Temperature limit of each part of the motor
(1) The temperature rise of the core in contact with the winding (thermometer method) shall not exceed the temperature rise limit of the winding insulation (resistance method), that is, Class A is 60 °C, Class E is 75 °C, Class B is 80 °C, Class F is 100 ° C and Class H is 125 ° C.
(2) The temperature of the rolling bearing should not exceed 95 °C, and the temperature of the sliding bearing should not exceed 80 °C. If the temperature is too high, the oil quality will change and the oil film will be destroyed.
(3) The practice of the case temperature is often based on the fact that it is not hot.
(4) The surface of the squirrel cage rotor has a large stray loss and a high temperature, which is generally limited to not jeopardize the adjacent insulation. It can be estimated by pre-painting non-reversible paint.
Elimination of motor heating faults
When the motor temperature exceeds the maximum operating temperature or the temperature rise exceeds the specified or the temperature rise does not exceed the specified, but the temperature rise suddenly increases at low load, the motor is faulty. The judgment and troubleshooting method are:
(1) The temperature rise under the rated load does not exceed the temperature rise limit, and the motor temperature exceeds the maximum allowable operating temperature only because the ambient temperature exceeds 40 °C. This phenomenon indicates that the motor itself is normal. The solution is to artificially reduce the ambient temperature. If it is not possible, the load must be reduced.
(2) The temperature rise under the rated load exceeds the nameplate specification. No matter what the situation is, the motor is faulty, and it must be stopped for inspection, especially when the temperature rise suddenly becomes larger.
The external reasons are: the grid voltage is too low or the line voltage drop is too large (more than 10%), the load is too heavy (more than 10%), the motor and the machine are not properly matched;
Internal reasons are: single-phase operation, turn-to-turn short circuit, phase-to-phase short circuit, stator grounding, fan damage or untightening, air duct blockage, bearing damage, stator-rotor phase rubbing, motor and cable joint heating (especially copper-aluminum or aluminum-aluminum Connection), the motor is corroded or damp.
In addition, in theory, the motor can be reversed, but some fans of the motor have directionality. If it is reversed, the temperature rise will exceed a lot. In short, troubleshooting must be addressed for each specific situation.
