ON-LINE STATOR TEMPERATURE MONITOR FOR SINGLE-PHASE INDUCTION MOTORS
W.L. Soong, A. Harris and C.H. Fong
Adelaide University
A. Kennewell, J. Botiuk and D. Gray
ITACS, Adelaide
Abstract
All electrical appliances containing electric motors sold in Australia must comply with the relevant
stator winding temperature rise specification in IEC Standard IEC 60335.1. The temperature rise
is commonly estimated based on the change in the DC resistance of the stator winding before and
after the operating test. This method is accurate and widely used but does not allow continuous
monitoring of the winding temperature during the test. This paper describes the design,
construction and preliminary testing of a monitoring device which allows continuous on-line
temperature measurement of single-phase AC induction motors with a direct readout of
temperature.
1
INTRODUCTION
1.1 Background
In domestic appliances containing electric motors it is
essential to keep the stator winding temperature rise
within safe limits at all times to prevent a risk of fire.
IEC Standard IEC 60335.1 [1] defines the appropriate
temperature limits for different classes of stator
insulation material and describes means for measuring
the actual temperature rise using thermometers,
thermocouples and stator resistance change.
The thermometer and thermocouple methods generally
require disassembly of the appliance and insertion of
the temperature probe(s). The probe also only
measures the local temperature and correct location of
the probes to find the hottest winding temperature is
difficult.
Note that the resistance method measures the average
temperature rise of the winding and that some parts of
the winding will be significantly hotter. This is
allowed for in the IEC test standard by using different
maximum allowable temperature rises dependent on
the measurement method.
1.2 Resistance Switching Measurement Method
The stator resistance measurement method is widely
used for temperature rise testing of induction motors in
appliances.
It is usually implemented using a
switching method similar to that shown in Figure 2. A
single multi-pole switch is used to allow the motor
terminals to be rapidly switched between the AC
mains and a DC source.
DC Source
Idc
DC
Resistance Change (%)
40%
AC
35%
30%
240Vac
25%
20%
DC
Vdc
DC
AC
IM
test
motor
AC
15%
Figure 2. Simplified diagram showing the resistance
switching measurement method.
10%
5%
0%
0
20
40
60
80
100
Temperature Rise (degK)
Figure 1. Graph showing the increase in copper
winding resistance with temperature.
The resistance measurement method is based on the
temperature co-efficient of the copper stator winding.
This is approximately 0.4% per degree Kelvin and is
illustrated in Figure 1. For example a 40 degree
temperature rise produces approximately 15% increase
in the stator resistance.
The DC resistance is determined from DC voltage and
current measurements. Note that the DC voltage
measurement is taken directly across the test motor
terminals. This is called a “four-wire” resistance
measurement technique and it avoids errors in the
measured resistance due to switch contact and lead
resistances.
The measurement method is as follows : initially the
multi-pole switch is set to the DC position and the
“cold” DC resistance of the motor is measured. The
motor is then switched to the AC mains and loaded
using a dynamometer to its rated operating condition.
After the temperature stabilises (approximately one or
two hours), the motor is switched back to the DC
source and the “hot” DC resistance is measured.
In some cases the stator winding temperature may
rapidly change after switching off the motor. In order
to compensate for the time delay between switching
off the motor and making the resistance measurement,
a series of resistance measurements can be performed
over a short period of time and the resulting curve
extrapolated back in time to estimate the winding
temperature at turn-off. An alternative approach is to
set the DC test current to approximate the AC running
current and hence try to maintain the same stator
copper losses during the resistance measurement.
1.3 Limitations of Existing Switching Method
The existing switching method for DC resistance
measurement is simple to perform. Its main limitation
is it does not allow continuous monitoring of the stator
temperature during the test.
This causes two
problems,
firstly an additional temperature
measurement device such as a thermocouple must be
used to determine when the motor reaches thermal
steady-state. This is time consuming as insertion of
the thermocouples requires disassembly of the
appliance.
A second problem is that it is difficult to determine the
temperature at which thermal cutout switches operate.
This is a normally-closed thermal switch wired in
series with the stator windings of some appliances
which open-circuits the winding when it reaches a
certain temperature. It is designed to protect the motor
under stall conditions when the stator current is high
and the motor rapidly overheats (see Figure 3).
Unfortunately using the existing switching method,
when the thermal switch becomes open-circuit it is
clearly no longer possible to measure the stator
resistance and hence determine the winding
temperature.
temperature
thermal
cutout trips
Normally thermal cut-out switches are tested by
performing an initial test run to obtain the tripping
time and then repeating the test but interrupting it just
before the trip time to determine the temperature. This
test procedure is difficult as, if the test is stopped too
early then the measured temperature will be too low,
however if the test is stopped too late, the thermal cutout will trip first and the test must be repeated when
the motor cools down.
Due to these limitations it is desirable to be able to
monitor and display directly the on-line stator winding
temperature rise for single-phase induction motors.
1.4 Requirements
Table 1 summarizes the main requirements for an online thermal monitor. For the desired accuracy of +/5degK it can be seen from Figure 1 that resistance
measurements of +/-2% accuracy are required.
Table 1. On-line thermal monitor initial requirements.
Parameter
Value
Power Supply
240V, 50Hz, 1ph
Motor Operating Current Range 0.1A to 10A
Maximum Temperature Rise
200 degK
Temperature Rise Accuracy
+/-5 degK
Temperature Monitoring
continuous
Figure 4 shows the basic concept for the on-line
thermal monitor. Appliances are simply plugged into
the unit which displays a continuous stator winding
temperature display. It is easy to use, avoids the need
for disassembly of the appliance, and allows
continuous temperature monitoring to easily determine
when the motor has reached steady-state and also
when thermal cut-out switches operate.
240Vac
supply
Thermal
Monitor
Appliance
+20.7deg
Figure 4. Basic concept for on-line thermal monitor
showing mains input, appliance power output and
temperature display.
2
ON-LINE TEMPERATURE
MEASUREMENT CIRCUIT DESIGN
The basic issue for an on-line temperature
measurement system based on stator resistance is
taking accurate DC resistance measurements on an AC
energised stator winding.
time
Figure 3. Stator temperature rise during overload
conditions showing operation of thermal cutout.
Figure 5 shows the initial concept. The key challenges
are to take DC resistance measurements in the
presence of a large AC signal, and to prevent the low
source impedance of the AC mains affecting the DC
resistance measurement.
AC
240Vac
the result. He used a DC constant current source to
generate a DC output voltage (Vdc) proportional to
resistance and hence temperature. A chart recorder
was used to record the temperature continuously.
to DC
Resistance
Measurement
Circuit
DC
IM
DC blocking
capacitor
X
Y
X
AC
240Vac
IM
DC
IM
In 1970, Johnson [3] extended Seely’s concept by
adding a second bucking transformer and separating
the DC current and DC voltage measurement circuits
(see Figure 7). This produces a “four-wire” resistance
measurement and eliminates the bucking transformer
secondary resistance and stray lead resistances from
IDC
The circuit used (see Figure 8) is basically a “twowire” resistance measurement version of that
described by Johnson. The “two-wire” approach is
used as it allows a much simpler connection to the
appliance and avoids the need to disassemble it to
allow direct access to the motor terminals for the DC
voltage drop measurement.
X
Y
X
Y
X
240Vac
The main issue with this circuit is that it uses a “twowire” resistance measurement which also includes the
secondary resistance of the bucking transformer.
+
VDC
-
2.2 Circuit Design
Y
Seely also used a “bucking” transformer to isolate the
DC resistance measurement circuit (based on a
Wheatstone bridge) from the AC circuit. This uses a
small isolation transformer with a turns ratio of 1:1
whose primary winding is connected to the mains and
whose secondary winding is connected in series with
the resistance measurement circuit. Ideally this should
reduce the AC voltage seen by the resistance
measuring circuit to zero however in practise there is a
residual AC signal of a few volts.
Y
Figure 7. Modified Seely bridge circuit using fourwire resistance method for more accurate
measurements.
to
Wheatstone
Resistance
Bridge
Figure 6. Seely Bridge circuit to allow continuous
DC resistance measurements in energised AC circuits.
X
Y
Figure 5. Initial concept to measure DC resistance on
energised AC circuits.
In 1955, Seely [2] described an innovative
arrangement in which a large blocking capacitor is
used to isolate the DC test current from the AC supply.
This is shown in Figure 6. The size of the capacitor is
chosen such that at the motor rated current, the AC
voltage drop across it is only a few volts.
Y
X
240Vac
2.1 Seely Bridge History
X
IM
+
VDC
-
IDC
Y
Figure 8. Circuit used in the thermal monitor. This is
a “two-wire” resistance measurement method variation
of the circuit in Figure 7.
The DC test current used during operation is typically
two orders of magnitude below the rated current.
This is to avoid extra stator heating (due to copper
losses) and extra rotor heating (due the resultant DC
injection braking effect of the test current). The DC
test current is provided by a DC constant current
source with five current values from 10mA to 200mA
for different motor current ratings. The exact value of
the test current is not critical but it must be highly
stable during the test so that the resultant DC voltage
across the motor is directly proportional to
temperature. In the prototype, a low-pass LC filter
was inserted between the isolation transformer and the
DC current source to reduce the residual AC voltage
seen by the DC current source.
Figure 9 shows the temperature readout circuit. The
DC output voltage (Vdc) from Figure 8 is passed
through a second-order low-pass RC filter before
being applied to the input of a variable gain amplifier.
When the motor is cold, the amplifier gain is adjusted
until the amplifier output voltage is equal to the
reference voltage (Vref = +5V). The readout will now
read zero at this point. Now for every 1 degree rise in
winding temperature, the resistance will increase by
about 0.4% and hence the output voltage will increase
by about 5V x 0.4% = 20mV. Hence a meter
measuring the voltage difference between the amplifier
output and the reference voltage can be calibrated to
read temperature rise directly. This approach is
readily applicable to a wide range of motor current
ratings.
temperature
rise readout
20mV=1K
+
-
IDC
+
motor
resistance
VDC
Figure 11. Internal circuitry of the thermal monitor
showing the large “bucking” transformer used for the
current source circuit.
low-pass
RC filter
variable
gain amplifier
+
-
VREF
Seely Bridge
Temperature Readout Circuit
Figure 9. The direct temperature readout circuit is
accurate and easily calibrated for a wide range of
motors.
3 CONSTRUCTION
Figures 10-12 illustrate the construction of the thermal
monitor.
Figure 12. Rear view of the thermal monitor showing
control and appliance power inputs and motor power
output along with the voltage and current outputs for a
chart recorder.
The largest component is the “bucking” transformer
used for the DC current source (see Figures 8 and 11).
This must be able to operate at rated voltage carrying
the maximum DC test current (200mA) without
saturating. Note that the “bucking” transformer for the
voltage sensing circuit can be much smaller as it does
not carry any significant DC current.
Figure 10. Front panel of thermal monitor showing
the DC current setting switch and the temperature
readout and nulling adjustment.
The DC blocking capacitor was also another major
component in the system (see Figure 13). Two pairs
of back-to-back polarized electrolytic capacitors were
used to simulate a bipolar capacitor with a sufficiently
large current rating and capacitance. The two antiparallel diodes bias the capacitors to allow them to
carry AC current without seeing reverse voltages. The
three series-connected bridge rectifiers protect the
capacitors against high motor starting currents. It was
found that it was necessary to switch out these bridge
rectifiers after starting to prevent them conducting the
DC test current.
Figure 13. DC blocking capacitor arrangement.
At the rear of the unit (see Figure 12), banana plugs
were used to make available the DC test current and
the DC voltage drop across the motor. These were
used in the calibration of the unit.
Table 2 summarizes the key parameters of the thermal
monitor.
Table 2. Key parameters of thermal monitor.
Parameter
Value
Power Supply
240V, 50Hz, 1ph
DC test current
10mA to 200mA
Maximum temperature rise
200 degK
Dimensions
16 x 25 x 30 cm
Weight
7 kg
4
TESTING
4.1 Procedure for Use
A significant advantage of the temperature monitor is
that it does not require disassembly of the appliance.
Its operation is as follows, firstly the appliance lead is
plugged into the monitoring instrument.
An
appropriate value of DC test current is then selected.
The DC test current is turned on and the amplifier gain
control adjusted until the readout reads 0.0degK. The
AC power is then turned on and after the starting
transient, the protective diodes across the DC blocking
capacitor are switched out. The unit then provides a
continuous display of the temperature rise of the
machine.
4.2 Stray Resistance Measurement
The “two-wire” resistance measurement approach
produces a very simple to use unit but is sensitive to
stray resistance both internal to the unit, as well as due
to appliance leads. To quantify these effects the stray
resistance of the unit was measured using a shorting
plug directly at the motor output of the monitor and
also at the end of a 2.4m cord. The results are shown
in Table 3 and show that the majority of the stray
resistance is associated with the appliance lead.
Table 3. Measured stray resistance of thermal
monitor system.
Condition
Stray
Resistance
Shorting plug placed directly at output <0.1Ω
of monitor
Shorting plug with 2.4m lead length
<0.4Ω
The effect of this stray resistance is to add an
additional resistance in series with the motor
resistance. Taking the worst-case situation where the
stray resistance stays at room temperature, then the
error in the calculated motor temperature rise is
proportional to the ratio of the stray resistance to the
motor cold resistance. Table 4 shows the worst case
temperature rise errors for motors of different current
ratings. It shows that the approach used should give
good results for small to medium sized motors but can
have significant errors for larger size machines.
Table 4. Calculated errors in temperature rise
measurement due to the stray resistance.
Small
Medium
Large
Motor current
0.1A
1A
10A
rating
Typical motor cold
200Ω
20Ω
2Ω
resistance
Stray resistance
0.4Ω
0.4Ω
0.4Ω
Maximum error in
0.2%
2%
20%
temperature rise
4.3 Initial Calibration Tests
A temperature rise test was conducted using a 240V,
0.43A, 27W dishwasher water pump motor which had
a cold resistance of 171Ω and the results are shown in
Table 4.
The motor was fitted with three
thermocouples wired in parallel to give an average
reading. The temperature rise is unusually large
because normally the pump motor is operated with
water passing through it. In this test no cooling water
was used.
Table 4. Results from the initial calibration test.
Measurement Method
Temperature
Rise
Three thermocouples wired in
118.2K
parallel
126.4K
Resistance calculated from
measured voltage and current
taken from the terminals at rear of
monitor
Thermal monitor display
128.1K
The thermal monitor display shows a good
correspondence with the value calculated from the
voltage and current readings. This indicates that the
readout calibration is accurate.
The ten degree difference between the thermocouple
readings and the stator resistance measurements is
common due to the difficulty of placing the
thermocouples in the hottest location of the stator.
Note that in the IEC test standard, the maximum
allowable temperature rise as measured by
thermocouples is 10 degrees lower than the maximum
allowable temperature rise as measured from
resistance.
From the preliminary test results, the unit is expected
to give good results for small and medium size
machines, however the effect of the design choice of
using a two-wire resistance method meant that the
desired accuracy of +/-5degK is not practical for larger
motors due to lead resistance errors. Despite this the
unit is still provides a useful indication of the stator
winding temperature for these motors during testing.
To properly validate the thermal monitor accuracy, a
series of tests comparing its results with that obtained
using the standard resistance switching approach is
planned for a range of motor sizes.
5 CONCLUSIONS
An instrument which provides a direct reading, online, stator winding temperature rise measurement for
single-phase induction motors was described,
constructed and tested. It offers a simple, convenient
means for conducting temperature rise tests.
Its main limitations is its use of a two-wire resistance
measurement which is sensitive to lead resistances
errors, especially for larger motors. Future work
includes validating the accuracy of the unit, modifying
the unit to allow the option for four-wire operation and
examining on-line measurement methods for other
types of motor such as three-phase induction motors
and universal motors.
6 ACKNOWLEDGMENT
The efforts of the workshop staff in the Department of
Electrical and Electronic Engineering at Adelaide
University in the construction and testing of the
thermal monitor is gratefully acknowledged.
7 REFERENCES
[1] IEC Standard IEC 60335.1, “Safety of household
and similar appliances”.
Part 1 General
requirements.
[2] R.E. Seely, “A circuit for measuring the resistance
of energised AC windings,” AIEE Transactions
Part 1, Communications and Electronics, Vol. 74,
May 1955, pp 214-218.
[3] G.J. Johnson, “Determination of temperature rise
of energised transformers by the resistance change
method,” IEEE Transactions on Power Apparatus
and Systems, Vol. 89, No. 3, Feb 1970, pp 336340.