What should I do if the motor gets damp?

What are the repair methods for motors that get wet due to water ingress? At the production site, due to motor selection, improper storage, maintenance, environmental factors and other factors, moisture enters the inside of the motor to form condensation or water enters directly into the motor, resulting in a decrease in the insulation resistance of the motor, affecting the normal use and operation safety of the motor. This article mainly analyzes and introduces the drying treatment methods of damp squirrel cage motors under different conditions on site, focusing on the principles and on-site application methods of current drying method and iron loss drying method.

 

Small capacity asynchronous motor gets damp

 

Drying method: It is more convenient to disassemble and disassemble a small-capacity asynchronous motor. It can be dried on-site or disassembled to a maintenance room for processing according to the on-site environment. Generally, the following two methods can be used to dry the motor.

 

Generally, small low-voltage asynchronous motors are suitable for external heating and drying methods, and the operation is relatively simple; the principle is to use the radiation, convection, and conduction of external heat sources to dry the motor during drying; generally divided into two methods:

 

1. Use light bulbs (or infrared light bulbs) and ovens for drying, and use hot air blowers for drying;

2. When using a light bulb or iodine tungsten lamp for drying, do not get too close to the coil to prevent the coil from being burned. Safety protection lamps must be used, and the temperature cannot exceed 100°C when using an oven.

 

Large and medium-sized asynchronous motors are affected by moisture

 

There are several drying methods:

 

1. Current drying method

 

The basic principle of the current drying method is to pass low-voltage current to the stator winding of the motor, lock the rotor, and use the temperature generated by the loss of the motor itself to dry the motor. During drying, the stator and rotor of the motor generate heat at the same time, and the drying speed is fast. It is generally used for applications with larger capacities. Large high and low voltage motors;

 

Note: Calculate the maximum current allocated to each phase winding of the locked-rotor current and it should not exceed 50% to 60% of the original rated current. Then you can select the voltage level for drying.

I=U*Iq/UN

U→voltage after drop

UN→Rated voltage

Iq→starting current, generally 5 to 8 times the rated current

 

Method 1: Directly connect the low-voltage three-phase AC power to the motor stator winding. There is no need to pull out the motor rotor. The motor stator and rotor are dried at the same time, which is convenient for on-site implementation. Large motors require a large power supply capacity, which may be limited by site conditions; 6 kV motor Generally, a 380 V power supply is supplied to the site for drying. If the motor insulation is low, the rotor can be blocked for drying. If the motor insulation is greater than 0.5Ω, a three-phase alternating current can be supplied and the motor can be rotated for drying.

 

Method 2: Connect the three-phase windings of the motor in series from end to end (or in series with one end to reduce the current), for a motor with 6 outlets; use an AC or DC welding machine or voltage regulator to adjust the current into the stator windings of the motor to dry the motor. , suitable for drying high and low voltage motors when the on-site power supply capacity is insufficient; when turning on or cutting off the current of the welding machine, the current should be adjusted to zero first to prevent high voltage from damaging the motor insulation; on-site processing does not require pulling out the motor rotor, which is convenient.

 

Method 3: The three-phase windings of the motor are connected in parallel (for motors with 6 terminals) or two phases are connected in series and then connected in parallel with another phase (for motors with 3 terminals); use an AC or DC welding machine or voltage regulator to adjust the current flow. Insert the motor stator windings to dry the motor.

Example of drying motor using current drying method

 

Take a motor model YKK710-10 with a capacity of 2 800 kW, a current of 335 A, and a voltage of 6 kV as an example: This motor is an outdoor motor. During the infrastructure construction period, the motor will be put into trial operation. The insulation resistance measured on site is 2 MΩ. Weather Moderate rain and high humidity. After 36 hours after the motor's built-in heater was put in, the insulation resistance was measured to have little change of about 3 MΩ. It was planned to increase the motor current drying method for drying while the original motor heating was continued to be put in.

 

Implementation method: Set up a temporary rainproof shed around the motor, connect to the on-site 380V power supply for drying, take the starting current Iq as 7 times the rated current 2 345 A, select 3+1 core 35 mm2 copper core cable for calculation, and measure the instrument: 2 500 V A megohmmeter, a clamp ammeter, an infrared thermometer, the motor's junction box, and a gap between the inspection holes on both sides to facilitate moisture dissipation. After 12 h, the insulation resistance stabilized at around 36 MΩ, achieving the purpose of drying. The actual measured current on site is 146 A. The temperature is monitored through on-site measurement and DCS screen. The maximum coil temperature is 85°C.

 

2. Motor iron loss drying method

 

The basic principle of the iron loss drying method is to wind an excitation coil on the stator winding iron core of the motor, pass in alternating current, so that the stator core generates magnetic flux, and use the temperature rise generated by the eddy current loss of the iron core to dry the motor.

 

Method 1: Wind the excitation coil around the motor stator core (through the stator bore), connect the 380 V (220 V) AC power supply, and dry the motor insulation. Scope of application: Large and medium-sized motors, motor rotor core extraction.

 

Method 2: Wrap the excitation coil around the motor stator shell, connect the power supply through the welding machine and voltage regulating transformer, and dry the motor insulation. Scope of application: large and medium-sized motors, the motor has been installed on site.

 

Examples of drying treatment methods for motor iron loss

 

Take a motor model Y2-355-4 as an example, with a capacity of 250 kW, a voltage of 6 kV, and a current of 30.4 A.

The core data measurement is: L=56.5 cm, n and Lf are 0, Da=59 cm, De=36 cm, h=tooth height 6.7 cm.

The average diameter of the stator core yoke: Dav=(Da-Hc)=(59-4.8)=54.2cm.

If the iron loss drying method is used to dry this motor, using a 220 V, 50 Hz power supply, and a magnetic density B = 1 T, the number of turns of the excitation coil to be wound is 38, and the generated excitation current is about 9.9 A.

 

Other things to note

 

1. For the motor with the rotor extracted, heating and drying should be carried out in clean air, and all parts of the motor should be cleaned before drying.

 

2. When the motor is dry, the winding temperature of the motor must be lower than the temperature required by the specified insulation level (to ensure safety, it is best to be 10°C lower than the specified temperature). Generally, the winding temperature is controlled at 70°C ~ 80°C during drying. for the best.

 

3. For motors with excessively wet windings, the DC current drying method cannot be used. DC electricity has an electrolytic effect.

 

4. When using current and iron loss method for drying, the motor shell must have a good grounding wire to prevent electric shock. When using light bulbs or iodine tungsten lamps for baking, care should also be taken to prevent electric shock.

 

5. For closed motors, when the rotor is not pulled out to dry, the junction box or inspection hole of the motor should be opened to a certain gap. For small low-voltage motors, the motor end cover can also be opened to facilitate the dispersion of moisture in the motor.

 

6. During the drying process, regularly measure the temperature and insulation resistance of each part of the winding and keep records. The initial drying stage is recorded every 15 to 30 minutes. In the next 1 to 2 hours, the temperature of the motor must be measured regularly to prevent damage caused by overheating at a certain point of the motor.

 

7. During drying, the heating temperature should be gradually increased. For humid motors, the temperature should be slowly heated to 50~60℃, maintained for 3~4 hours, and then gradually increased.

 

8. In the initial stage of motor drying, due to the increase in temperature and the emission of moisture, the insulation resistance will decrease, then gradually increase, the increase speed will slow down, and finally reach stability. At a constant temperature, the insulation resistance value will remain for more than 3 to 4 hours. When it remains unchanged, the drying work can be completed.

 

9. During the drying process of the motor without extracting the rotor, if the conditions are met, rotate the motor rotor to 180°C regularly to prevent deformation caused by uneven heating of the rotor and also facilitate the dissipation of moisture.

 

In fact, there are many specific methods for drying the motor after it gets damp. At the production site, a suitable drying method should be selected according to the specific conditions to dry the motor. However, no matter which method is used to dry the motor, it must be noted that the motor temperature cannot exceed its allowable value. It will cause new damage to the motor insulation. During the drying period, pay attention to the safety protection of the equipment and people. The production site often requires the motor to be put into operation in time and run safely. The environment in the south is rainy and humid. In order to ensure that the motor is not affected by moisture or water intrusion, it For operation or safe operation, specific moisture-proof measures for motors should be formulated and attention should be paid to motor selection for humid environments.