#MOTOR WINDING SYSTEM FULL#

2 shows the voltage that a VFD, running at almost a 6-pulse mode, will send to the motor. The frequent use of variable frequency drives (VFDs) can result in detrimental effects to electric motors because of the condition of power in manufacturing facilities. For example, if a 100 hp motor has an unbalance factor of 3 percent, the motor should be derated to 0.88 or 88 percent of capacity, 88 hp. According to the derating curve, the higher the level of unbalance, the lower the acceptable level of steady state load. 1 shows the NEMA derating curve for percentage of unbalance. NEMA MG-1 Sections II and IV specify what voltage quality, as a function of balance and distortion, allows what level of percentage load. Only accurate load level calculations can give reliable measurements of excessive losses and overheating in the motor.Д®lectric motors in manufacturing plants generally need to be derated because of poor power conditions in order to maximize their useful life. This justifies the need for accurate knowledge of operating load level. These losses are a destructive factor to windings and a significant reason for bearing damage. Losses, not current levels alone, are the true source of heat. In industrial applications, perfect voltage conditions are rare. The repair for each of these three motors ran into the hundreds of thousands of dollars. This explains why these motors were failing. However, after looking at the true load to the motor, an overload of nearly 20 percent was discovered. A mild overload was identified by examining the stator current of this motor. Two identical applications had undergone unscheduled outages in the previous 12 months.

#MOTOR WINDING SYSTEM FULL#

Under full load conditions, this is the difference between life and death to a motor.ДЇor example at a coal-fired power plant in the United States, a 7000 hp 6.6kV motor was running with only 7 percent overcurrent, but an 8 percent overvoltage. A load error of more than 10 percent can be introduced by relying on stator current readings to access probable load and heat levels. It has been shown that for motors ranging from 10-200 hp, operating at a 10 percent overvoltage would typically decrease losses by only 1-3 percent.Д®ven though the motor current may vary when applying overvoltages, the excessive damaging heat in the motor will not improve. A common mistake is made in operating at an overvoltage to reduce the stator current and to reduce the introduction of heat. Stator current is frequently used to measure load level, but load level can easily be masked by an overvoltage condition. There are typically five main reasons for overheating-overload, poor power condition, high effective service factor, frequent stops and starts, and environmental reasons. Articles have been published stating that a significant cause of bearing deterioration is overheating. Leading standardization organizations have concluded that 30 percent of motor failures are attributed to insulation failure and 60 percent of these are caused by overheating.

As an example, if a motor that would normally last 20 years in regular service is running 40 C above rated temperature, the motor would have a life of about 1 year.

The common rule states that insulation life is cut in half for every 10 C of additional heat to the windings. Maintenance experts agree that excessive heat causes rapid deterioration of motor winding insulation. On-line technologies permit assessment of the entire motor system to facilitate troubleshooting.

Motor winding system