AUTOMATIC THREE-PHASE SQUIRREL-CAGE INDUCTION MOTOR TEST ASSEMBLY FOR MOTOR THERMAL BEHAVIOR STUDIES

ABSTRACT

This paper presents an experiment to show to the students the importance of the temperature on the performance of the three-phase squirrel cage induction motor. Is is presented a laboratory assembly to perform, automatically, three-phase squirrel-cage induction motor load tests. Besides the motor, mechanical load (fan), drive and electrical protection equipment, the assembly comprises a 12 bits data acquisition system, a software controlled electronic driving system, temperature sensors and a microcomputer. It is useful for fast and reliable motor part temperatures acquisition during any proposed load test, via computer keyboard, allowing for the data storage for motor thermal behavior studies. The performance of the assembly was checked by performing load tests according to the duty types proposed in [1].

INTRODUCTION

The IEC 34-1 [1] defines a duty as "the statement of the load(s) to which the machine is subjected, including, if applicable, starting, electric braking, no-load and rest and de-energized periods, and including the durations and sequence in time". Rest and de-energized is defined as "the complete absence of all movement and of all electrical supply or mechanical drive".

IEC 34-1 defines also the duty type as being continuous, short-time, periodic duty and non-periodic duty. Periodic duty includes constant load(s) with specified duration that constitute a duty cycle itself along the time and non-periodic duty includes load and speed variations in the operating range. It is also defined thermal equilibrium as "The state reached when the temperature rises of the several parts of the machine do not vary by more than 2K over a period of 1 hour" and cyclic duration factor as "the ratio between the period of loading, including starting and electric braking, and the duration of the duty cycle, expressed as a percentage".

The duty types defined by [1] are:

1. Continuous running duty - duty type S1 - "Operation at constant load of sufficient duration for thermal equilibrium to be reached" (corresponding to a multiple of thermal time constants). The motor must be so rated, that its nominal output power equals or exceeds the continuous load (Figure 1a). In this duty type, t_N= constant load operation time.
2. Sort-time duty - duty type S2 - "Operation at constant load during a given time, less than that required to reach thermal equilibrium, followed by a rest and de-energized period of sufficient duration to re-establish machine temperature within 2K of the coolant" (Figure 1b). In this duty, t_N= constant load operation time. Under this type of duty, machine can be overloaded during t_N until the temperature of the hottest point reaches the permissible limit according to its thermal classification (insulation class). This means that for a load power P_L a motor rated less than P_L can be chosen. Coalant means the refrigeration fluid like air in the most cases.
3. Intermittent periodic duty - duty type S3 - "A sequence of identical duty cycles, each including a period of operation at constant load and a rest and de-energized period. In this duty, the cycle is such that the starting current does not significantly affect the temperature rise" (Figure 1c). In this duty type, t_R= rest and de-energized time, t_C= duty cycle duration time (t_C=t_N + t_R). The cyclic duration factor is CDF=t_N/t_C. After some cycles the thermal equilibrium is reached.
4. Intermittent periodic duty with starting - duty type S4 - "A sequence of identical duty cycles, each cycle including a significant period of starting, a period of operation at constant load and a rest and de-energized period" (Figure 1d). In this duty type, t_D= starting time, t_C = t_D + t_N + t_R, CDF= (t_D + t_N/t_C.
5. Intermittent periodic load with electric braking - duty type S5 - "A sequence of identical duty cycles, each cycle consisting of a period of starting, a period of operation at constant load, a period of rapid electric braking and a rest and de-energized period" (Figure 1e). In this duty type, t_F= electrical braking time, t_C = t_D + t_N + t_R, CDF = (t_D + t_N + t_F)/t_C.
6. Continuous-operation periodic duty - duty type S6 - "A sequence of identical duty cycles, each cycle consisting of a period of operation at constant load and a period of operation at no-load. There is no rest and de-energized period" (Figure 1f). In this duty type, T_NL = no-load operation time, t_C = t_N + t_v, CDF = t_N/t_C.
7. Continuous-operation periodic duty with electric braking - duty type S7 - "A sequence of identical duty cycles, each cycle consisting of a period of starting, a period of operation at constant load and a period of electric braking. There is no rest and de-energized period" (Figure 1g). In this duty type, t_C = t_D + t_N + t_F, CDF = 1.
8. Continuous-operation periodic duty with ratio load/speed changes - duty type S8 - "A sequence of identical duty cycles, each cycle consisting of a period of operation at constant load corresponding to a predetermined speed of rotation, followed by one or more periods of operation at other constant loads corresponding to different speeds of rotation (carried out, for example, by means of a change of the number of poles in the case of induction motors). There is no rest and de-energized period" (Figure 1h). In this duty type, t_C = t_D + t_N1 + t_F1 + t_N2 + t_F2 + t_N3, CDF1 = (t_D + t_N1)/t_C, CDF2 = (t_F1 + t_N2)/t_C, CDF3 = (t_F2 + t_N3)/t_C.

   9. Duty with non-periodic load and speed variation - duty type S9 - "A duty in which generally load and speed are varying non-periodically within the permissible operating range. This duty includes frequently applied overloads that may greatly exceed the full loads" (Figure 1i). In this duty type, t_L = operation under various loads time, t_S = operation under overload time.

Figure 1. Duty types - IEC-34-1 1 - load x time. up

The load duty for some drive may be described by one of the duty types among the nine types presented in [1] or by another duty specification by the motor purchaser. It is a purchaser responsability to specify the duty as well as possible to the manufacturer to manufacture the right motor.

If the duty is not specified by the purchaser, duty type S1 is imposed and the class of rating assigned shall be maximum continuous rating.

If machine shall be applied to a S2 to S9 duties the rating shall be based on the correspondent duty.

In [1] it is mentioned that machine load test, in manufacturer plant, can be done using the equivalent continuous rating for machines complying with S3 to S9 duties. The standard [1] does not specify how to calculate the equivalent continuous rating but the rms power value is frequently used according to the agreement between manufacturer and purchaser.

The equivalent rms power value can be calculated for any duty like that presented in Figure 2 as an example. For Figure 2a, the rms is given by (1)

Figure 2. Equivalent power example - load x time. up

(1)

For Figure 2b the rms is given by (2)

(2)

When it is necessary to specify a motor for a specific drive, there are two possibilities: a) to follow the IEC or equivalent standards recommendations specifying a special machine to the manufacturer. This procedure is currently used to specify large motors (larger than 200 HP) that are not series manufactured but it is generally avoided to specify small motors (less than 200 HP) because it results in special motors and so very expensive when compared with series manufactured motors (catalog motors), b) to specify a catalog motor is what people use to do when possible.

Papers [2] and [3] present a simulation method that can be used to help the designer for the duty definition, catalog motor selection and, if necessary, for the equivalent power calculation. To do the researches reported in [2] it was necessary to develop the automatic test assembly reported in this paper. This test assembly can be used also by motor manufacturers in their test area, by O&M having a large number of motors, in workshops, in electrical machine technology research centers and also in educational electrical machine laboratories.

AUTOMATIC TEST ASSEMBLY DESCRIPTION

For thermal behavior tests of three-phase squirrel-cage induction motors, fed directly from the grid or from power electronic converters, it is necessary: a) to install temperature sensors in several parts of the machine, b) to run the machine according to the duty type specified by IEC [1], as mentioned before, or by the user, c) to record instantaneous temperature values during machine run.

The automatic test assembly is an experimental electrical machine assembly (Figure 3) developed to accomplish with the described subject mentioned above. It is composed by the motor and the mechanical load (fan), switches, contactors, electrical protection devices, inverter, data acquisition card with sensing circuits, temperature sensors for motor and ambient air, contactor relays control circuits and a microcomputer.

Figure 3. Experimental electrical machine assembly. up

System is automatized and controlled by the microcomputer allowing, through a specific software, for selection of: a) power source type (grid or converter), b) starting type (no-load or rated-load), c) type of duty, d) duration of each part of the operation cycle for the different duties (time-on, time-off including starting, stopping and braking), e) number of temperature values to be acquired and storaged. Its output allows for data table and plot construction.

The automatic test assembly allows for a sequence of any number of tests since it is programmed in the computer.

The selection of power source and the operation time interval (time-on, time-off, starting, stopping and electrical braking) are done by the microcomputer that send pulses to operate the contactors and relays through the relays control circuits. Each relay is operated according to the developed software.

The temperatures, acquired by temperature sensors, in the stator winding, in the stator and rotor cores and in the ambient air are conditioned, filtered, converted into digital in a 12 bits A/D converter and stored in files in the computer HD. When test or tests finish motor is stopped automatically and data can be edited in tables, plots or treated numerically in a proper software.

Computer program allows for real time system operation and details of temperature sensors, sensor circuits, conditioning circuits, data acquisition card, relays control circuits and software are provided as follows.

TEMPERATURE SENSORS AND SIGNAL CONDITIONING

Figure 4 shows a motor section with the four temperature sensor positions. The sensors used are the very small metal encapsulated integrated circuit Intersil type AD 590JH.

Figure 4. Motor sensor positions. up

This device provides a linear output current related to the temperature. The ratio current/temperature is 1µA/°K between 218°K and 429°K and it is possible to calibrate it.

The device is insensible to voltage drops on the conductors due to its very high impedance to the output current, so that it can be installed far from the reading circuit.

To put the sensor in the stator and rotor cores, holes were drilled in the machine stator and rotor lamination packs without disassembly them. To put the sensor in the stator winding it was necessary to dismount it partially to erase the coils.

As the tests were conducted in a 3 HP motor it was not possible to insert a sensor in the rotor bar. This would be possible in larger machines. To get the rotor core temperature, a couple of slip rings and brushes were used without problem because the sensor is free from voltage drop on the brushes across the measurement circuit.

To use the voltage signal, got from the temperature sensor, it is necessary to condition it properly to the AD converter voltage range.

The AD converter used is the Analog Devices AD674. The circuit of Figure 5 permits to deal with analog input in the range of 0 V to +10 V (pin 13) and 0 V to +20 V (pin 14) and uses 3 operational amplifiers, that are instrumentation amplifiers in the non-inverter configuration, as a signal conditioning circuit.

Figure 5. Sensing circuit. up

The gain is got just adjusting the potenciometer P₂ and, next to the signal conditioning, there is a signal filter.

The filter is an active lowpass 2^nd order (Butterworth) that presents a good response from 0 Hz to the determined cutoff frequency (fc) where the gain of the storage felt to 0,707, in other words, -3 dB. In the case of Figure 5, where R1 = R2 = 240 k, R3 = R4 = R5 = R6 = 10 k, R7 = R8 = 240 k, all of them 1 % tolerance, C1 = C2 = 47 nF it has fc = 14 Hz. The filter is necessary to eliminate the high frequency signals, electrical and electromagnetic interference noises.

The complete sensing circuit uses the integrated circuit NE5514 from Phillips that has 4 encapsulated op-amp. Filter output is directly connected to one of the multiplexer input channels in the data acquisition card.

DATA ACQUISITION CARD

The data acquisition card could be bought from any manufacturer but it was designed and constructed by the authors. Figure 6 shows the data acquisition card drawing that comprises the multiplexing, sample & holding and the A/D conversion stages. The output of each sensing circuit (in this case there are 4) are connected to one of the multplexer inputs; the multiplexer selects, in a sequential way, among their input channels, the one which is the desired at each time and connects it to its output during a specific time interval.

During this time interval a sample & hold circuit freeses the instantaneous value of this signal while the A/C converter converts it to the digital form. This digital instantaneous value is sent to the microcomputer or to a microprocessor (in this case it was used a PC AT 486 microcomputer).

The multiplexer used is the AD 7506, the sample & hold is the AD 583 HC, and the A/D converter is the AD674AJ, all of them from Analog Devices. A reading enabling circuit using two AND gates (7427) and three inverters (7486) is necessary to permit that any input channel could be selected and be able at the multiplexer output.

Figure 6. Data acquisition card drawing with relay drives. up

RELAYS CONTROL CIRCUIT

To make the system automatized it is necessary to have a relays control circuit. The AC motor control is shown in Figure 7. The contactor CM1 is used for motor starting and stopping. CM2 is used in tests with electrical braking that is done by phase inversion with CM2; CM3 is used to select the power source between the grid or the inverter.

Figure 7. AC motor control. up

To operate the contactors in order to automatize the system the five relays control circuits are used, each one with a specific task (Figure 8).

The contactors control logic is shown in Table 1, where the logic level 0 is used to operate the desired relay.

Table. Contactors control logic.

The relay RL4 is used to select the frequency of operation of the inverter when it is to be used. It can be selected 2 different frequencies 60 Hz and 30 Hz as shown in Figure 8.

If the relay RL4 is not operated the voltage frequency remains 60 Hz. The reference voltage source (V_REF) is internal to the inverter and the voltage V_S on the resistor R_S is proportional to the chosen frequency.

The fifth relay RL5 is used to control the motor load though the control of the air opening of the fan using a solenoid.

Figure 8. Relay positions for motor operation. up

The relays control circuit schematic is shown in Figure 9. The drive block is constituted by the register 74LS373 of 8 bits with parallel input and output (unit U1) and by buffer 74LS07 with six drivers, unit U16.

Figure 9. Relay positions for frequency selection. up

The register is a relay selection unit. Through a control byte stored in unit U1 the desired relay is selected. This process consists in the input a low level "0" in the driver unit U16 (Figure10).

Figure 10. Relay control circuit. up

The optocouplers are used because it is required an electrical insulation between the drive unit and the relay.

The relay operation generates electromagnetic interference noise able to cause the misoperation of any contactor.

DATA ACQUISTION AND RELAYS CONTROL SOFTWARE

The hardware shown before is supervised by the microcomputer that, through a software, a permits its operation. The software establishes a communication between the user and the microcomputer.

The programming language used is the Pascal 6.0 and the software permits: a) the testing motor temperatures acquisition according to the number of points given as input, b) to handle the temperature data in tables or plots, c) to automatize the system allowing that all the motor type duties prescribed by [1] could be performed via computer keyboard, d) the test performing in real time with accurate and reliable results. e) to treat mathematically the output signals. The block diagram is presented in the Figure 11.

Figure 11. Software block diagram. up

RESULTS

Figure 12 shows the stator winding, stator core and rotor core temperature rise x time curves for duty type S2 with t_N = 20 secs.

Figure 13 shows the same temperature rise for duty type S4 with t_N = 20 secs, t_D = 5 secs and t_R = 5 secs.

Figure 12. S2 duty type (experimental). up

Figure 13. S4 duty type (experimental). up

Figure 14. S4 duty type (simulation). down

Figure 14 shows the stator winding temperature rise x time curve for duty type S, with the same times as in Figure 13, obtained by simulation using the dynamic mathematical model presented in [2] and [3].

Results presented in [2] were checked also against test results measured using analog ampermeter and all of them were quite equal showing that results got from this assembly are the best at the best price.

CONCLUSIONS

This laboratory assembly can shows to electrical machine students the importance of the temperature considerations in electrical machine loading.

Figures 12 to 14 shows that for any type of the load duty the temperature in each part of the machine reaches its steady-state value. This steady-state value can not be higher than that specified for each part of the machine.

In case of class B machines the stator winding temperature can not be higher than 120 K. So, for a maximum ambient temperature of 40 K. The stator winding temperature rise can not be higher than 80 K. In the case of figure 13 it did not reach 60 K showing that it is possible to use the same machine operating with a higher load in the same duty.

Figure 12 also shows the temperature behavior during starting and stopping. It is possible to do blocked-rotor test, successive starting tests and speed reversion tests that are tests that force the machine termically.

ACKNOWLEDGEMENTS

The authors would like to thank FAPESP and CNPq for financial support of this research.

REFERENCES

1. IEC Publ. 3-1, Rotating electrical machines, Part 1: Rating and Performance, 8^th edition, 1983.
2. E. Ruppert Filho and E. Avólio, "Squirrel-cage induction motor dynamics simulation using an electrical and thermal mathematical model based on manufacturer technical bulletins data and on technical standard statements", Journal of Energy Systems, IASTED, Canada, USA and Switzerland, vol. 1, 1994.
3. E. Ruppert Filho, E. Avólio and R.R. Riehl, "Three-phase squirrel-cage induction motor dynamic thermal mathematical model, Trans. IEE, Japan, vol. 116-D, no. 7, 1996.

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