Motor overheat protection standards and measures

This article discusses the overheating danger of motors . Motors are often used in daily life and in various industries. If no protective measures are taken for motors, safety accidents may occur.

The possible dangers of motor-driven electrical appliances during use include electric shock, fire and overheating. There are many reasons for motor overheating, such as overload, inappropriate selection, cooling failure, lack of monitoring and necessary maintenance leading to premature insulation aging, etc. Motors will generate losses during operation. On the one hand, these losses reduce the efficiency of the motor, and on the other hand, the losses are converted into motor heat, which increases the temperature of the motor winding. The service life of the winding insulation material is related to its operating temperature. If the temperature is too high, the insulation material will accelerate aging, causing the insulation performance to drop sharply, greatly shortening the service life of the motor, and even causing fire and electric shock hazards.

Therefore, the purpose of motor overheat protection is mainly to adopt protective measures in the design, manufacturing, installation and use of the motor. When the motor works under certain load and heat dissipation conditions, the temperature of the winding does not exceed the standard allowable value.

Motor protection equipment example diagram

Relevant standards for motor overheat protection

Each country or each individual standard has different requirements for motor overheat protection. We use the Canadian Electrical Code, Part I, Safety Standard for Electrical Installations and A National Standard of Canada CAN/CSA-C22.2 NO.68 Motor-Operated Appliances (Household and Commercial) as examples to briefly interpret the standard requirements. The above two standards are standards formulated by CSA (Canadian Standards Association) and adopted by the Canadian government as national standards.

The classification and usage of the main motor protectors are shown in Table 1:

Table-1 Comparison of motor protection measures

Methods for implementing motor overheating protection

Using Canadian National Standard/CSA Standard CAN/CSA-C22.2 NO.68 as an example, let’s understand how to implement motor overheating protection for motor-driven electrical appliances to meet the requirements of the standard. The standard lists the following ways or possible situations:

(1) Use a motor-mounted thermal protector that complies with CSA Standard C22.2 No.77 Motor with Inherent Overheating Protection. These motor overheating protectors are listed in the table above. When using them, it is important to note that the protector cannot only respond to the motor current, but also needs to have an appropriate rated operating voltage, current value and temperature preset protection value, and the thermal protector needs to be suitable for the motor. The tests required by the standard include Running Heating Temperature test, Locked-Rotor Temperature testand Locked-Rotor Endurance test. If the motor meets this standard, it can be marked "THERMALLY PROTECTED". That is to say, when the motor complies with the complete electrical appliance standard CSA68 and the motor standard CSA77, the motor meets the standard requirements under normal operating rated load (Rating), overload (Running Heating) and locked-rotor application conditions, and the motor is protected from overheating.

(2) If the motor-driven appliance is equipped with an overload protection device, overheating protection is not necessarily required. In this case, it is necessary to comply with the requirements of CEC Part I. This type of overload protector is different from the overheating protector mentioned above. It only relies on the current of the motor to achieve protection. It can be independent of the motor or attached to the motor. For the selection of tripping current, the ratio of the tripping current of the protector to the rated current of the motor generally does not exceed 1.15. If Fuses are selected as overload protectors independent of the motor, this type of fuse needs to use a time-delay fuse of the type ("D" Fuse).

(3) For some commercial electrical appliances that need to be permanently connected to the building's power distribution line, it is not necessary to install an overload protector in the appliance, but it is necessary to ensure that the power distribution line provides overload protection when the appliance is installed and connected, and the appliance must have a warning label (CAUTION).

(4) For some specific electrical appliances, the standard exempts motor overheating protectors. For example, electrical appliances that use momentary contact switches to control motors, handheld electrical appliances, and household appliances with manned intermittent working systems and built-in fuses. In addition, for household appliances with manned intermittent working systems, if they pass the stall test evaluation, they can also be exempted from motor overheating protectors.

Case

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

Table-2 Motor nameplate information

1. Test purpose

The purpose of the stall temperature test is to evaluate whether the temperature of the winding does not exceed the standard allowable value when the motor is running in a stall state, and the overheat protector can protect the motor from overheating.

2. Test method

According to the standard requirements, the resistance method or thermocouple method is applicable to measure the stall temperature of the motor. In general, the appropriate test method should be selected according to the type of motor or the conditions of the laboratory.

Table-3 Comparison between resistance method and thermocouple method

3. Test conditions

◆ The standard requires type testing, and representative samples (combination of motor and protector) are selected for testing.

◆ The motor sample needs to use wood or other heat-insulating materials as a fixture to fix the motor, and the motor shaft needs to be blocked.

◆ Unless otherwise specified, the motor must be installed in a position that places the protector at the bottom to the greatest extent, which is called the "Worst Case". Because according to the convection principle of heat conduction, the temperature above the winding is usually higher than that below. When the protector trips at the bottom, the temperature above the winding will be the highest.

◆ In order to determine the hottest point of the winding temperature, it is necessary to lay the thermocouple on the actual working winding. In the example, for the capacitor-operated single-phase asynchronous motor, it is necessary to lay it on the actual working main winding. Generally, at least 4 thermocouples are laid, such as one before and after the winding, and one before and after the bottom.

◆ The motor test voltage is based on the rated voltage on the motor nameplate. Generally, the corresponding AC nominal voltage specified by the standard is selected. For example, if the nameplate voltage is 110-120 V, the test voltage is 120 V. In this example, the motor nameplate voltage is 115 V, so the test voltage is 120 V.

◆The metal shell 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 shell is live.

◆The data that needs to be prepared and recorded include voltage, frequency, locked-rotor current, ambient temperature and continuous winding temperature.

4. Test procedures and result judgment

Figure 1 Curve of time and stalled-rotor winding temperature
(example of single-phase motor and automatic reset thermal protector combination)

Take the combination of a single-phase motor and an automatic reset thermal protector as an example. For other situations, please refer to the relevant standards. The test is carried out at room temperature and the motor is blocked for 72 hours. During this period, the thermal protector must disconnect the circuit and automatically reset due to the increase in winding temperature. The instrument continuously records the winding temperature. The schematic diagram in Figure 1 is obtained from experience. The entire process must meet the following results at the end: the winding temperature does not exceed the limit value in Table 4. In this example, the insulation level of the motor is Class B; the grounded fuse cannot be blown; the motor cannot catch fire; the insulation material of the motor cannot be severely damaged such as peeling, brittleness, carbonization, etc.; the motor cannot have electrical or mechanical failures; and it can pass the withstand voltage test.

Table-4 Maximum locked-rotor temperature limit of winding