected
by a variable resistance 72 to the inverting input
of operational amplifier 73, The noninverting
input of amplifier 73 is connected to the mid-point
of a pair of series resistances 74 and 75 connected
from positive conductor 62 to ground. Resistance 75
is variable. The output of amplifier 73 is connected
to the base of a transistor 76. The emitter of
transistor 76 is connected to the positive conductor
62 by a resistance 77, and is connected to the in-
verting input of amplifier 73 by a parallel com-
bination of resistor 78 and capacitor 80. The inverting
input of amplifier 73 is connected to ground by
resistance 81, The collector of transistor 76 is
connected to ground by a series path comprising
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1~87Z77
resistance 82 and load 83. The amplifier 73 and
transistor 76 form a voltage-to-current converter
whose gain is controlled by variable resistance 72
and whose zero level is controlled by variable
resistance 75.
Table II below shows the figures for the
previous Table I extended to show the output.
TEMP R41 VOUT Freq. V-Convert 23 OUTPUT
( C) (ohms) (volts) _hertz) (volts) (ma)
0 9.035 o 0 0 4.0
10.966 2 2000 3.33 9.33
100 12.897 4 4000 6.66 14.66
150 14.828 6 6000 10.0 20.0
The telemetry converter llreceives its power
from a regulated DC supply 25 which may, for example,
be a BURR BROWN 552 power supply providing + 15 -
volts, 0 and -15 volts on conductors 62, 63 and 64
as was previously mentioned. A transformer 84, con-
nected to AC supply 12, provides a voltage for bridge
rectifier having diodes 85 and a capacitor 86. This is
DC supply 26 (Figure 1). The positive output of
the bridge rectifier is connected by conductor 87
to a choke 88 to the brush and slip ring 14. The
diode 90 is provided to dissipate over voltage if
- the connections should be broken when the power
' is on. A diode 91 is connected to the ring 14
for protection against a reversal of connection to
the brushes or slip rings 14 and 15. The power from
slip ring 14 is again filtered by choke 92, protected
as before by diode 93 against over voltage which
could result from disconnection with voltage applied.
The choke 93 is connected to a pre-regulator 94
~ 14
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lQ~372~ Case 2492
which reduce~ the unregulated DC voltage to 24 volt~
DC ~uitable for DC to DC converter 30. A filter
capacitor 95 is connected between choke 92 and pre-
regulator 94 to ground.
The DC to DC converter 30 may, for example.
be a BH Industries 2055-24-15 converter which provide~
+ and -15 volts on conductors 53 and 54.
It is believed the operation of Figure 2
will now be clear and no further description i~
required.
The telemetry system provides a reliable
transfer of a temperature signal from a rotor of a
dynamoelectric machine by direct contact over slip
rings and brushes. Changes in resistance between
81ip rings and brushes, or radio frequency inter-
ference, will not affect the accuracy of the trans-
mission of temperature signals.
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