How can I control the speed of an induction motor: an overview of the ways

Changing the speed of a squirrel-cage rotor

There are several ways:

1. Rotation control by changing the electromagnetic field of the stator: frequency regulation and changing the number of pairs of poles.

2. Change in the slip of the electric motor due to a decrease or increase in voltage (can be used for BP with a phase rotor).

Frequency regulation

In this case, the adjustment is made using a device connected to the engine for frequency conversion. For this, powerful thyristor converters are used. The process of frequency regulation can be considered on the example of the transformer EMF formula:

U₁= 4.44w₁k₁fΦ

This expression means that in order to maintain a constant magnetic flux, which means preserving the overload capacity of the electric motor, it is necessary to adjust the level of the supply voltage simultaneously with the frequency conversion. If the expression calculated by the formula is saved:

U₁/ f₁= U ’₁/ f ’₁

this means that the critical moment is not changed. And the mechanical characteristics correspond to the figure below, if you do not understand what these characteristics mean, then in this case the adjustment occurs without loss of power and torque.

The advantages of this method are:

• smooth regulation;
• change of rotor speed up and down;
• tough mechanical characteristics;
• profitability.

The downside is the need for frequency converter, i.e. increase in the cost of the mechanism.By the way, on the modern market there are models with single-phase and three-phase input, the cost of which with a power of 2-3 kW lies in the range of 100-150 dollars, which is not too expensive for the full adjustment of the drive of machines in a private workshop.

Switching the number of pole pairs

This method is used for multi-speed motors with a complex winding that allows you to change the number of pairs of its poles. The most widely used are two-speed, three-speed and four-speed blood pressure. The principle of adjustment is easiest to consider based on a two-speed blood pressure. In such a machine, the winding of each phase consists of two semi-windings. The rotation speed changes when they are connected in series or in parallel.

In a four-speed electric motor, the winding is made in the form of two independent parts. When changing the number of pairs of poles of the first winding, the speed of the electric motor is changed from 3000 to 1500 rpm. Using the second winding, the rotation is adjusted to 1000 and 500 rpm.

When the number of pole pairs changes, the critical moment also changes. To keep it unchanged, it is necessary to simultaneously regulate the supply voltage along with changing the number of pole pairs, for example, by switching star-triangle schemes and their variations.

Advantages of this method:

• rigid mechanical characteristics of the engine;
• high efficiency.

Disadvantages:

• step adjustment;
• heavy weight and overall dimensions;
• high cost of an electric motor.

Methods of controlling the speed of the HELL with a phase rotor

Changing the speed of rotation of the blood pressure with a phase rotor is made by changing the slip. Consider the main options and methods.

Supply voltage change

This method is also used for blood pressure with a short-circuited rotor. The induction motor is connected via an autotransformer or LATR. If reduce supply voltage, engine speed will decrease.

But this mode reduces the overload capacity of the engine. This method is used to regulate within the voltage range not higher than the rated voltage, since an increase in the rated voltage will lead to the motor failure.

Active resistance in a rotor chain

When using this method, a rheostat or a set of constant high-power resistors is connected to the rotor circuit. This device is designed to gradually increase resistance.

The slip increases in proportion to the increase in resistance, and the rotation speed of the motor shaft decreases.

Advantages:

• wide range of regulation in the direction of lowering the speed of rotation.

Disadvantages:

• decrease in efficiency;
• increase in losses;
• deterioration of mechanical characteristics.

Asynchronous valve stage and dual power machines

Changing the speed of asynchronous electric motors in these cases is performed by changing the slip. In this case, the rotation speed of the electromagnetic field is unchanged. Voltage is applied directly to the stator windings. The adjustment is due to the use of sliding power, which is transformed into the rotor circuit, and forms an additional Emf. Such methods are used only in special machines and large industrial devices.

Soft start of asynchronous electric motors

BP in addition to the obvious advantages, have significant disadvantages. This is a jerk at the start and large inrush currents, 7 times the nominal. For a soft start of the electric motor, the following methods are used:

• switching windings according to the star-delta scheme;
• turning on the electric motor through an autotransformer;
• the use of specialized devices for soft start.

Most frequency controllers have a soft start function. This not only reduces inrush currents, but also reduces the load on the actuators.Therefore, frequency control and soft start are pretty much connected.

How to make a device for changing the speed of rotation of the electric motor with your own hands

To adjust low-power single-phase blood pressure you can use dimmers. However, this method is unreliable and has serious drawbacks: reduced efficiency, serious overheating of the device and the risk of engine damage.

For reliable and high-quality speed control of electric motors at 220V, frequency regulation is best suited.

The diagram below allows you to assemble a frequency device for adjusting electric motors with power up to 500 watts. The change in rotation speed is made in the range from 1000 to 4000 revolutions per minute.

The device consists of a variable frequency master oscillator, consisting of a multivibrator assembled on a K561LA7 chip, a counter on a K561IE8 chip, a half-bridge controller. The output transformer T1 decouples the upper and lower transistors of the half-bridge.

The damping circuit C4, R7 dampens voltage surges that are dangerous for power transistors VT3, VT4. The rectifier, the voltage doubler of the supply network, includes diode bridge VD9, with a filter capacitor on which there is a doubling of the supply voltage of the half-bridge.

Primary winding voltage: 2x12V, secondary winding 12V. The primary winding of the key management transformer consists of 120 turns of copper wire with a cross section of 0.7 mm, with a tap from the middle. Secondary - two windings, each of 60 turns of the occasion with a cross section of 0.7 mm.

Secondary windings must be insulated as reliably as possible from each other, since the potential difference between them reaches 640 V. The output windings are connected to the gate valves in antiphase.

So we examined how to adjust the speed of induction motors. If you have questions, ask them in the comments under the article!

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• How to connect a three-phase motor for 220 and 380 volts
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