Several ways to control a single-phase asynchronous motor

Advantages and disadvantages of various methods of controlling asynchronous motors. Conclusions from practical experience.

Currently, squirrel-cage induction motors are widely used. This is due to the fact that such machines do not have a brush assembly, their rotor is made of aluminum and is technologically very simple, which means that the design itself is very reliable. Consider several ways to control a single-phase asynchronous motor.

Capacitor Single Phase Electric Motor

Most common type asynchronous single phase motor is an engine with two stator windings. The first and second windings are identical in the number of turns, but in series with one of the windings include a capacitor. The capacitor provides a phase shift between the windings to form a rotating magnetic field for the rotor.

Frequency control method

The main way to control such an engine, currently used, is the frequency method. This method is implemented using special devices called PWM inverters. These inverters, in turn, are single-phase and three-phase, which is determined by the number of pairs of power outputs for controlling the motor windings. To control a single-phase motor, both single-phase and three-phase inverters can be used. An example of a homemade design -do-it-yourself frequency converter.

Single-phase PWM inverter control

With this control, both motor windings are connected in parallel. Two outputs of the inverter are connected to the connection points of the windings. The inverter generates voltage with a variable frequency and with a linear dependence of voltage to frequency. You can adjust the frequency both down and up. The adjustment range usually does not exceed 1:10, because the capacitance of the capacitor in one of the windings is directly dependent on the frequency.

Advantages

The main advantages of this method are the ease of commissioning, which does not require redesign of the engine structure; reliable work, as the frequency converter is specially designed to control these types of motors; good characteristics (PID controllerpreset speeds, low inrush current, protective functions, etc.)

disadvantages

The disadvantages include: only unidirectional rotation; higher cost and deficit of single-phase converters compared to three-phase ones, due to their low output.

Three-phase PWM inverter control

In this case, the motor windings are connected in series. The outputs of the three-phase converter are connected to the midpoint and to the ends of the motor windings. In this case, the capacitor is excluded from the circuit (some alteration of the motor is required) Since the motor windings are shifted 90 degrees and the inverter gives a phase shift of 120 degrees, the field will not be perfectly circular and this will negatively affect the control parameters.

The field will be pulsating. Since the switching order of the terminals of the inverter can be changed programmatically, it is easy to achieve a change in the alternation of voltages on the windings, therefore, to change the direction of rotation of the motor rotor.

Advantages

The advantages include: market accessibility and a relatively low price; the possibility of reverse operation of a conventional non-reverse engine; wider adjustment range than that of a single-phase converter; the possibility of programmable functions like a single-phase inverter or even wider due to a larger number of switched outputs.

disadvantages

Disadvantages are: reduced and pulsating moment of a single-phase motor; increased heat; not all standard converters are ready for such work, because Some manufacturers expressly prohibit the use of their products in this mode.

Phase control with a triac controller (dimmer)

This method is the most "ancient", it is due to the absence until recently in the wide sale of frequency controllers and their relatively high price. With this control, the motor windings remain connected in parallel. One of the windings is connected in series with the phase-shifting capacitor. To the points of parallel connection of the windings is connected triac controller.

At the output of this regulator, a single-phase voltage is generated with a constant frequency (50 Hz) and an adjustable rms value. This is due to the regulation of the opening voltage of the triac, i.e. the open time of the triac changes over the period of the mains voltage.

The moment on the motor shaft, with this regulation, will decrease in proportion to the voltage, the critical slip will be unchanged.

Advantages

Main advantages: exceptional simplicity of the control device; the ability to assemble and repair such a device with any radio amateur; an order of magnitude or even several orders of magnitude lower price compared to frequency drives.

disadvantages

The main disadvantages are: regulation of revolutions only for lowering; range of regulation using a dimmer only 2: 1; speed stability is only satisfactory; permissible load decreases sharply with decreasing speed; engine overheating at low speeds, as lack of performance of the built-in engine fan; the need to overestimate engine power.

conclusions

Based on all of the above, it is imperative to recommend the use of frequency drives to control asynchronous motors. Such drives (PWM inverters) in addition to the undoubted convenience of management, allow to obtain high efficiency of plants and achieve growth power factor (cos phi) to 0.98, i.e. implement an energy conservation program.

See also: Single-phase connection of a three-phase motor and Collector Motor Speed ​​Controllers