Inductor voltage issues in different topologies

shaorc

[If you don't understand, just ask] The voltage balance equation in motor science is U=E+IR+jIX. Here we take a permanent magnet synchronous AC motor as an example. U is the voltage applied to the three-phase stator winding, that is, the voltage output by the drive inverter. E is the back electromotive force generated by the rotor's rotating magnetic field on the stator winding, which is 4.44Nkfψ. The last two terms are the voltage drop caused by the resistance and the leakage inductance. [1] Now if the motor is running at 2000 rpm and suddenly stops, you will find that the DC bus voltage of the drive will soar to a value higher than the rated value. We know that this is caused by the spike in the inductor voltage. But I want to analyze the cause of the spike from the above formula. Is it because when the speed suddenly changes to 0, U suddenly becomes 0, and the back electromotive force on the stator winding is "backflowing" into the drive, which is reflected in the increase in the DC bus value? 【2】In the following figure, the single-ended flyback switching power supply of Figure 1 has a clamping circuit in parallel with the primary winding Np to prevent the spike generated when the switch is turned off. In the typical DCDC buck and boost circuit of Figure 2, only a diode is added to provide a circuit for consuming the inductor voltage. Why is there such a difference in the peak voltage when the switch is turned off? 【3】If a three-phase 380VAC is drawn from the mains by a circuit breaker, it passes through a three-phase reactor and then connected to the rectifier circuit. If the circuit breaker is suddenly turned off, where will the voltage spike of the three-phase reactor appear? I don’t see any anti-spike measures Figure 1 Figure 2

maychang

[1] "Now if the motor is running at 2000 rpm and suddenly stops" Explain how to do an "emergency stop". Do you use a mechanical brake to hold the motor shaft? Do you use your driver to apply a voltage to the motor to make it rotate in the opposite direction?

maychang

[1] If the drive suddenly stops, the DC bus voltage will soar to a value higher than the rated value. Which inductor is responsible for the spike in inductor voltage? The leakage inductance of the motor winding? The filter inductor on the DC bus? The leakage inductance of the transformer supplying power to the rectifier filter circuit?

maychang

【2】In the single-ended flyback switching power supply in Figure 1, a clamping circuit is connected in parallel to the primary winding Np to prevent the spike generated when the switch tube is turned off. In the typical DCDC buck and boost circuits in Figure 2, only a diode is added to provide a consumption loop for the inductor voltage. The working principles of the three circuits are completely different. The diodes in the buck and boost circuits are not used to provide a "consumption loop". You must first understand how these switching circuits work under ideal conditions before you can talk about "the same spike voltage when turning off" (in fact, there may not be such a spike voltage).

maychang

【3】If a three-phase 380VAC is drawn from the mains by a circuit breaker, passed through a three-phase reactor, and then connected to the rectifier circuit, if the circuit breaker is suddenly turned off, where is the voltage spike of the three-phase reactor reflected? It is reflected in the sudden change of voltage at both ends of the reactor. This suddenly changing voltage is superimposed on the power supply voltage and may be applied between the circuit breaker contacts, generating an arc.

maychang

[3] If you use a circuit breaker to draw three-phase 380VAC from the mains, pass it through a three-phase reactor, and then connect it to the rectifier circuit, you made it yourself, right? I have never seen the mains connected to the rectifier circuit through a reactor.

maychang

In fact, the so-called "inductor voltage" should be called "induced electromotive force". The law for determining the direction of induced electromotive force is Lenz's law. The law for determining the magnitude of induced electromotive force is Faraday's law of electromagnetic induction. That's all.

bigbat

1. Problem: It was mentioned in a book on electrical machinery that motors and engines can be converted into each other in some cases, so there is no strict boundary between the two! Ideal inductors and capacitors can store energy. The motor is just using mechanical movement to convert the energy stored in the inductor. So when your motor stops, the energy stored in the inductor coil will be released in the form of electrical energy. It becomes a generator! So this is a very "dangerous" thing for large motors. Usually, it is necessary to use a "capacitive" load to consume it, otherwise it may affect the power grid! Some people say: using diodes and resistors is enough, practice tells you that it is too late! For example: equipment like train locomotives is an engineering masterpiece of our country. 2. Problem: For a single-ended flyback switching power supply, the frequency is increased to reduce the size and cost of the transformer. Therefore, a large amount of energy will be stored in the inductor when working. The larger the inductance, the more energy is stored. However, the stronger the inertia of the inductor that needs to be solved, the stronger the inertia, the lower the frequency, so it is very contradictory. Each power supply is to find a balance point, so "the circuit cannot consume all the energy, otherwise the power supply is meaningless because of low efficiency!" 3. The three-phase reactor is mainly a protective measure to protect the power grid from the influence of large loads, which can also be understood as "balanced phase voltage". Therefore, the function of each phase is to prevent sudden changes in voltage, so it does not store a lot of energy when working, but only prevents the voltage of the load from flowing back!

maychang

bigbat posted on 2019-1-16 12:28 1. The question is, it is said in a book on electrical engineering that electric motors and engines can be converted into each other in some cases, so there is no strict distinction between the two...

"It is said in a book on electrical engineering that electric motors and engines can be converted into each other in some cases" "Electric motors and engines" should be "electric motors and generators". Electric motors can be used as generators, and generators can be used as electric motors.

shaorc

maychang published on 2019-1-16 11:07 【3】If you use a circuit breaker to draw three-phase 380VAC from the mains, pass through a three-phase reactor, and then connect it to the rectifier circuit you made yourself through a three-phase reactor...

Here I provide a picture to illustrate that the three wires at one end of the reactor are connected to the 380VAC mains on the wall, and the other end is connected to L1, 2, and 3 in another picture, which is the rectifier. This is what I mean, the expression is inappropriate

bigbat

maychang posted on 2019-1-16 12:53 "It is said in a book on electrical engineering that electric motors and engines can be converted into each other in some cases." "Electric motors and engines" should be " ...

The boss's reminder is right! The boss is very rigorous in answering questions every time. I admire him very much!

maychang

bigbat posted on 2019-1-16 13:51 The boss is right! The boss is very rigorous in answering questions every time, I admire him very much!

If you have time, please help me see what the reactor mentioned on the 10th floor is. The first photo on the 10th floor is not a simple three coils, one end of each coil is led out from the top, and the other end of the coil is led out from the bottom, right?

shaorc

maychang posted on 2019-1-16 13:56 If you have time, please help me see what the reactor on the 10th floor is about. The first photo on the 10th floor is not a simple three coils, each coil has one end from the top...

Yes, the reactor is divided into two rows, each coil is introduced from the top and led out from the bottom

shaorc

maychang posted on 2019-1-16 10:55 【1】"Now if the motor is running at 2000 rpm and suddenly stops suddenly" Explain clearly how to "stop suddenly". It is to use mechanical brakes to hold the motor shaft...

Emergency stop means setting parameters in the driver to directly change the command speed to 0, so that the motor reaches 0 speed in a short time (1-2 seconds) without using mechanical means to stop.

shaorc

maychang published on 2019-1-16 10:57 【1】When the motor is suddenly stopped, the DC bus voltage of the drive will soar to a value higher than the rated value. I wonder if this is caused by the inductor voltage spike...

The inductor voltage here refers to the back electromotive force generated on the stator winding of the motor.

zhuyebb

[1] If the drive is suddenly stopped, the DC bus voltage will soar to a value higher than the rated value. This is caused by the inductor voltage spike.

shaorc

maychang published on 2019-1-16 11:01 【2】In the single-ended flyback switching power supply of Figure 1, a clamping circuit is connected in parallel with the primary winding Np to prevent the spike generated at the moment the switch tube is turned off, while the typical...

In the single-ended flyback switching power supply, the primary winding and the secondary winding are not turned on at the same time. When the primary winding stores energy, the secondary diode is reverse biased and does not conduct. When the primary MOS is turned off, the secondary is turned on, but at this time the primary side generates a spike voltage in the boost and buck circuits. The former is in the MOS shutdown stage, and the inductor transfers the stored energy to the output, and the latter is completed in the turn-on stage! This is the principle I understand.

maychang

shaorc posted on 2019-1-16 16:30 In a single-ended flyback switching power supply, the primary winding and the secondary winding are not turned on at the same time. When the primary winding stores energy, the secondary diode is reverse biased and does not conduct, and the primary MOS is turned off...

The secondary diode in a single-ended flyback circuit has the same function as the diode in a Buck or Boost circuit, both of which supply power to the load when the power tube is turned off. However, the diode on the primary side of the transformer in a single-ended flyback circuit does not have this function. The primary diode has nothing to do with the output. In a single-ended flyback circuit, there is always leakage inductance between the primary and secondary sides of the transformer, which means that part of the magnetic flux generated by the primary current fails to couple to the secondary side. Then when the secondary diode is turned on, the magnetic energy stored in the primary leakage inductance cannot be released through the secondary diode. The magnetic energy stored in this part of the leakage inductance will generate a very high electromotive force, which may break down the power tube and cause damage to the power tube. The magnetic energy stored in the primary leakage inductance to the secondary side can only be released by the diode and the voltage regulator connected to the primary side. This part of the circuit can also be composed of diodes, capacitors, and resistors, with the same purpose, which is to release the energy stored in the leakage inductance. Buck circuit and Boost circuit are single inductors, and there is no leakage inductance mentioned above.

shaorc

maychang published on 2019-1-16 16:45 The secondary diode in the single-ended flyback circuit has the same function as the diode in the Buck or Boost circuit, both of which supply power to the load when the power tube is turned off. But...

I understand, the leakage inductance forms a spike voltage, and a single inductor has no leakage inductance

maychang

shaorc posted on 2019-1-16 15:41 Emergency stop means setting parameters in the driver to directly change the command speed to 0, so that the motor can reach 0 speed in a short time (1-2 seconds), without using mechanical methods...

Since we don’t know the structure of the driver and the structure of the permanent magnet synchronous motor winding, it is difficult to specifically analyze how the back electromotive force on the motor stator winding is "backflowed" to the DC bus. But it is easy to explain from the perspective of energy conversion. The motor originally has a certain speed, so it has a certain rotational mechanical energy (the moment of inertia cannot be zero). When the circuit method is used to stop the motor, the mechanical energy of the motor must be converted into electrical energy, and the electrical energy can only flow into the DC bus, which will inevitably increase the DC bus voltage. If the motor is stopped mechanically, the mechanical energy of the motor will be converted into heat energy, which will not increase the DC bus voltage. It is sometimes more convenient and more universal to look at the problem from the perspective of work and energy.