Motor Load Matching – Motor Current
Using motor application parameters to support root cause analysis
There is a machine in your facility whose motor fails every couple of years. Your job is to find out why. Conducting a root cause analysis of the failures is the place to start. You need an understanding of the factors that should be considered in choosing the correct motor for the application so you can tell if the motor is the right one for the machine.
In the last post (http://blog.machineryhealthcare.com/bid/100112/Motor-Load-Matching-The-Motor) we talked about the torque parameters of the motor. This week we’ll discuss several current parameters.
The next several blog’s will attempt to introduce some of the major factors that could be causing the motor to fail. You can check each of these for your application as you search for a solution.
- 1. The motor
We need to understand some of the characteristics of motors; torque, horsepower, current, service factor, load profile among others.
- 1. Motor Current
In addition to the relationship of torque, motor current is also related to speed. Figure 3 shows this.
There are two points of interest here. The first is full-load current.
- Full-load current
Full-load current is the steady state current that flows when the motor is operating at full load torque and speed.
- Locked-rotor current
Locked-rotor current is the steady-state current of a motor with the rotor locked and with rated voltage applied. Operating at locked-rotor condition for more than 20 seconds can result in insulation damage due to excessive heat.
- 4. Service factor
Service factor is defined as the permissible amount of overload a motor will handle within defined temperature limits. When voltage and frequency are maintained at nameplate rated values, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate. However, locked-rotor torque, locked-rotor current and breakdown torque are unchanged. Operating under these conditions will cause a shorter motor life because of the higher temperatures causing the insulation to deteriorate faster.
- 5. Motor temperature
Heat is a major consideration when selecting a motor. If the motor runs too hot, the life of the insulation will breakdown much faster than it should. A rule of thumb states that for each 10°C temperature rise above rated temperature, the insulation life is cut in half. High temperatures can also cause breakdown of the grease in the motor’s bearings, causing early bearing failure. Bearing or gear lubricant life is reduced by half for every 25°F (approximately 14°C) increase in temperature.
The motor is subjected to two sources of heat: internal and external. The ambient temperature for the application should be determined in degrees centigrade (°C). Most motors are designed to operate in an ambient of 40°C. If the ambient is greater then 40°C, then applications requiring frequent starting and/or frequent overloads may require special motors to compensate for the increase in total temperature.
- Motor cooling
The intent of motor cooling is to increase the rate of heat transfer from the motor to the surroundings, thus cooling the motor. There are several ways of doing this.
Motor enclosure
The type of enclosure determines the airflow patterns over the motor.
Frame surface area
Ribbed motors have a higher surface area and transfer more heat.
Airflow over the motor
Increasing the speed of the air flowing over the motor increases the cooling.
Ambient air density
A decrease in air density decreases the heat transfer. Most motors are rated for up to 3300 feet altitude. Motors with a service factor greater than 1.0 may be operated up to 9900 feet at a 1.0 service factor.
- Insulation class vs. temperature
Motor Insulation is classified by NEMA by its ability to withstand high temperature. The total temperature is the sum of ambient plus motor temperature rise. If the motor has a temperature probe embedded in the insulation, the maximum temperature can be 10°C higher.
Key takeaways:
- Understanding motor currents is critical to troubleshooting.
- Motor tempature has a significant impact of motor application.
How have you used these parameters in sizing and troubleshooting motors?
How do you select a replacement motor in your plant? Do you consider these parameters?
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