Electric motors impact almost every aspect of modern living. Refrigerators, vacuum cleaners, air conditioners, fans, computer hard drives, automatic car windows, and multitudes of other domestic appliances and devices all use electric motors to convert electrical energy into useful mechanical energy. In addition to running the commonplace appliances, electric motors are also responsible for a very large portion of industrial processes. Electric motors are used at some point in the manufacturing process of nearly every conceivable product that is produced in modern factories. Because of the nearly unlimited number of applications for electric motors, it is not hard to imagine that there are million motors of various sizes in operation across the world. This enormous number of motors and motor drives has a significant impact on the world because of the amount of power they consume.
The systems that controlled electric motors in the past suffered from very poor performance and were very inefficient and expensive. In recent decades, the demand for greater performance and precision in electric motors, combined with the development of better solid-state electronics and cheap microprocessors has led to the creation of modern adjustable speed drive. An adjustable speed drive is a system that includes an electric motor as well as the system that drives and controls it. Any adjustable speed drive can be viewed as five separate parts: the power supply, the power electronic converter, the electric motor, the controller, and the mechanical load.
The power supply can provide electric energy in the form of AC or DC at any voltage level. The power electronic converter provides the interface between the power supply and the motor. Because of this interface, nearly any type of power supply can be used with nearly any type of electric motor. The controller is the circuit responsible for controlling the motor output. This is accomplished by manipulating the operation of the power electronic converter to adjust the frequency, voltage, or current sent to the motor. The controller can be relatively simple or as complex as a microprocessor. The mechanical load is the mechanical system that requires the energy from the motor drive. The mechanical load can be the blades of a fan, the compressor of an air conditioner, the rollers in a conveyor belt, or nearly anything that can be driven by the cyclical motion of a rotating shaft.
Electrical Motor Drives
Today, with advancements in power electronics, control electronics, microprocessors, microcontrollers, and digital signal processors (DSPs), electric drive systems have improved drastically. Power electronic drives are more reliable, more efficient, and less expensive. In fact, a power electronic drive on average consumes 25 per cent less energy than a classic motor drive system. The advancements in solid-state technologies are making it possible to build the necessary power electronic converters for electric drive systems. The power electronic devices allow motors to be used in more precise applications. Such systems may include highly precise speed or position control. Systems that used to be controlled pneumatically and hydraulically can now be controlled electrically as well.
More advanced electric motor drives are now replacing older motor drives to gain better performance, efficiency, and precision. Advanced electric motor drives are capable of better precision because they use more sophisticated microprocessor or DSP controllers to monitor and regulate motor output. They also offer better efficiency by using more efficient converter topologies and more efficient electric motors. The more advanced drives of today also offer a performance boost by utilizing superior switching schemes to provide more output power while using lighter motors and more compact electronics.
Electrical Motor Losses
Motor efficiency may be increased by reducing losses. Motor energy losses can be divided into various categories, each of which is influenced by design and construction of the motor. One design consideration is the size of air gap between the rotor and the stator. Large air gaps tend to maximize efficiency at the expense of power factor, while small air gaps slightly compromise efficiency while significantly improving power factor. The efficiency of a motor is determined by intrinsic losses that can be reduced only by changes in motor design. Intrinsic losses are of two types: fixed losses – independent of motor load, and variable losses – dependent on load. Fixed losses consist of magnetic core losses and friction and windage losses. Magnetic core losses (sometimes called iron losses) consist of eddy current and hysteresis losses in the stator. They vary with the core material and geometry and with input voltage. Friction and windage losses are caused by friction in the bearings of the motor and aerodynamic losses associated with the ventilation fan and other rotating parts.
Variable losses consist of resistance losses in the stator and in the rotor and miscellaneous stray losses. Variable losses depend upon motor load. Resistance to current flow in the stator and rotor result in heat generation that is proportional to the resistance of the material and the square of the current (I2R). Where R is the stator winding resistance for stator resistance loss and for rotor resistance loss it can be used as rotor winding resistance.
Stray losses arise from a variety of sources and are difficult to either measure directly or to calculate, but are generally proportional to the square of the rotor current. No load losses such as core losses and friction and windage losses both are about 15% of the total losses that occur in the motor while under loaded condition. Part-load performance characteristics of a motor also depend on its design. Both η and PF fall to very low levels at low loads.
Energy Efficient Motors
Electric motors are of utmost importance in industrial as well as agriculture sector. These motors found their application as constant speed drives with very low rating as well as variable speed drives with very high rating. Energy efficiency and energy conservation are very closely related to each other. With increase in demand of energy and due to uncertainties in oil supply and fluctuating price of conventional fuels, efficiency and conservation of energy has become an important aspect of industrial as well as rural development. A large amount of electrical energy is consumed by induction motor used for irrigation in rural sector and industrial purpose in urban sector. In country like India agriculture and industrial sector is developing rapidly, in same way electrical energy consumption is increasing. A study indicated that a 5 per cent improvement in overall efficiency of induction motor would save enough energy that would be comparable to energy produced by a new power plant of few hundred megawatts.
Energy-efficient motors are the ones in which, design improvements are incorporated specifically to increase operating efficiency over motors of standard design. Design improvements focus on reducing intrinsic motor losses. Improvements include the use of lower-loss silicon steel, a longer core (to increase active material), thicker wires (to reduce resistance), thinner laminations, smaller air gap between stator and rotor, copper instead of aluminum bars in the rotor, superior bearings and a smaller fan, etc. Energy-efficient motors now available in India operate with efficiencies that are typically 3 to 4 percentage higher than standard motors.
The suitable selection of copper conductor size will reduce the resistance. Reducing the motor current is the most readily accomplished by decreasing the magnetising component of current. This involves lowering the operating flux density and possible shortening of air gap. Rotor I2 R losses are a function of the rotor conductors (usually aluminium) and the rotor slip. Utilisation of copper conductors will reduce the winding resistance. Motor operation closer to synchronous speed will also reduce rotor I2 R losses. Core losses are those found in the stator-rotor magnetic steel and are due to hysterisis effect and eddy current effect during 50 Hz magnetisation of the core material. These losses are independent of load and account for 20 – 25 per cent of the total losses. The hysterisis losses which are a function of flux density, are to be reduced by utilising low loss grade of silicon steel laminations. The reduction of flux density is achieved by suitable increase in the core length of stator and rotor. Eddy current losses are generated by circulating current within the core steel laminations. These are reduced by using thinner laminations.
Friction and windage losses result from bearing friction, windage and circulating air through the motor and account for 8 – 12 per cent of total losses. These losses are independent of load. The windage losses also reduce with the diameter of fan leading to reduction in windage losses. Stray load losses vary according to square of the load current and are caused by leakage flux induced by load currents in the laminations and account for 4 to 5 per cent of total losses. These losses are reduced by careful selection of slot numbers, tooth/slot geometry and air gap.
Energy efficient motors cover a wide range of ratings and the full load efficiencies are higher by 3 to 7 per cent. The mounting dimensions are also maintained to enable easy replacement. As a result of the modifications to improve performance, the costs of energy-efficient motors are higher than those of standard motors. The higher cost will often be paid back rapidly in saved operating costs, particularly, in new applications or end-of-life motor replacements.
Factors affecting the Motor Efficiency
Most electric motors are designed to run at 50 per cent to 100 per cent of rated load, the maximum efficiency is achieved usually near 75 per cent of rated load. Thus, a 10-hp motor has an acceptable load range of 5 to 10 hp with maximum efficiency is at 7.5 hp. A motor’s efficiency tends to decrease dramatically below about 50 per cent of the rated load. However, the range of efficiency varies with individual motors and tends to extend over a broader range for larger motors. A motor is considered under loaded when it is in the range where efficiency drops significantly with decreasing load.
Overloading of motors can decrease efficiency. Many motors are designed with a service factor that allows short time overloading.
Power factor is an important attribute relating to efficiency of AC induction motors. As the load on the motor comes down, the magnitude of the active current reduces. However, there is no corresponding reduction in the magnetising current, which is proportional to supply voltage with the result that the motor power factor reduces, with a reduction in applied load. Induction motors, especially those operating below their rated capacity, are the main reason for low power factor in electric systems. Motors, like other inductive loads, are characterized by power factors less than one. As a result, the total current draw needed to deliver the same real power is higher than for a load characterised by a higher PF. An important effect of operating with a PF less than one is that resistance losses in wiring upstream of the motor will be higher, since these are proportional to the square of the current. Thus, both a high value of PF close to unity are desired for efficient overall operation in a plant.
Effect of Harmonics
Harmonics are ac voltages and currents with frequency that are integer multiples of the fundamental frequency. In earlier years, harmonics were not prevalent in most of the industries due to balance linear loads using three phase induction motors along with incandescent lighting, resistivity etc., but the rapid advancement of power electronics in industrial application makes industrial loads non-linear type. These non-linear loads draw non-sinusoidal current from the sinusoidal voltage waveform. The distortions thus produced in the voltage and current waveforms from the sinusoidal waveforms are called harmonic disorders.
Harmonics are generated due to increasing number of non-linear loads occurred when the system voltage is linear but the load is non-linear, the current will be distorted and become non-sinusoidal. The actual current will become higher than the current measured by an ammeter or any other measuring instrument at the fundamental frequency. It also occurred when the supply system itself contains harmonics and the voltage is already distorted, the linear loads will also respond to such voltage harmonics and draw harmonic currents against each harmonic present in the system and generate the same order of current harmonics. When the system voltage and loads are both non-linear (a condition which is more common) the voltage harmonics will magnify and additional harmonics will be generated, corresponding to the non-linearity of the load and hence will further distort an already distorted voltage waveform.
Energy Efficiency and Environment
It is well known that environment and efficiency are closely interlinked with each other. Electric drive systems are largely responsible for the largest part of the electricity consumption. Therefore, an increase in efficiency in motor will result in large energy savings and reduction in CO2 emission into our environment. Machine designers over many years have tried their best to respond to the need for improved efficiency of induction motors. Real driver for the evolution of higher efficiency motor is to save the environment through reduction of energy consumption. An improvement in energy efficiency will lead to reduction of CO2 emissions. Today, in India millions of the induction motors are manufactured every year and they combined to consume about 50 per cent of the total energy generated. By improving the efficiency, considerable amount of energy can be saved and it also leads to save environment because to meet the load of these machines power generating stations are releasing millions of tons of greenhouse gases into the atmosphere every year. There is requirement of sustaining the constantly increasing demand of energy and at the same time reducing environmental pollution, then automatic increase in the efficiency of energy conversion will have to substantially improve in order to produce more power from the same or less material.
The optimum design gives a motor having uniformly high efficiency over a wide range of load and supply voltage. It is seen that, within the same frame size, the full load efficiency of the new motor is about 2.5 per cent more than that of the standard. The active material cost of the energy efficient motor is slightly more than that of the standard motor, but the extra cost is paid back within a reasonable period.
Dy. Director (Generation),
M.P. Electricity Regulatory Commission