Motor circuit Troubleshooting. Reasons for Motor overloading

Motor circuit Troubleshooting. Reasons for Motor overloading

In the process industry motors are used all over. Thousands of motors in the single process industry are not a big number. Such electric motors are used to drive different types of mechanical loads like pumps, fans, belt conveyors, mixers, grinders, or cutters. Mostly Induction motors are used in industry. They can be single-phase or three-phase. Three-phase motors are very common in the industry due to their efficient operation and robust design. In normal conditions motors are running smoothly without giving any trouble. But sometimes when load running conditions are not even, the motors trip on overload. This motor tripping on overload is necessary to prevent the motor from blowing and burning. If the motor is not tripped in case of an overload, the motor takes more current which heats up the motor, and eventually motor burns due to much current consumption. To prevent the motor from such catastrophic burning an overload protection relay is used in the motor circuit. The function of the motor overload relay is to monitor the motor current all the time and trip the motor in case of any overload detected.

Three-phase motor starter circuit diagram

Now in the case of an overloaded motor, the service technician or engineer must check the motor as well as the motor circuit. There should be no obvious damage to the motor or its circuit. All three phases should available in the case of a three-phase system. In the case of a single-phase motor, it should be confirmed that the phase and neutral both are present. Also, the motor should be checked that there should no restriction to motor rotation. In the case of a small motor, the motor shaft could be rotated with hand along with load connected. But in the case of a very big motor, the motor shaft can't be rotated with a hand. It's a must to confirm motor and load bearings. Sometimes the motor or load bearings are damages due to which the motor is overloaded. Also, the mechanical process load must be evaluated so that there should be no excessive load on the motor. If the load is too much, it is recommended to decrease the load before giving a start to the motor after resetting. At the restart, the motor load current must be checked and measured with the help of a clamp meter. In the case of a single-phase motor only one phase current would be measured. Whereas in the case of a three-phase motor all three-phased would be measured. The motor-rated current could be obtained from the motor nameplate.

motor current measurement

All three phases current in the case of a three-phase motor should be closely the same. The large difference is the current shows motor internal winding short circuit which could be phase to phase winding short circuit or phase to ground short circuit. In case of any abnormality in the current reading, the service technician must evaluate the case of the problem and must solve the problem before putting back the motor into service. Motor overload settings are made on 100% to 125% of the motor rated current depends upon the motor and the process conditions.

Motor overload tripping reason is not only the motor's high current consumption. Thermal overload responds to heat. The tripping principle of the thermal overload is based on heating. So any heat source can activate the thermal overload and can cause a false motor overload trip. In such a case the service technician should evaluate the reason for the heat source and should solve the overheating reason of the overload protection relay. The main cause of the heating is the surrounding of the motor or ambient. The main reason for overheating can be a loose connection. Any loose terminals on the motor starter cause overload terminal overheating which further trips the overload relay. Tightening the motor terminals and starter terminals would solve the issue. Also if the starter is located in the location where excessive heat is acting on the thermal overload relay, the heat caused by the motor current and the external heating would be enough to trip the overload. In such a situation an exhaust fan blowing air around the starter would help to reduce the ambient heating on the thermal overload. Mostly manual starters which are installed in a closed box affect more due to heating problems. In such a case special ventilation arrangement is a must to keep things cool.