Electric Motor Basics

1. How is the temperature resistance of insulating materials divided?

A: Our country is divided into six levels, namely A, E, B, F, H, C.

(1) The maximum allowable working temperature of Class A insulating material is 105℃

(2) The maximum allowable working temperature of Class E insulating material is 120℃

(3) The maximum allowable working temperature of Class B insulating material is 130℃

(4) The maximum allowable working temperature of Class F insulating material is 155℃

(5) The maximum allowable working temperature of Class H insulating material is 180℃

(6) The maximum allowable working temperature of Class C insulating material is above 180℃.

2. Briefly describe the structure and working principle of induction motor.

Answer: The working principle of the induction motor is as follows. When the three-phase stator windings pass through the three-phase symmetrical alternating current, a rotating magnetic field is generated. This rotating magnetic field rotates in the inner bore of the stator, and its magnetic lines of force cut the wires on the rotor. Induced current. Due to the interaction force between the stator magnetic field and the rotor current, the electromagnetic torque is generated, so the stator rotating magnetic field drags the rotor with the current-carrying wire to rotate.

3. Why is the current large when the induction motor starts? And the current will become smaller after starting?

Answer: When the induction motor is in a stopped state, from an electromagnetic point of view, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed-circuit rotor winding is equivalent to the short-circuited secondary coil of the transformer; There is no electrical connection between the stator winding and the rotor winding, only the magnetic connection, and the magnetic flux forms a closed circuit through the stator, the air gap, and the rotor core. At the moment of closing, the rotor has not yet rotated due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed - synchronous speed, so that the rotor winding induces the highest potential possible. The current produces magnetic energy that cancels the stator magnetic field, just as the secondary magnetic flux of the transformer needs to cancel the effect of the primary magnetic flux.

In order to maintain the original magnetic flux that is compatible with the power supply voltage at that time, the stator automatically increases the current. Because the current of the rotor is very large at this time, the stator current also increases greatly, even as high as 4~7 times of the rated current, which is the reason for the large starting current.

Why is it small after starting: As the speed of the motor increases, the speed of the stator magnetic field cutting the rotor conductor decreases, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases, so the stator current is used to offset the rotor current. The part of the current affected by the magnetic flux is also reduced, so the stator current is from large to small until it is normal.

4. Is the starting current dangerous? Why do some induction motors need starting equipment?

Answer: Generally speaking, since the starting process is not long, a large current flows in a short period of time, and the heat generation is not too severe, the motor can withstand it, but if the normal starting conditions are destroyed, for example, a motor with a light load starting is required to start with a heavy load. If the speed cannot be raised normally, or when the voltage is low, the motor cannot reach the rated speed for a long time, and the motor is started several times in a row, the motor windings may be overheated and burned.

The large starting current of the motor has an impact on other electrical equipment on the same power bus. This is because a large starting current is supplied to the motor, and the voltage drop of the power supply line is very large, which greatly reduces the voltage of the busbar connected to the motor, which affects the normal operation of other electrical equipment. release etc.

As far as the induction motor itself is concerned, it is allowed to start directly, that is, it can be started with rated voltage.

Because the capacity of the motor does not match the capacity of the power supply it is connected to, the induction motor may not be able to start because the voltage at the line terminal is too low and the starting torque is not enough when starting. In order to solve this problem and reduce the impact on other electrical equipment on the same bus, some motors with larger capacity must use starting equipment to limit the starting current and its impact.

Whether it is necessary to start the equipment or not, the key lies in the comparison of the power supply capacity and the motor capacity. The larger the power plant or grid capacity, the larger the motor capacity that allows direct starting. Therefore, in the newly built medium and large power plants, almost all induction motors except the winding type are directly started. Only in the old and small power plants, there are motors started by various starting equipment.

For squirrel cage motors, the purpose of using starting equipment is nothing more than to reduce the starting voltage, so as to achieve the result of reducing the starting current. Depending on the step-down method, the start-up method (1) Y/△ conversion start-up method. During normal operation, the stator winding is connected to a delta-shaped motor, and it is connected to a Y-shaped connection when it is started, and it is changed to a delta-shaped connection after it is started. (2) Start method with autotransformer. (3) Use the reactor to start the method.

5. The end of the three-phase winding of the motor is reversed, what happens when it starts? How to find it?

Answer: If the three-phase winding and one-phase winding of the motor are connected in reverse, when starting:

(1) Difficulty starting.

(2) One-phase current is large.

(3) Vibration may cause loud sound.

The general search method is:

(1) Carefully check the head and tail marks of the three-phase windings.

(2) Check the polarity sequence of the three-phase windings. If it is not N and S, it means that one phase winding is reversely connected.

6. Why can't the stator winding of the induction motor start when one phase is broken?

Answer: For the three-phase star-connected stator winding, when one phase is disconnected, the motor is at the line voltage where only the two-phase line is terminated with the power supply, forming a series circuit and becoming a single-phase operation.

During single-phase operation, the following phenomena will occur: the electric motor that was originally stopped will not start, and it makes a "mum" sound. If you dial the rotor shaft with your hand, it may be able to rotate slowly. The rotating speed of the original rotating motor slows down, the current increases, the motor heats up, and even burns out.

7. What are the abnormal phenomena of broken rotor bars during the operation of squirrel cage induction motor?

A: The rotor of the squirrel-cage induction motor is broken during operation, the motor speed will slow down, the stator current will fluctuate periodically, and the body will vibrate, which may emit a rhythmic "humming" sound.

8. What are the abnormal phenomena of single-phase grounding during the operation of induction motor stator windings?

Answer: For a 380-volt low-voltage motor, it is connected to the neutral point grounding system. When a single-phase grounding occurs, the current of the grounding phase increases significantly, the motor vibrates and makes abnormal noises, and the motor heats up, which may cause the The fuses of the phases are blown, and the windings may also be damaged by overheating.

9. What effect does the frequency change have on the operation of the induction motor?

A: When the deviation of the frequency exceeds ±1% of the rated current, the operation of the motor will deteriorate, affecting the normal operation of the motor. When the operating voltage of the motor is constant, the magnetic flux is inversely proportional to the frequency, so the change of the frequency will affect the magnetic flux of the motor.

The starting torque of the motor is inversely proportional to the cube of the frequency, the maximum torque is inversely proportional to the square of the frequency, and the maximum torque is inversely proportional to the square of the frequency, so the change of the frequency also affects the motor torque.

The change of frequency will also affect the speed and output of the motor.

When the frequency increases, the stator current usually increases. When the voltage decreases, the frequency decreases and the reactive power absorbed by the motor decreases. Due to the change of frequency, it will also affect the normal operation of the motor and make it heat up.

10. Under what circumstances will the induction motor overvoltage?

A: In the running induction motor, the operating overvoltage of the inductive load is prone to occur at the moment when the switch is turned off. In some cases, the operating overvoltage can also be generated when the switch is closed. For a wound motor with a voltage of more than 3,000 volts, if the rotor is open, the magnetic flux will suddenly change at the moment of closing, and overvoltage will also be generated.