3.2. Electrical Drives

Power electronics is a key factor for energy efficiency in electrical systems. Power electronics is basic technology for such renewable energy sources as photovoltaic, wind energy and fuel cell systems. The basic purpose of power electronics is to control energy flow while transforming the energy. As the process of transforming electrical energy to mechanical energy is necessary in most industrial technologies the electrical drives are of top importance in energy saving.

Losses

The converter, the motor and the mechanical transmission system are the main losses producers in variable speed drives. The motor control method can deal with converter and motor losses. The control method cannot help to reduce the losses from transmission losses and those generated in the grid because of the rectifier as they are not related to the motor flux level and current. Energy optimal control methods are focused on the operation of both converter and motor at the most optimal point reducing thus the losses. Energy optimal control methods try to find the energy optimal stator current and rotor flux in order to produce the same amount of torque. Thus, selecting the desired flux level is a balance between efficiency and not reducing the performance of the drive.

The converter losses are due to non-ideal nature of switching devices. They are divided into conduction losses and switching losses. The resistive behavior of the semiconductor power devices (transistors and diodes) when they are conducting produces these losses. These losses depend on the circulating current, the power factor of the converter and parameters of semiconductors affected in their turn by many other parameters such as temperature and voltage. The switching losses linearly depend on frequency. To reduce these losses an adequate modulation technique should be used. It can reduce the number of commutations and, therefore, the switching losses, without reducing the performance of the drive. The losses of drive include also the motor losses considered before.

In order to increase efficiency it seems reasonable to transfer only a necessary amount of energy changing the rotational speed of the motor. This can be done by means of variable speed drive that can adjust the frequency and the voltage amplitude needed for the system in real-time. Modern available variable speed drives are more than just systems to change the frequency and amplitude of the voltage applied to the motor. They are also used as protection devices for the motor; they also act as a soft-starter, reducing stress on input lines and transformers. These technologies can give significant operational cost savings and a reduction of CO2 emissions.

From the converter point of view there are three main factors influencing the system efficiency:

• converter topology – some topologies are able to reduce current harmonics in the motor, reducing losses and torque ripple, in this case the multilevel and matrix converters are the key topologies, the multilevel converters offer great advantages such as lower harmonic distortion, lower voltage stress on loads, less electromagnetic interference; multilevel converters were basically developed to increase a rated power in the converter;

• modulation technique – depending on the realized mt the efficiency of the motor and the converter can be improved;

• semiconductor devices (SD) – new materials applied in the production of SD, such as SiC (silicon carbide), GaN (gallium nitride) and even diamond, have lower losses and a higher working temperature capability. Semiconductors in power electronics are used in saturation mode only, i.e. in ON or OFF states, but not in the linear operating regime. In the semiconductors there two main sources of losses: conduction and switching losses.

5-6lpp.

Control methods

Three control methods for induction motors and PMSM are of greatest importance in industrial electronics: V/f control, vector control and DTC (direct torque control).

V/f control

V/f control method is based on the well-known equivalent per phase model of an induction motor (Fig.2) operating in steady state.

Fig.2. Equivalent per phase model of an induction motor

In the V/f control flux linkage λ_s of the motor is supposed constant within the whole speed range up to the base speed (ω_b). The applied voltage versus the applied frequency is a linear relation, and can be seen in Fig.3. Above base or rated speed the voltage is maintained constant while the frequency increases because the voltage capability of the converter is limited. At low speed for maintaining a constant flux some extra voltage must be applied to the motor. This voltage depends on the current level (load) of the motor.

Fig.3.

Figure 4 demonstrates the block diagram of the V/f control scheme. The algorithm computes the voltage amplitude proportional to the desired frequency. The voltage and frequency are used in the modulator to synthesize the switching signals for the voltage source inverter.

Fig.4. Block diagram of the V/f control scheme

The direct connection of a motor to the grid is a particular case of V/f control. By means of VSD this case can be extended to the whole speed range adding the features of soft-starting, protection and speed regulation. That is why V/f method is now one of the most applied control methods for induction motors.

To assure the operation in the stable region of the motor characteristics, limiting the maximum slip, a PI controller can be added in an external loop. This controller also can operate as current protection for the motor.

In the case of PMSM the V/f control strategy cannot be applied directly because of instability at low speeds. The use of PMSM with V/f control requires the use of a squirrel cage rotor combined with permanent magnets or the measurement of the rotor mechanical speed in order to synchronize stator currents and rotor position, but this solution increases the cost and reduces reliability of the speed sensors. This type of control is mostly used in continuous running applications such as pumps, fans and HVAC which are at the focus of energy saving.

Vector control

Vector control is based on the instantaneous decomposition of the electrical equations of the motor in order to obtain a relationship between the stator currents and rotor flux with the torque produced like in a DC motor. The decomposition is based on the well-known Park transformation and the result is the decomposition of the currents producing flux and torque corresponds to the armature current.

The principles of vector control are equally applied in synchronous and asynchronous motors. The main difference is that in the case of PMSM the rotor flux is produced by permanent magnets, which yield to an increase of efficiency of PMSM in front of an asynchronous motor because there is no need to create rotor flux from stator currents.

Fig.5. .

In this control strategy the torque and the flux can be independently controlled, fig.5. In addition, current limitation is directly implemented in the control scheme. It allows an inherent protection of the motor and converter. This limitation can be adapted depending on the motor operating conditions, such as temperature, allowing the use of the motor in different ranges.

DTC

In Direct Torque Control (DTC) the torque and the stator flux are controlled simultaneously by inverter voltage space-vector selection using a look-up table. The main advantage of DTC compared it with vector control is its simplicity. It allows DTC to implement a speed sensorless control system. The main drawbacks of DTC are the high torque ripple and the variable switching frequency caused by the hysteresis controllers.

The DTC is based on the fact that the stator flux vector can be known from the voltage applied to the stator. On the other hand the produced torque can be computed knowing the stator flux vector and stator current vector.

Fig.6…

DTC is a sensorless torque controller technique. The speed close loop system requires a speed sensor but like in the case of vector control speed sensorless techniques can be used with DTC. In the case of this control method it is hard to predict the converter losses because the conduction intervals and switching frequency are not constant.

In the case of V/f control scheme, where the rotor speed and flux are not decoupled, the main disadvantage in flux reduction is that for a given operating point (torque and speed) the stator frequency must be increased, and then the maximum torque is reduced, making the system more sensitive to the load disturbance.

In the case of vector control and DTC, where the torque and flux are decoupled, the flux can be independently optimized giving the better performance.

Modulation techniques

The conversion from DC to AC can be realized using different modulation techniques that are the turning of the switches of the inverter on and off with a high frequency in such a way that the average value of the inverter output voltage equals to that of the voltage reference. The longer the switch is on the higher the average output voltage is. There are several these techniques having advantages and disadvantages each. That one should be selected that best fits the demands of the application. The most widely used techniques are:

• Square-Wave Modulation;

• Sinusoidal Carrier-Based PWM;

• Third Harmonic Injection PWM;

• Space Vector PWM.

The comparison of these techniques is presented in Table 1.

Table 1

MT	idea	ad	disad
Square-Wave Modulation (six-step mod.)	- turning on and off each leg of the inverter at the frequency of the reference output voltage.	the simplest obtains the maximum fundamental output voltage	high level of THD restricted for control the speed of IMs in the area of max torque and speed
Sinusoidal Carrier-Based PWM (SPWM)	- the idea is in comparison of a carrier signal (usually a triangular) with a modulated signal (a sine-wave control signal)	- easily implemented in digital manner	- non-ability of motor to operate at rated speed results in the increment of the losses of the motor and a drop of its efficiency
Third Harmonic Injection PWM (THIPWM)	- an adding of an adequate third harmonic wave-form to each of the reference three-phase voltages	- increasing of the fundamental maximum output voltage for about 15% higher than in SPWM	- zero-sequence components are not always of sine-wave
Space Vector PWM (SVPWM)	- using of a complex voltage vector on the generalization of the Park strategy; it includes the discretization in eight unique vectors which are the voltage different states that can impose three-phase inverter

Effective utilization of energy

The solving of the task of effective utilization of energy will allow reduce a consumption of power and financial resources within the industrial and agricultural production to decrease high expenses of both state and people, to improve ecology of the environment. As a basic consumer of electrical energy electric drive (ED) play a key role in this task. The energy saving can be realized in ED itself as well as in the applied technologic process, at the stage of both design and assembling and its usage. The design and assembling of ED should be defined with the modern level of theory and practice development and segregation of sciences – electrical mechanics, electronics, automatics, and information technologies. The basic methods of energy saving at this stage are the following:

1. A proved calculation of a required motor power and the mechanism taking into account all operational conditions. As it is known a motor with an overstated power has low power operational indises. In this case an underexpoitation of its initial resources takes place. On the other hand application of a motor with not enough power reduces the effectiveness of the technology resulting in the motor overloading and its early death.

2. Selection of ED components having minimum power loses. First of all it relates to the motors with max possible efficiency factor and power factor.

3. Selection of the approaches and technical means of the ED parameters regulation when the power and energy loses are reduced to nimimum, etc.

While maintaining EDs the energy saving is achieved by means of proper maintaining and opportune modernization. YThe modernization of ED should provide technical measurements for energy saving improvement while maintaining it: replacing of lower loaded motors, limitation or full exclusion of their idle-time operation, IM short voltage decreasing with low loads, reactive power compensation, etc. Any work with the equipment operation improvement should be accompanied with the evaluation of expected technical economy from it.

As it was mentioned before a highly effective method is to regulate ED by means of semiconductor converters. Special principles of ED with IMs (induction motors) realization can result in minimization of IM consumed current and therefore the electric energy losses in it. This can be illustrated with the characteristics of stator current I_s dependence on voltage U₁ for the cases of different loading torque M_st. The graphs constructed correspondingly for M_st1<M_st2<M_st3<M_st4 demonstrate that each torque has a voltage resulting in the minimum consumed network current (fig.7a). The dotted line in the graph drawn across the currents’ minimum points for each load determines the law of voltage regulation regarding the current, when with any M_st this current is minimum. The circuit realizing the minimization of current is in fig. 7 b.

Fig.7. IM speed regulation changing the supply voltage

Besides the motor (4) it includes also voltage regulator (3) with control system (2), sensors of current (5) and voltage (6) and comparator (1). The required control law is realized by means of positive feedback of current. Three-phase sensor 5 produces a signal propotional to the current. The auxiliary feedback of voltage if negative, supporting the necessary quality of the transient processe. Besides the minimization of the energy losses the regulation of the voltage of IM stator with low loads and idle-time operation result in decreasing of current, power consumed from the network, power losses and therefore increasing of efficiency and power factors.

The theoretical calculations and experimental data prove that these regulation approaches allow increasing of power indices of ED for some percentage that provides a high economic effect when IMs are widely used. Fig.8 contains an example of characteristics of η, cos , relative stator current I^*=I_s/I_sN and power losses ΔP*=ΔP/ΔP_N in accordance with the relative stator voltage U^*_s=U_s/U_sN for a 30kW motor and for the torque loading of 20% from the highest values of these parameters; the characteristics are achieved with the voltage of 60-80% of the rated.

Fig.8. Characteristics of η, cos , relative stator current I^*=I_s/I_sN and power losses ΔP*=ΔP/ΔP_N in accordance with the relative stator voltage U^*_s=U_s/U_sN

The extreme values of these parameters are achived when IM is operating at a particular value of slip. Thus providing minimum of a consumed current with any level of loading its slip should be maintained as s=R₂^’/X_μ. The slip maintained at a required optimal level is provided ima system with negative speed feedback.

Let us consider as an example of energy saving the regulation of ED of a centrifugal pump. The basic characteristics of a pump is the dependence of its pressure H on the consumption of water Q. Fig.9 demonstrates such characteristics H=f(Q) for two speeds of the driven motor – rated ω_N and decreased ω₁.

Fig.9. Characteristics H=f(Q) for two speeds of the motor

The specific feature of many pump installations is the changing amount of liquid delivered to the consumer. For example, consumption of water from pumps in the apartment houses is changed within day-night perios having two maximums, fig.10. The opportunity of energy saving can be demonstrated with so-called relative formulas of water consumption Q, pressure H and consumed power P relation to the pump rotation speed. For two speed values ω₁ and ω₂ the following is valid:

.

Formulas show that the pressure and consumed power will be decreased by means of propotional decreasing of the pump rotation speed. Therefore providing with necessary amount of liquid the pump will operate with lower pressure and energy consuming.

Fig.10….

Liquid consumption Q, pressure H and power P are related to each other with the expression:

,

where ρ is the density of liquid, kg/m³; g=9.8m/s² – gravitation acceleration; η_N, η_ED – efficiency factors of pump and electric drive correspondingly; pressure h in this is measured in meters, liquid consumption Q – in m³/s.

Electric energy consumed within the time period t_p by ED is:

.

The economy of electric energy during time tp with decreasing of pressure from value H₂ with constant speed to level H₁ with decreasing of its speed is:

.

Calculation example…

References

1. A. Sumper, A. Baggini. Electrical Energy Efficiency: Technologies and Applications, - WILEY, 2012

2. T. D. Eastop, D. R. Croft. Energy Efficiency for Engineers and Technologists, - Pearson Education, 1990

3. Anibal de Almeida, Paolo Bertoldi, Werner Leonhard. Energy Efficiency Improvements in Electric Motors and Drives, - Springer, 2000

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