Rabu, 13 April 2016

How to fix overheated electric motor

Care experts agree that excessive heat causes rapid deterioration of the motor winding insulation. The general rule states that the life of the insulation is cut in half for every 10 C additional heat to the rolls. For example, if a motor is usually the last 20 years in regular service runs 40 C above the rated temperature, the motor will have a life of about 1 year. 

Leading standardization organizations have concluded that 30 percent of motor failure associated with insulation failure and 60 percent is caused by overheating. The article was published stating that a significant cause of damage to bearings overheating.There are usually five main reasons for excessive overheating, electrical conditions are poor, high effective service factor, frequent stops and starts, and environmental reasons.Overload condition 

Stator is now often used to measure the load level, but the level of the load can be easily covered by overvoltage conditions. A common error made in operating at voltages to reduce the stator currents and to reduce the introduction of heat. It has been proven that for 10-200 hp motor starting, operating at voltages of 10 percent will usually reduce losses by only 1-3 per cent.Although the motor current can vary when applying voltage, excessive heat in the motor damage will not improve. An error loading more than 10 percent can be introduced by stator current readings rely on access to the load and heat level of possibility. In full load conditions, this is the difference between life and death for the motor. 

For example, in coal-fired power plants in the United States, the 7000 hp 6.6kV motor running with only 7 percent of the current, but the voltage is 8 percent. Two identical applications have undergone a scheduled outage in the previous 12 months. A lightweight advantages identified by examining the stator current of this motor. However, after seeing the correct load to the motor, an excess of nearly 20 percent were found. This explains why the motor failed. Fixes for each of the three motors is running into hundreds of thousands of dollars. 

In industrial applications, the voltage condition is perfectly rare. Losses, current levels are not alone, is the true source of heat. This loss is a damaging factor rolls and a significant reason for bearing damage. 

This justifies the need for accurate knowledge of the level of operating expenses. Just load level calculations can accurately provide reliable measurement of excessive losses and electric motor overheating. 

Electrical conditions 
Electric motors at the plant should generally be derated due to poor power conditions to maximize their lifespan. NEMA MG-1 Part II and IV specify what voltage quality, as a function of balance and distortion, allowing what percentage rate burden. Fig. 1 shows the NEMA derating curve for the percentage of imbalance. According to the derating curve, the higher the degree of imbalance, the lower the acceptable level of steady state load. For example, if a 100 hp motor has an imbalance factor 3 percent, the motor must be derated to 0.88, or 88 percent of capacity, 88 hp. 

Often using variable frequency drive (VFD) can lead to adverse effects of electric motors for electrical conditions at the manufacturing facility. Fig. 2 shows the voltage that the MCC, running at nearly 6-pulse mode, will be sent to the motor. distorted current is the motor reaction to poor power conditions. severe distortions clear. This scenario shows the NEMA derating of 0.7 which allows the motor to be operated at only 70 percent of output. 

Effective service factor 
The key to finding the most common cause of overheating is the accuracy in estimating the load level. This can be identified by looking at only the current and voltage. The formula for calculating the effective service factors are:

Effective service factor prediction care professionals provide solid conclusions stress on the motor load certain applications. 

In another example, data collected using a dynamometer showed 300 hp motor that was tested ran almost a full load, 99.7 percent. Voltage distortion poor because silicon controller rectifier previously unknown defect in the power supply. so the NEMA derating factor of 0.85 results in effective service factor of 1.17, which indicates an alarm condition. 

Apart from service factors nameplate, any motor that operates above the 1.0 service factor is under stress. Factors indicating a higher service to the excess motor abilities for short periods of time, the ability of steady state operation is not higher. often poor voltage conditions and can be caused by various reasons. NEMA determine the load level allowed for a poor voltage conditions. On-line monitoring tool that is able to accurately calculate the load operation ensures plant operation within acceptable limits. 

Start and stop often
Table 1 displays the maximum number of starts and stops for line-operated motors as a function of and their speed. Limiting the startup frequency, the most stressful part of the operation of the motor, it is very important. 

Many well-documented case of motor failure repeatedly addressed by increasing the horsepower rating of the motor which shortens the time between failures. However, the root cause of the failure of the actual frequency of starts and stops. The key is to monitor the number of starts and hours for medium and small motor or daily for larger motor. 

On-line testing can ensure compliance with professional standards. It can be used in identifying the reason for failure in the operation that does not comply with the standards by incorporating these standards in the operation of long-term unattended monitoring. 

State of the environment
Thermography is often used to determine the conditions under which the electric motor is used. poor cooling due to high ambient temperatures, clogged drain, etc., are typical examples of nonelectrically temperature stress induced in both motor and insulation systems. chemical abrasive substances in the air, wet operation, and operation of high altitude are some general environmental pressures. 

Standardized tests
Bearing and winding failure is the most common motor failure. The fundamental reason is usually excessive heat. preventive maintenance practices often restrict on-line electrical measurements to interpret the current levels. While important, this method does not conclusively identify winding failures caused by excessive heat. The best way to ensure a successful preventive maintenance and monitoring is to test in accordance with NEMA and other professional standards. Automatic votes necessary to effectively ensure the health of the motor.

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