Efficiency of the rotating machinery (that is, the ratio between mechani- cal output power and electric input power for motor operation; the ratio between electric output power and mechanical input power for generator operation) is generally in the range of 85–95 %, the higher values for medium- and high-power machines (medium-power machines between 50 and 500 kW; high-power ones above these values). Efficiency also depends on the output power as percentage of the rated power as shown in Fig.7.5. Smaller machines usually have lower values of efficiency.
Efficiency can be improved (1) in the design phase, with the so-called high- efficiency motors where efficiency is 2–3 % points higher than in normal-efficiency motors, by using low-loss materials, in particular low-loss iron, and (2) in the operating phase by allowing the machine to operate around the maximum efficiency point (see also Chap.5for transformers).
Efficiency of the entire drive is lower because of power converter effi- ciency, which however is about 95 %. Overall efficiencies of 85–90 % can be assumed as actual values for many electrical drives.
Fig. 7.5 Efficiency versus output power for electric motors
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7.4.1 Basic Principles and Operating Modes of Electrical Machines
Electrical machines are traditionally classified as alternating current machines (synchronous machines, asynchronous or induction machines, commutator machines) and direct current machines (commutator machines) according to the voltage of the feeding line. The stationary structure (stator) and the rotary structure (rotor) are both generally provided with windings. One structure has to generate a magnetic flux by means of a particular set of windings (field windings in commuta- tor machines and synchronous machines) or permanent magnets, and the other structure has the main windings (armature windings) through which power flows from the electric line to the shaft or vice versa. Asynchronous machines (also called induction machines) have polyphase stator windings (generally two or three phase) which generate a rotating flux and carry the electric power fed by the line; rotor windings are usually polyphase squirrel-cage windings.
A.c. (alternating current) machines can be fed directly from the line at medium or low voltage depending on the machine power (medium voltage for high-power machines, more than 500 kW). Synchronous machines must rotate at a synchronous speed related to the line frequency (synchronous speed: in r/min¼60f/pair of poles;
in rad/s¼2πf/pair of poles where the frequencyfis expressed in Hz) to effect the energy transformation, so they have to rotate at a constant speed and are not self- starting. All the other a.c. machines (except the synchronous ones) may work at any speed and are self-starting.
D.c. (direct current) machines require a d.c. voltage line that is not generally available in industrial applications. Power converters from a.c. to d.c. voltage are therefore necessary.
As a general rule, shaft torque can be expressed as the product of magnetic flux (generated by a current flowing inside a field winding or inside a polyphase winding or by a permanent magnet in one struc- ture) multiplied by current flowing inside the winding in the other structure (stator or rotor). This relationship is the basis of all torque control methods.
At steady state, torque-speed characteristics can be used to represent the output of the machines when operating as motors and the input when operating as generators. The working point is the intersection between the torque-speed curve of the machine and the load curve as shown in Fig.7.6where the characteristics of the various classes of machine are represented.
If the machine is fed directly from the line at constant voltage and frequency, there is only one torque-speed characteristic (that is, the locus of working points of the machine) and therefore only one working point is allowed for each load. Thus, at constant voltage and frequency, torque and speed values at the working point are imposed by the load and may change only if one of the load parameters varies.
Rheostatic control, which changes the speed of a motor by introducing a variable amount of resistance into the stator or into the rotor circuit, always involves waste of energy when the motor runs at reduced speed. In fact, a voltage drop occurs at the terminals of the rheostat, thus provoking Joule effect losses in it. Notice that these systems cannot change the synchronous speed already defined, because they do not change the frequency of the supply. Once widely used for controlling d.c. and induction machines with wound rotors, rheostats have gradually become obsolete because of the introduction of power electronic converters which exercise a better control with lower losses.
Fig. 7.6 Torque-speed characteristics for electrical machines at given input voltage and current 108 7 Electric Distribution Systems from Facilities to End Users
In conclusion, energy saving at end-user level is possible only by a correct choice of the nominal power of the machine, so that the working point is set in the maximum efficiency zone, and by installing high-efficiency machines instead of standard machines. Once the installation has been made, it is the load that sets the working point, unless more complex systems, such as power electronic converters, are introduced between the supply and the electrical machine.
7.4.2 Basic Principles and Operating Modes of Electrical Drives Unlike electrical machines directly connected to the supply, electrical drives allow a set of working points inside a working area instead of one fixed point.
The electric input variables (voltage, current, frequency) can be modified within a given range by controlling the power electronic converter properly, so that the mechanical output quantities (torque, speed, shaft position) are regulated indepen- dently within defined boundaries. In this way, the external characteristics of the machines can be continuously modified, giving rise to a family of curves which allow drive operation in an entire region of the torque-speed plane. Any point can then be reached with an appropriate control law, by varying the motor voltage (or current), the frequency, or both. The working point of the drive is no longer confined, for a given load, to the single intersection with the electrical machine characteristic resulting from a fixed-parameters source.
Two working regions are generally defined: (1) constant-torque region as a set of operating points where any torque can be delivered by the drive at any speed, within the thermal and insulation limits of the drive, and (2) constant-power region as an operating region where the torque is delivered only within the power limit of the drive (remember that shaft power¼shaft torquespeed), so that its value decreases as the speed increases.
Operation at constant torque is normally performed with a constant value of the machine flux (by permanent magnets or by constant field current) and implies that any torque, within the motor’s capabilities, can be delivered at any speed by action on the proper control variables, usually the current. When torque is flux multiplied by current, when the flux and the current are kept constant, a constant value of torque can be delivered at any speed by varying the voltage (and the frequency in a.c. machines), according to the control system requirements.
Operation at constant power occurs beyond the constant-torque region and higher speeds are reached at the expense of the torque within the limits of the maximum power delivered by the machine. Operation is bounded by the hyperbola of the equation power¼torquespeed¼constant. In this region, voltage is kept
constant (at its rated value or lower), while the machine flux is weakened. A final limit on the speed axis can be either a mechanical constraint related to the rotor type of construction or an electrical limitation (converter or motor current). Because of the constant-voltage value, constant power at any speed corresponds to a constant current from the line (neglecting losses).
Figure7.7shows constant-torque and constant-power regions for various drives;
both motor and generator operating modes are illustrated.
Notice that electrical drives may work in a reversible mode by transforming mechanical energy into electric energy without significant modification of their structure. This capability is used mainly to effect regenerative braking by transforming the energy stored inside rotating masses at the shaft into electric Fig. 7.7 Constant-torque and constant-power region for drives with synchronous (a), asynchro- nous (b), and d.c. machines (c)
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energy, which is fed back to the network. Accurate braking control as well as energy recovery are the main reasons for using this mode of operation both in industry and in traction.
Electrical drives play an important role in energy saving when machines such as pumps, compressors, and fans are controlled by speed variation as an alternative to other dissipative methods (see Chaps.10,11,12,13).
In addition, electrical drives are among the main components of industrial automation systems and allow improvements in production quality and quantity, thus generally determining a decrease in specific energy consumption per unit of production. Interfacing with the plant centralized control, diagnostics and monitor- ing are other factors that make electrical variable-speed drives attractive for indus- trial applications.
7.4.3 Application Problems
Electrical variable-speed drives may be a source of thermal, mechanical, and electrical malfunctioning which must be investigated carefully in order to guarantee a safe operation of the drive itself and of the electric network.
The power factor at the drive feeding node changes according to the operating mode: generally, the lower the actual load, the lower the power factor. That depends on the internal structure of the power converter and on the current and voltage harmonics, but it can be assumed as a general rule. A control system is usually required to keep the power factor value within the accepted range.
Derating of electrical machines, when fed by power converters, is necessary because of the increase in losses due to the harmonics and because of the reduction in efficiency of cooling systems at low speed. A derating of 10–15 % depends on the harmonics; higher derating, up to 50 %, is due to thermal phenomena caused by the reduced speed range. In any case, an independent cooling system is installed, generally a fan driven by an independent electric motor.