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In the daily equipment management of cranes, the faults and maintenance of electric motors (specifically referring to winding motors) have always been a tricky problem for users. In response to this situation, I would like to discuss the causes of the problems and effective solutions.
In the ordinary control scheme of electric motors, the purpose of connecting a resistance in series with the rotor is to adjust the speed and starting torque to optimize the starting characteristics of the motor. After starting, the connected resistance should be gradually removed to allow the motor to operate normally.
Commonly used resistors are gradually removed using time relay control, but this method can cause certain damage to the main equipment on the crane!
Taking the lifting mechanism as an example, it is well known that the lifting mechanism of a crane is a potential load. When lifting a load, the load acts as resistance, and when the load descends, the load acts as an accelerating force. Therefore, the acceleration of the lifting mechanism during lifting and descending is completely different; at the same time, different loads will also affect the acceleration of the lifting mechanism.
Due to the different accelerations when lifting or descending, and under heavy or light loads, the time for the motor to accelerate to full speed will also differ. Therefore, the timing for cutting off the motor rotor resistor should also be different. However, time relays cannot judge acceleration, and people usually adjust the time setting of the time relay based on experience, which can lead to different cutting points for the motor rotor resistor during lifting and descending under the same load.
ØWhen the time adjustment value is large, it is beneficial for the lifting mechanism to ensure sufficient time to accelerate fully under a certain motor characteristic before cutting off the rotor resistor, switching the motor characteristics, and ensuring that the motor's impact current does not exceed the allowable maximum value when the resistor is cut off. However, during the descent of the lifting mechanism, due to the large acceleration, within the same time setting, the motor has already accelerated to the generator braking state, operating at a speed significantly exceeding the synchronous speed, which also exceeds the allowable speed of related reducers and other transmission equipment, causing damage to the equipment and the motor.
ØWhen the time adjustment value is small, it is beneficial for the lifting mechanism to quickly cut off the resistor during acceleration to prevent overspeed. However, during lifting, due to insufficient acceleration time, when the motor rotor resistor is cut off, the motor does not get enough acceleration, and the impact current when the resistor is cut off will far exceed the motor's allowable maximum current value, generating large current and torque that can damage the motor and mechanical transmission equipment.
Similarly, a fixed time setting value will also produce different resistor cutting points under different loads, affecting the motor and mechanical transmission equipment.
In the figure, using a time relay as the condition for cutting the rotor resistance, the switching torque and current impact that the motor bears during the switching of the rotor resistance are extremely irregular (see ab, cd, ef, ghsegments), and in most cases, they are far greater than the motor's rated value. Therefore, thisis one of the main reasons for the motor's easy burning and the reduced lifespan of mechanical transmission equipment..
To solve the hazards caused by the time relay switching of the motor rotor resistance, the most practical method is to change from the time switching principle to the speed switching principle, that is, based on the selection of the resistor in the system design, set the speed switching point of the motor rotor resistor. As shown in the figure below, regardless of whether the mechanism is lifting, descending, under heavy load, or light load, we design to cut off the resistance at the set motor speed point, thus avoiding the aforementioned hazards. At the same time, if for some reason the motor load is blocked, and the speed does not reach the switching speed setting point, the motor rotor resistor will not be gradually switched, avoiding motor burning.
The speed switching principle requires monitoring the operating speed of the motor, and the simplest method is to use a frequency relay. Since the rotor frequency of the AC asynchronous winding motor corresponds one-to-one with the motor's operating speed, using a frequency relay to detect the motor rotor frequency in real-time allows for the detection of the motor's operating speed, thus achieving the speed principle switching of the motor rotor resistance.
Meiheng Company produces WQP0101, WQP0104two types of frequency relays, which can meet the needs of this scheme.
WQP0101, WQP0104Digital frequency relays are independently developed by Meiheng Company, using frequency as the control condition. They use stable performance imported CMOSdevices, with high detection accuracy and stability. Frequency settings and digital display are arranged on the panel for easy adjustment and observation.
In addition to being used for cutting off the rotor resistance of AC asynchronous winding motors, they can also be used in other industrial occasions for power frequency control.
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