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Design Case of Automatic Reset Thermal Protector for Air Compressor Motor

Article Source:Thermal protectorAuthor:YaxunPopularity:Published time:2019-06-08 15:44

For example, a manufacturer produces air compressors, and the sales market is in North America (Canada and the United States), applying for CSA product safety certification to comply with North American standards. The standard applicable to this type of product is CAN/CSA-C22.2 NO.68. The motor nameplate information used in the air compressor is as follows:  The motor uses CSA-certified loaded thermal protector, which is an automatic reset motor thermal protector. For the above thermal protection method, the applicable method (1), the motor must comply with the CSA standard C22.2 No. 77. The following will give an example of the motor stall temperature test required by this standard: Motor locked temperature test (LOCKED ROTOR TEMPERATURE)

Motor nameplate information
Table 2  Motor nameplate information



1. Testing purposes
The purpose of the stall temperature test is mainly to evaluate the motor running in the stalled state. The temperature of the winding does not exceed the standard allowable value, and the overheat protector can protect the motor from overheating.

2. Test Methods
According to the standard requirements, the motor stall temperature is measured by the resistance method or the thermocouple method. In general, the appropriate test method should be selected according to the type of motor or laboratory conditions.

Table-3 Comparison of resistance method and thermocouple method
Comparison of resistance method and thermocouple method

3. Test conditions
◆ The standard requires a type test, and a representative sample (a combination of a motor and a protector) is selected for testing.
◆Motor samples require wood or other insulation materials to be used as fixtures to hold the motor and to block the motor shaft.
◆ Unless otherwise specified, the motor must be installed in such a position that the protector is placed at maximum below, called the “Worst Case”. Because according to the convection principle of heat conduction, the upper part of the winding is usually higher than the temperature below, and when the protector is tripped below, the temperature above the winding will be the highest.
◆ In order to determine the hottest temperature of the winding, it is necessary to lay the thermocouple on the actual working winding. In the example, the capacitor operates a single-phase asynchronous motor, which is placed on the main winding of the actual work. Generally, at least 4 thermocouples are placed. For example, cloth is placed on the front and back of the winding, and the bottom and bottom are below.
◆Motor test voltage, based on the rated voltage of the motor nameplate, generally select the corresponding nominal voltage of the mains. For example, the nameplate voltage is 110-120 V and the test voltage is 120 V. In this example, the motor nameplate voltage is 115 V, then the test voltage is 120 V.
◆ The metal housing of the motor is connected in series with a 3 A fuse to the power ground. If the 3 A fuse blows, it can be checked that the motor insulation is damaged and the case is energized.
◆ The data that needs to be recorded is voltage, frequency, locked current, ambient temperature and continuous winding temperature.

4. Test procedure and result determination
Time and curve of locked rotor temperature
figure 1  Time and curve of locked rotor temperature
(Example single-phase motor combined with automatic reset thermal protector)


 
Take the combination of a single-phase motor and an automatic reset thermal protector as an example. For other cases, please refer to the relevant standards. Tests carried out at room temperature, the stall for 72 hours. In the meantime, the thermal protector must be automatically reset due to the increase of the winding temperature, and the instrument continuously records the winding temperature. The schematic of Figure-1 is obtained empirically. From the end of the process to the end, the following results must be met: The winding temperature does not exceed the limit of Table-4, the insulation class of the motor in this example Class B; The grounded fuse cannot be blown; The motor cannot catch fire; The insulation material of the motor shall not be damaged, such as peeling, brittleness, carbonization, etc.; The motor must not have electrical or mechanical faults; it can pass the withstand voltage test.
 Winding maximum stall temperature limit
Table 4 Winding maximum stall temperature limit