How to determine the temperature of the windings of AC motors by their resistance
Winding temperature measurement during motor warm-up tests
The temperature of the windings is determined by testing the motor for heating. Heating tests are performed to determine the absolute temperature or the temperature rise of the winding or parts of the motor relative to the temperature of the cooling medium at rated load. Electrical insulating materials used in the construction of electrical machines age and gradually lose their electrical and mechanical strength. The rate of this aging depends mainly on the temperature at which the insulation operates.
Numerous experiments have established that the durability (service life) of the insulation is reduced by half if the temperature at which it works is 6-8 ° C higher than the limit for a given class of heat resistance.
GOST 8865-93 establishes the following heat resistance classes of electrical insulating materials and their characteristic limiting temperatures:
Heat resistance class — Y A E B F H C Limit temperature, respectively — 90, 105, 120, 130, 155, 180, over 180 gr. S
Heating tests can be performed under direct load and indirect (heating from core losses). They are carried out to the established temperature with practically unchanged load. The steady-state temperature is taken into account, which within 1 hour changes by no more than: 1 °C.
As a load in heating tests, various devices are used, the simplest of which are various brakes (shoes, bands, etc.), as well as loads provided by a generator operating with a rheostat.
During the heating tests, not only the absolute temperature is determined, but also the temperature rise of the windings above the temperature of the cooling medium.
Table 2 Maximum permissible temperature increases of engine parts
Parts for electric motors
Maximum permissible pre-increase in temperature, ° C, with insulation material class of heat resistance
Temperature measurement method
A
E
V
F
H
Variable winding current of the motors 5000 kV-A and more or with the length of the sickle house 1 m and more
60
70
80
100
125
Resistance or temperature in the detectors arranged by the grooves
Same but less than 5000 kV A or s core length 1m and more
50*
65*
70**
85**
105***
Thermometer or cooposition
Rod windings of asynchronous rotor motors
65
80
90
110
135
Thermometer or cooposition
Slip rings
60
70
80
90
110
Thermometer or temperature in the speakers
Cores and other steel parts, contact coils
60
75
80
110
125
Thermometer
The same, without contact separating from the windings
The temperature rise of these parts must not exceed values that would create a risk of damage to insulating or other related materials
* When measuring by the resistance method, the permissible temperature is increased by 10 ° C. ** The same, at 15 ° C. *** The same, at 20 ° C.
As can be seen from the table, GOST provides different methods of temperature measurement, depending on the specific conditions and parts of the machines to be measured.
The thermometer method is used to determine the surface temperature at the point of application. (housing surface, bearings, windings), ambient temperature and air entering and exiting the motor. Mercury and alcohol thermometers are used. Only alcohol thermometers should be used near strong alternating magnetic fields, as they contain mercury eddy currents are induceddistortion of measurement results. For better heat transfer from the node to the thermometer, the tank of the latter is wrapped in foil and then pressed against the heated node. For thermal insulation of the thermometer, a layer of cotton wool or felt is applied to the foil, so that the latter does not fall into the space between the thermometer and the heated part of the engine.
When measuring the temperature of the cooling medium, the thermometer must be placed in a closed metal cup filled with oil and protecting the thermometer from radiant heat emitted by surrounding heat sources and the machine itself, and accidental air currents.
When measuring the temperature of the external cooling medium, several thermometers are located at different points around the examined machine at a height equal to half the height of the machine and at a distance of 1 — 2 m from it. The average arithmetic value of the readings of these thermometers is taken as the temperature of the cooling medium.
Thermocouple method, widely used for temperature measurement, is mainly used in AC machines. Thermocouples are placed in the gaps between the layers of the coils and at the bottom of the slot, as well as in other hard-to-reach places.
To measure temperatures in electrical machines, copper-constantan thermocouples consisting of copper and constantan wires with a diameter of about 0.5 mm are usually used. In a pair, the ends of the thermocouple are soldered together. The junction points are usually placed at the place where it is necessary to measure the temperature ("hot junction"), and the second pair of ends is connected directly to the terminals of the sensitive millivoltmeter with high internal resistance… At the point where the unheated end of the constantan wire connects to the copper wire (at the terminal of the measuring device or the transition terminal), the so-called "cold junction" of the thermocouple is formed.
On the contact surface of two metals (constantan and copper) an EMF occurs, proportional to the temperature at the point of contact, and a minus is formed on the constantan and a plus on the copper. EMF occurs at both the "hot" and "cold" junctions of the thermocouple.However, since the temperatures of the junctions are different, then the EMF values are different, and since in the circuit formed by the thermocouple and the measuring device, these EMFs are directed to each other, the millivoltmeter always measures the difference in EMF of the «hot» and « the cold» junctions corresponding to the temperature difference.
It was found experimentally that the EMF of a copper-constantan thermocouple is 0.0416 mV per 1 ° C of the temperature difference between the «hot» and «cold» junctions. Accordingly, the millivoltmeter scale can be calibrated in degrees Celsius. Since the thermocouple only records the temperature difference, to determine the absolute "hot" junction temperature, add the "cold" junction temperature measured with the thermometer to the thermocouple reading.
Resistance Method — Determining the temperature of windings from their DC resistance is often used to measure the temperature of windings. The method is based on the well-known property of metals to change their resistance depending on temperature.
To determine the temperature rise, the resistance of the coil is measured in the cold and heated state and calculations are made.
It should be borne in mind that from the moment the engine is turned off until the start of measurements, some time passes, during which the coil has time to cool down. Therefore, in order to correctly determine the temperature of the windings at the time of shutdown, i.e. in the operating state of the engine, after switching off the machine, if possible, at regular intervals (according to the stopwatch), several measurements are made.These intervals should not exceed the time from the moment of shutdown to the first measurement. The measurements are then extrapolated by plotting R = f (t).
The resistance of the winding is measured by the ammeter-voltmeter method. The first measurement is carried out no later than 1 minute after the engine is turned off for machines with a power of up to 10 kW, after 1.5 minutes — for machines with a power of 10-100 kW and after 2 minutes — for machines with a power of more than higher than 100 kW.
If the first resistance measurement is made no more than 15 — 20 from the moment of disconnection, then the largest of the first three measurements is taken as resistance. If the first measurement is taken more than 20 s after switching off the machine, then a cooling correction is set. To do this, make 6-8 resistance measurements and build a graph of the resistance change during cooling. On the ordinate axis are plotted the corresponding measured resistances, and on the abscissa is the time (exactly to scale) elapsed from the moment the electric motor is switched off until the first measurement, the intervals between the measurements and the curve shown in the graph as a solid line. This curve then continues to the left, maintaining the nature of its change, until it intersects the y-axis (shown by a dashed line). The segment along the ordinate axis from the beginning of the point of intersection with the dashed line determines with sufficient accuracy the desired resistance of the motor winding in the hot state.
The main nomenclature of motors installed in industrial enterprises includes insulation materials of classes A and B.For example, if class B mica-based material is used to insulate the groove and to wind PBB wire with class A cotton insulation, then the motor belongs to the heat resistance class. to class A. If the temperature of the cooling medium is below 40 ° C (the standards for which are given in the table), then for all classes of insulation the permissible temperature increases can be increased by as many degrees as the temperature of the cooling medium is below 40 ° C, but not more than 10 ° C. If the temperature of the cooling medium is 40 — 45 ° C, then the maximum permissible temperature increases indicated in the table are reduced for all classes of insulating materials by 5 ° C, and at temperatures of the cooling medium 45-50 ° C — at 10 ° C. The temperature of the cooling medium is usually taken as the temperature of the surrounding air.
For closed machines with a voltage of no more than 1500 V, the maximum permissible temperature increase of the stator windings of electric motors with a power of less than 5000 kW or with a core length of less than 1 m, as well as of windings from rod rotors at measuring temperatures by the resistance method can be increased by 5 ° C. When measuring the temperature of the windings by the method of measuring their resistance, the average temperature of the windings is determined. In reality, when the engine is running, the individual winding areas tend to have different temperatures. Therefore, the maximum temperature of the windings, which determines the durability of the insulation, is always slightly higher than the average value.