In industry, over 60% of electricity is consumed by asynchronous electric drives - in pumping, compressor, ventilation and other installations. This is the simplest, and therefore cheapest and most reliable type of engine.
The technological process of various industrial productions requires flexible changes in the rotation speed of any actuators. Thanks to the rapid development of electronic and computer technology, as well as the desire to reduce electricity losses, devices have appeared for economical control of electric motors of various types. In this article we’ll talk about how to ensure the most efficient control of an electric drive. Working in a company (company group ), I see that our customers are paying more and more attention to energy efficiency
Most of the electrical energy consumed by manufacturing and process plants is used to perform some kind of mechanical work. To drive the working parts of various production and technological mechanisms, asynchronous electric motors with a squirrel-cage rotor are predominantly used (in the future we will talk about this type of electric motor). The electric motor itself, its control system and the mechanical device that transmits motion from the motor shaft to the production mechanism form an electric drive system.
The presence of minimal electricity losses in the windings due to regulation of the motor rotation speed, the possibility of a smooth start due to a uniform increase in frequency and voltage - these are the main postulates of effective control of electric motors.
After all, previously there were and still exist such methods of engine control as:
- rheostatic frequency control by introducing additional active resistances in the motor winding circuits, sequentially short-circuited by contactors;
- change in voltage at the stator terminals, while the frequency of such voltage is constant and equal to the frequency of the industrial AC network;
- step regulation by changing the number of pole pairs of the stator winding.
But these and other methods of frequency regulation carry with them the main drawback - significant losses of electrical energy, and step regulation, by definition, is not a flexible enough method.
Are losses inevitable?
Let us dwell in more detail on the electrical losses that occur in an asynchronous electric motor.
The operation of an electric drive is characterized by a number of electrical and mechanical quantities.
Electrical quantities include:
- mains voltage,
- motor current,
- magnetic flux,
- electromotive force (EMF).
The main mechanical quantities are:
- rotation speed n (rpm),
- rotating torque M (N•m) of the engine,
- mechanical power of the electric motor P (W), determined by the product of torque and rotational speed: P=(M•n)/(9,55).
To denote the speed of rotational motion, along with the rotation frequency n, another quantity known from physics is used - the angular velocity ω, which is expressed in radians per second (rad/s). There is the following relationship between the angular velocity ω and the rotation frequency n:
taking into account which the formula takes the form:
The dependence of the engine torque M on the rotational speed of its rotor n is called the mechanical characteristic of the electric motor. Note that when an asynchronous machine operates, so-called electromagnetic power is transmitted from the stator to the rotor through the air gap using an electromagnetic field:
Part of this power is transmitted to the rotor shaft in the form of mechanical power according to expression (2), and the rest is released in the form of losses in the active resistances of all three phases of the rotor circuit.
These losses, called electrical, are equal to:
Thus, electrical losses are determined by the square of the current passing through the windings.
They are largely determined by the load of the asynchronous motor. All other types of losses, except electrical ones, change less significantly with load.
Therefore, let us consider how the electrical losses of an asynchronous motor change when the rotation speed is controlled.
Electrical losses directly in the rotor winding of an electric motor are released in the form of heat inside the machine and therefore determine its heating. Obviously, the greater the electrical losses in the rotor circuit, the lower the efficiency of the engine, the less economical its operation.
Considering that stator losses are approximately proportional to rotor losses, the desire to reduce electrical losses in the rotor is even more understandable. That method of regulating the engine speed is economical, in which the electrical losses in the rotor are relatively small.
From the analysis of the expressions it follows that the most economical way to control motors is at a rotor speed close to synchronous.
Variable Frequency Drives
Installations such as variable-frequency drives (VFDs), also called frequency converters (FCs) ). These settings allow you to change the frequency and amplitude of the three-phase voltage supplied to the electric motor, due to which a flexible change in the operating modes of the control mechanisms is achieved.
High Voltage Variable Frequency Drive
VFD design
Here is a brief description of existing frequency converters.
Structurally, the converter consists of functionally related blocks: input transformer block (transformer cabinet); a multi-level inverter (inverter cabinet) and a control and protection system with an information input and display unit (control and protection cabinet).
The input transformer cabinet transfers energy from the three-phase power supply to a multi-winding input transformer, which distributes the reduced voltage to a multi-level inverter.
A multilevel inverter consists of unified cells - converters. The number of cells is determined by the specific design and manufacturer. Each cell is equipped with a rectifier and a DC link filter with a bridge voltage inverter using modern IGBT transistors (insulated gate bipolar transistor). The input AC current is initially rectified and then converted into alternating current with adjustable frequency and voltage using a solid-state inverter.
The resulting sources of controlled alternating voltage are connected in series into links, forming a voltage phase. The construction of a three-phase output power system for an asynchronous motor is carried out by connecting links according to the “STAR” circuit.
The protection control system is located in the control and protection cabinet and is represented by a multifunctional microprocessor unit with a power supply system from the converter's own power source, an information input/output device and primary sensors of the converter's electrical operating modes.
Saving potential: counting together
Based on data provided by Mitsubishi Electric, we will evaluate the energy saving potential when introducing frequency converters.
First, let's see how power changes under different engine control modes:
Now let's give an example of a calculation.
Electric motor efficiency: 96,5%;
Variable frequency drive efficiency: 97%;
Fan shaft power at nominal volume: 1100 кВт;
Fan characteristics: H=1,4 p.u. with Q = 0;
Full working time per year: 8000 hours.
Fan operating modes according to the schedule:
From the graph we get the following data:
100% air consumption – 20% of operating time per year;
70% air consumption – 50% of operating time per year;
50% air consumption – 30% operating time per year.
The savings between operation at rated load and operation with the ability to control the motor speed (operation in conjunction with a VFD) are equal to:
7 kWh/year - 446 kWh/year= 400 kWh/year
Let's take into account the electricity tariff equal to 1 kWh / 5,5 rubles. It is worth noting that the cost is taken according to the first price category and the average value for one of the industrial enterprises of the Primorsky Territory for 2019.
Let's get the savings in monetary terms:
3 kWh/year*600 rub/kWh= 000 rub/year
The practice of implementing such projects allows, taking into account the costs of operation and repairs, as well as the cost of the frequency converters themselves, to achieve a payback period of 3 years.
As the figures show, there is no doubt about the economic feasibility of introducing VFDs. However, the effect of their implementation is not limited to the economy alone. VFDs smoothly start the engine, significantly reducing its wear, but I’ll talk about this next time.
Source: habr.com