ONLINE STATOR TEMPERATURE MONITOR FOR SINGLEPHASE INDUCTION MOTORS W.L. Soong, A. Harris and C.H. Fong Adelaide University A. Kennewell, J. Botiuk and D. Gray ITS, 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 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 online temperature measurement of singlephase 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. Resistance Change (%) 40% 35% 30% 25% 20% 15% 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 coefficient 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. 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 multipole switch is used to allow the motor terminals to be rapidly switched between the mains and a source. Source Idc Vdc test motor Figure 2. Simplified diagram showing the resistance switching measurement method. The resistance is determined from voltage and current measurements. Note that the voltage measurement is taken directly across the test motor terminals. This is called a fourwire 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 multipole switch is set to the position and the cold resistance of the motor is measured. The motor is then switched to the 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 source and the hot 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 turnoff. An alternative approach is to set the test current to approximate the 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 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 steadystate. 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 normallyclosed thermal switch wired in series with the stator windings of some appliances which opencircuits 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 opencircuit it is clearly no longer possible to measure the stator resistance and hence determine the winding temperature. temperature thermal cutout trips time Figure 3. Stator temperature rise during overload conditions showing operation of thermal cutout. Normally thermal cutout 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 online stator winding temperature rise for singlephase 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. Online 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 online 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 steadystate and also when thermal cutout switches operate. supply Thermal Monitor 20.7deg Appliance Figure 4. Basic concept for online thermal monitor showing mains input, appliance power output and temperature display. 2 ONLINE TEMPERATURE MEASUREMENT CIRCUIT DESIGN The basic issue for an online temperature measurement system based on stator resistance is taking accurate resistance measurements on an energised stator winding. Figure 5 shows the initial concept. The key challenges are to take resistance measurements in the presence of a large signal, and to prevent the low
source impedance of the mains affecting the resistance measurement. to Resistance Measurement Circuit Figure 5. Initial concept to measure resistance on energised circuits. 2.1 Seely Bridge History In 1955, Seely [2] described an innovative arrangement in which a large blocking capacitor is used to isolate the test current from the supply. This is shown in Figure 6. The size of the capacitor is chosen such that at the motor rated current, the voltage drop across it is only a few volts. blocking capacitor to Wheatstone Resistance Bridge Figure 6. Seely Bridge circuit to allow continuous resistance measurements in energised circuits. Seely also used a bucking transformer to isolate the resistance measurement circuit (based on a Wheatstone bridge) from the 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 voltage seen by the resistance measuring circuit to zero however in practise there is a residual signal of a few volts. The main issue with this circuit is that it uses a twowire resistance measurement which also includes the secondary resistance of the bucking transformer. In 1970, Johnson [3] extended Seely s concept by adding a second bucking transformer and separating the current and voltage measurement circuits (see Figure 7). This produces a fourwire resistance measurement and eliminates the bucking transformer secondary resistance and stray lead resistances from the result. He used a constant current source to generate a output voltage (Vdc) proportional to resistance and hence temperature. A chart recorder was used to record the temperature continuously. V Figure 7. Modified Seely bridge circuit using fourwire resistance method for more accurate measurements. 2.2 Circuit Design The circuit used (see Figure 8) is basically a twowire resistance measurement version of that described by Johnson. The twowire 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 voltage drop measurement. V Figure 8. Circuit used in the thermal monitor. This is a twowire resistance measurement method variation of the circuit in Figure 7. The 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 injection braking effect of the test current). The test current is provided by a 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 voltage across the motor is directly proportional to temperature. In the prototype, a lowpass LC filter was inserted between the isolation transformer and the current source to reduce the residual voltage seen by the current source. I I
Figure 9 shows the temperature readout circuit. The output voltage (Vdc) from Figure 8 is passed through a secondorder lowpass 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. motor resistance I V lowpass RC filter variable gain amplifier temperature rise readout 20mV=1K V REF Figure 11. Internal circuitry of the thermal monitor showing the large bucking transformer used for the current source circuit. 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 1012 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 current source (see Figures 8 and 11). This must be able to operate at rated voltage carrying the maximum 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 current. Figure 10. Front panel of thermal monitor showing the current setting switch and the temperature readout and nulling adjustment. The blocking capacitor was also another major component in the system (see Figure 13). Two pairs of backtoback 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 current without seeing reverse voltages. The three seriesconnected 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 test current. Figure 13. blocking capacitor arrangement. At the rear of the unit (see Figure 12), banana plugs were used to make available the test current and the 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 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 test current is then selected. The test current is turned on and the amplifier gain control adjusted until the readout reads 0.0degK. The power is then turned on and after the starting transient, the protective diodes across the 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 twowire 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 worstcase 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 temperature rise 0.2% 2% 20% 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 Resistance calculated from 126.4K 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. Part 1, Communications and Electronics, Vol. 74, May 1955, pp 214218. [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 336 340. 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 twowire 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 singlephase 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 twowire 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 fourwire operation and examining online measurement methods for other types of motor such as threephase induction motors and universal motors. 6 KNOWLEDGMENT 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 windings, AIEE Transactions