Nissan 

Electric motor speed controller: principle of operation. Regulating the rotation speed of asynchronous electric motors by switching the number of pole pairs

The direction of rotation of the motor shaft sometimes needs to be changed. This requires a reverse connection diagram. Its type depends on what kind of motor you have: direct or alternating current, 220V or 380V. And the reverse of a three-phase motor connected to a single-phase network is arranged in a completely different way.

To reversibly connect a three-phase asynchronous electric motor, we will take as a basis the circuit diagram for connecting it without reversing:

This scheme allows the shaft to rotate only in one direction - forward. To make it turn into another, you need to swap places of any two phases. But in electrics it is customary to change only A and B, despite the fact that changing A to C and B to C would lead to the same result. Schematically it will look like this:

To connect you will additionally need:

  • Magnetic starter (or contactor) – KM2;
  • Three-button station, consisting of two normally closed and one normally open contacts (a Start2 button has been added).

Important! In electrical engineering, a normally closed contact is a state of a push-button contact that has only two unbalanced states. The first position (normal) is working (closed), and the second is passive (open). The concept of a normally open contact is formulated in the same way. In the first position the button is passive, and in the second it is active. It is clear that such a button will be called “STOP”, while the other two are “FORWARD” and “BACK”.

The reverse connection scheme differs little from the simple one. Its main difference is the electric locking. It is necessary to prevent the motor from starting in two directions at once, which would lead to breakdown. Structurally, the interlock is a block with magnetic starter terminals that are connected in the control circuit.

To start the engine:

  1. Turn on the machines AB1 and AB2;
  2. Press the Start1 (SB1) button to rotate the shaft clockwise or Start2 (SB2) to rotate the shaft in the opposite direction;
  3. The engine is running.

If you need to change direction, you must first press the “STOP” button. Then turn on another start button. An electrical lock prevents it from being activated unless the motor is switched off.

Variable network: electric motor 220 to network 220

Reversing a 220V electric motor is only possible if the winding terminals are located outside the housing. The figure below shows a single-phase switching circuit, when the starting and working windings are located inside and have no outputs to the outside. If this is your option, you will not be able to change the direction of rotation of the shaft.

In any other case, to reverse a single-phase capacitor IM, it is necessary to change the direction of the working winding. For this you will need:

  • Machine;
  • Push-button post;
  • Contactors.

The circuit of a single-phase unit is almost no different from that presented for a three-phase asynchronous motor. Previously, we switched phases: A and B. Now, when changing direction, instead of a phase wire, a neutral wire will be connected on one side of the working winding, and on the other, a phase wire will be connected instead of a zero wire. And vice versa.

You have to face the issue of adjusting the speed when working with power tools, driving sewing machines and other devices in everyday life and at work. It makes no sense to regulate the speed by simply lowering the supply voltage - the electric motor sharply reduces the speed, loses power and stops. The optimal option for adjusting the speed is to regulate the voltage with motor load current feedback

In most cases, power tools and other devices use universal commutator electric motors with sequential excitation. They work well on both AC and DC current. A feature of the operation of a commutator electric motor is that when switching the armature windings on the commutator lamellas during opening, pulses of self-induction counter-EMF occur. They are equal to the supply ones in amplitude, but opposite to them in phase. The back-EMF displacement angle is determined by the external characteristics of the electric motor, its load and other factors. The harmful effect of back-EMF is expressed in sparking on the collector, loss of engine power, and additional heating of the windings. Some of the back-EMF is suppressed by capacitors that shunt the brush assembly.

Let's consider the processes occurring in the regulation mode with the OS, using the example of a universal scheme (Figure 1). The resistive-capacitive circuit R2-R3-C2 provides the formation of a reference voltage that determines the rotation speed of the electric motor.

As the load increases, the rotation speed of the electric motor drops, and its torque decreases. The back-EMF arising on the electric motor and applied between the cathode of the thyristor VS1 and its control electrode decreases. As a result, the voltage at the control electrode of the thyristor increases in proportion to the decrease in back-EMF. The additional voltage on the control electrode of the thyristor causes it to turn on at a smaller phase angle (cut-off angle) and pass more current to the electric motor, thereby compensating for the decrease in rotation speed under load. There is, as it were, a balance of pulse voltage on the control electrode of the thyristor, composed of the supply voltage and the self-induction voltage of the motor. Switch SA1 allows, if necessary, to switch to full voltage power supply, without adjustment. Particular attention should be paid to selecting a thyristor based on the minimum turn-on current, which will ensure better stabilization of the motor rotation speed

The second scheme (Fig. 2) is designed for more powerful electric motors used in woodworking machines, grinders, and drills. In it, the principle of adjustment remains the same. The thyristor in this circuit should be installed on a radiator with an area of ​​at least 25 cm2.

For low-power electric motors and, if it is necessary to obtain very low rotation speeds, the circuit on an IC can be successfully applied (Fig. 3). It is designed for 12V DC power supply. In the case of a higher voltage, the microcircuit should be powered through a parametric stabilizer with a stabilization voltage no higher than 15V.

Speed ​​adjustment is carried out by changing the average voltage of the pulses supplied to the electric motor. Such pulses effectively regulate very low rotation speeds, as if continuously “pushing” the electric motor rotor. At high rotation speeds, the electric motor operates normally.

A very simple scheme (Fig. 4) will allow you to avoid emergency situations on the (toy) railway line and will open up new possibilities for train management. An incandescent lamp in the external circuit protects and signals a short circuit on the line, while limiting the output current.

When it is necessary to regulate the speed of electric motors with high torque on the shaft, for example in an electric winch, a full-wave bridge circuit (Fig. 5) can be useful, providing full power to the electric motor, which significantly distinguishes it from the previous ones, where only one half-wave of the supply voltage worked.

Diodes VD2 and VD6 and quenching resistor R2 are used to power the trigger circuit. The phase delay in opening the thyristors is ensured by the charging of capacitor C1 through resistors R3 and R4 from a voltage source, the level of which is determined by the zener diode VD8. When capacitor C1 is charged to the operating threshold of the unijunction transistor VT1, it opens and starts the thyristor at the anode of which there is a positive voltage. When the capacitor discharges, the unijunction transistor turns off. The value of resistor R5 depends on the type of electric motor and the desired depth of feedback. Its value is calculated using the formula

where Im is the effective value of the maximum load current for a given electric motor. The proposed schemes are highly repeatable, but require the selection of some elements depending on the characteristics of the motor used (it is almost impossible to find electric motors similar in all parameters, even within the same series).

Literature

1. Electronics Todays. Int N6

2. RCA Corp Manual

3.IOI Electronic Projects. 1977p93

5. G. E. Semiconductor Data Hand book 3. Ed

6.Graph P. Electronic circuits. -M World, 1989

7. Semenov I.P. Power regulator with feedback. - Radio Amateur, 1997, N12, C 21.

For a long time, unregulated electric drives based on AM have been used in industry, but recently there has been a need forspeed regulation of asynchronous motors.


The rotor speed is

In this case, the synchronous rotation speed depends on the voltage frequency and the number of pole pairs

Based on this, we can conclude that the speed of the blood pressure can be adjusted by changing the slip, frequency and number of pole pairs.

Let's look at the main adjustment methods.

Speed ​​control by changing active resistance in the rotor circuit

This speed control method is applicable inmotors with wound rotor. In this case, a rheostat is connected to the rotor winding circuit, which can gradually increase the resistance. As resistance increases, engine slip increases and speed decreases. This ensures that the speed is adjusted downwards from the natural characteristic.

The disadvantage of this method is that it is uneconomical, since as slip increases, losses in the rotor circuit increase, therefore, the engine efficiency decreases. Plus, the mechanical characteristics of the engine become flatter and softer, due to which a small change in the load torque on the shaft causes a large change in the rotation speed.


Speed ​​control in this way is not effective, but despite this it is used in motors with a wound rotor.

Regulating motor speed by changing the supply voltage

This control method can be implemented by connecting an autotransformer to the circuit, in front of the stator, after the supply wires. At the same time, if you reduce the voltage at the output of the autotransformer, the engine will operate at a reduced voltage. This will lead to a decrease in engine speed, at a constant load torque, as well as a decrease in the overload capacity of the engine. This is due to the fact that when the supply voltage decreases, the maximum motor torque decreases by a factor of square. In addition, this torque decreases faster than the current in the rotor circuit, which means that losses also increase, with subsequent heating of the motor.

The method of regulation by changing the voltage is only possible downward from the natural characteristic, since it is impossible to increase the voltage above the nominal one, because this can lead to large losses in the engine, overheating and failure.

In addition to the autotransformer, you can use a thyristor voltage regulator.

Speed ​​control by changing power frequency

With this control method, a frequency converter (FC) is connected to the motor. Most often this is a thyristor frequency converter. Speed ​​control is carried out by changing the voltage frequency f, since in this case it affects the synchronous speed of rotation of the motor.

As the voltage frequency decreases, the overload capacity of the motor will drop; to prevent this, it is necessary to increase the voltage U 1 . The value by which you need to increase depends on the drive. If regulation is carried out with a constant load torque on the shaft, then the voltage must be changed in proportion to the change in frequency (as the speed decreases). When increasing the speed, this should not be done, the voltage should remain at the rated value, otherwise it may cause damage to the engine.

If speed control is carried out with constant engine power (for example, in metal-cutting machines), then the change in voltage U 1 must be made proportional to the square root of the change in frequency f 1.

When regulating installations with a fan characteristic, it is necessary to change the supplied voltage U 1 in proportion to the square of the change in frequency f 1.

Regulation by changing the frequency is the most acceptable option for asynchronous motors, since it provides speed control over a wide range, without significant losses and reducing the overload capabilities of the motor.

Regulation of blood pressure speed by changing the number of pole pairs

This control method is possible only in multi-speed asynchronous motors with a squirrel-cage rotor, since the number of poles of this rotor is always equal to the number of stator poles.

In accordance with the formula discussed above, the speed of the motor can be adjusted by changing the number of pole pairs. Moreover, the speed change occurs in steps, since the number of poles takes only certain values ​​- 1,2,3,4,5.

Changing the number of poles is achieved by switching the coil groups of the stator winding. In this case, the coils are connected using various connection schemes, for example “star-star” or “star-double star”. The first connection diagram gives a change in the number of poles in a ratio of 2:1. This ensures constant engine power during switching. The second circuit changes the number of poles in the same ratio, but at the same time provides constant motor torque.

The use of this control method is justified by maintaining efficiency and power factor during switching. The downside is the more complex and enlarged design of the engine, as well as an increase in its cost.

It follows that regulation of the rotation speed of asynchronous electric motors can be carried out:

changing the frequency of the supply current;

changing the number of poles of the stator winding;

introducing additional resistances into the rotor winding circuit.

The first two methods are used to regulate the rotation speed of electric motors with a squirrel-cage rotor, and the last one is used for electric motors with a wound rotor (with slip rings).

Regulating the rotation speed by changing the frequency of the supply current is used very rarely, since this method is applicable only when the electric motor is powered from a separate generator. In this case, to regulate the speed, it is necessary to change the rotation speed of the supply generator in the same proportion as the speed of the controlled electric motor should change. If the electric motor is powered from a three-phase current network, then it is impossible to regulate its speed by changing the frequency. In practice, speed control by changing frequency is used only in... AC rowing electric installations, in which powerful rowing electric motors are powered by separate generators and therefore the frequency of the supply current can be adjusted arbitrarily.

Most often in practice, the second method is used, which makes it possible to quite simply carry out stepwise control of the rotation speed of asynchronous electric motors with a squirrel-cage rotor. If it is possible to change the number of pole pairs of the stator winding [ see formula (80)] then, therefore, it is possible to stepwise regulate the rotation speed of the electric motor, since the number of pole pairs can be equal to 1, 2, 3, etc. Electric motors that allow switching the number of pole pairs must have either several independent windings in the stator slots, or one winding with a special switching device. The domestic industry produces two-, three- and four-speed electric motors, used mainly in marine transport and on some cranes. When the numbers of poles differ significantly from each other, two-speed axis electric motors are manufactured with two independent windings. One, for example, can be performed on 2 R= 2, and the second by 2 R= 8 poles. Then, when the first winding is connected to the network, the magnetic field of the stator will rotate at a speed n 1 = 60·50 / 1 = 3000 about /min, and when connecting the second winding to the network - at a speed n 1 = 60·50 / 4 = 750 about /min. The rotation speed of the rotor will change accordingly. n 2 = n 1 (1-s).

Often, one winding is placed in the stator slots of a two-speed electric motor, but it is made in such a way that it can be turned on in a triangle if necessary (Fig. 49, A) and a double star (Fig. 49, b). When such a winding is connected with a triangle, the number of poles is 2 R = 2A, and when turned on by a double star 2 R = A(Where A- any integer), i.e., when moving from a triangle to a double star, the number of pairs of poles of the stator winding is halved, and the speed of the electric motor doubles.

Regulation by switching the number of pole pairs is used only for an electric motor with a squirrel-cage rotor, because electric motors with a wound rotor have one

temporarily, when switching the stator winding, it is necessary to switch the rotor winding, which complicates the design of the electric motor and the switching device. This method of speed control is highly economical, but it is not without its drawbacks. In particular, speed control does not occur smoothly, but in jumps; a rather complex switching device is required, especially when the number of speeds is more than two; when moving from one speed to another, the stator circuit breaks, and current and torque shocks are inevitable; the power factor at lower speeds is lower than at higher speeds due to increased magnetic flux dissipation.

Speed ​​control by introducing additional resistances into the rotor circuit is possible only with electric motors with a wound rotor. According to equation (97), when different active resistances are introduced into the rotor circuit, the rigidity of the characteristics changes (Fig. 50), i.e., under the same load, the speed of the electric motor will be different. Obviously, the higher the value of additional resistance, the softer the artificial characteristic and the lower the speed of the electric motor.

Let's say the electric motor is running at a steady speed. n 1 on natural characteristics A at the point 1 , developing some torque M 1 = M c . When introducing some resistance into the rotor circuit R 1 the electric motor will switch to operation according to the characteristic b, whose equation

Since at the moment the resistance is turned on, the speed of the electric motor will practically not change, the transition from the characteristic A for characterization b will happen horizontally 1 -2 , and the torque of the electric motor will decrease to M 2 , which is less than the moment of resistance of the mechanism M , therefore, the speed of the electric motor will decrease and the slip will increase. As slip increases, the moment, according to expression (92), increases until the torque of the electric motor again becomes equal to the moment of resistance of the mechanism, after which equilibrium of the moments will occur and the motor will rotate at a new steady speed n 3 (dot 3 ).

If necessary, additional resistance can be included R 2 . Then the speed of the electric motor will decrease to the value n 5 . When the resistances are turned off, the speed of the electric motor will increase, and the transition from one characteristic to another occurs in the reverse order, as shown in Fig. 50.

The latter method allows you to obtain a wide range of speeds, but is extremely uneconomical, since with an increase in the active resistance of the rotor circuit, energy losses in the electric motor increase, which means its efficiency decreases. The control rheostats themselves, especially for powerful electric motors, turn out to be bulky and emit a lot of heat.

It must also be borne in mind that most electric motors are now self-ventilated.

As a result, when the rotation speed decreases, cooling deteriorates and the electric motor cannot develop the rated torque.