Dig deep into the motor stall

13/07/2019

The lack of phase operation is the number one killer of the motor. However, the damage caused by the motor stall can not be ignored. For the assembly line, once the motor is blocked for a long time, it will burn out the motor and even damage the equipment, causing irreparable damage. Therefore, the motor's stall protection is necessary, and the protection setting starts from the motor's stall test.

Motor stalling is a state in which the motor still outputs torque at zero speed, which is generally caused by foreign matter, mechanical damage or man-made. When the motor load is too large, the foreign matter is stuck, the mechanical failure of the dragging device, the bearing damage, etc., the motor will not start or stop rotating.
 
When the motor rotates normally, the rotating magnetic field generated by the stator drives the rotor to rotate in the direction of rotation of the magnetic field. During the rotation of the rotor, the magnetic induction line is cut to generate an induced current. The magnetic field generated by the induced current rotates with the rotor and reverses in the stator. Inductive current, thereby suppressing the current in the stator windings.

If the motor is blocked, the reverse induced current cannot be generated in the stator, that is, the voltage applied to the winding coil is greatly increased as the input voltage, and therefore, the current in the winding is greatly increased. At this time, the power factor of the motor is extremely small, and the blocking current increases rapidly. According to the capacity of the motor and the processing technology, the blocking current can reach 5 to 12 times of the rated current. Therefore, the motor will burn out when the stalling time is slightly longer. .

In order to prevent serious damage caused by blockage, the motor should generally be equipped with an overcurrent protection device. When the motor is closed for a long time, the motor cannot be rotated, and the current cannot be lowered. The overcurrent protection device should be able to jump off the power switch of the motor in time to protect the motor. To prevent overheating. The setting of the current needs to be completed by means of testing. This process is indispensable for the development of new motors, and the blocking test is especially important for the factory test of the motor.

The stall test is to measure the stall current Ik and the stall torque Tk and the stall loss Pk at the rated voltage. For the three-phase asynchronous motor, it is also possible to simultaneously measure the stall current, the stall torque, and the relationship between the locked input power and the input voltage, that is, the stall characteristic curve, through the magnitude of the stall current and the balance of the three phases. The analysis can reflect the rationality and some quality problems of the motor, stator winding and stator, and the magnetic circuit composed of the rotor. It can provide relevant measured data for improved design and process, and help the faulty motor find the cause and determine the repair content.
 

The MPT1000 motor test platform has a complete solution for the stall current test. The stall test is divided into a conventional stall test and a national standard stall test. The routine stall test is a market-tested, easy-to-use and willing to accept solution with a simple process and low cost. When the equipment is ready, give the motor under test the rated voltage to operate in the no-load state. After the operation is stable, load the motor and gradually increase the load until the measured motor speed decreases and stalls; or use the MPT motor test system. The custom-made blocker directly blocks the motor and applies the rated voltage to the motor.

In the short time of motor stall, the high-speed, high-precision data acquisition system is used to collect the torque, voltage, current and other data of the motor under test and calculate the measurement parameters and calculation parameters of the motor, and pass the power analyzer and the industrial computer. Displayed on the screen and stored, the stored data can be easily exported and printed, which greatly facilitates the analysis of motor stall data. Thanks to the high-speed and high-accuracy data acquisition module, the entire stall test can take a short time to perfectly protect the motor to prevent the motor from burning out.

The national standard blocking test refers to the test carried out according to the test method specified in Chapter 9 "Determination of the locked current and the locked rotor torque" of the national standard "GB/T 1032-2012 three-phase asynchronous motor test method", and this test The method is only for three-phase motors. The main difference between the national standard blocking test and the conventional blocking test is the applicable type and whether additional voltage regulating equipment needs to be added. If the three-phase AC power supply with programmable control is used, the entire national standard blocking test can also be automated.

Whether it is the conventional blocking test or the national standard blocking test, it can provide real and reliable data for the motor stall research, provide the basis for the motor design, and also test the motor's stalling performance. With these data, the motor's stall protection will be based, and the fault diagnosis of the motor becomes more convenient. Therefore, the motor stall test is necessary for motor production and design.