Variable Speed Drives and Variable Voltage Control of Induction Motors

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1 Variable Speed Drives and Variable Voltage Control of Induction Motors 1. Introduction A large portion of electrical energy is consumed in electric machines. It ranges from 60% to 80% in various countries. The application includes elevator, escalator, airconditioning system, ventilation, industry drives, electric vehicle, washing machine, etc. In the past, DC motors was used in conventional system because of easy control. Induction motor is later on emerged and together later with higher performance vector control, it becomes well accepted by many users. The method to drive the motor is pulse width modulation (PWM). The driving circuit is called inverter which is formed 6 transistors for many 2-level systems. Most of the motor and inverter has a high efficiency. In normal operation, the efficiency of motor is around 80-95% whereas the efficiency of inverter is 85-97%. 2. BACKGROUND OF VARIABLE SPEED DRIVES 2.1 Background The motor drives for both induction motors and DC motors required VSD. This section will concentrate on Induction motor as it is popular nowadays. The implementation of VSD in induction motor is usually carried by an inverter. A typically inverter drive is shown in Fig 2. The energy saving for VSD motor drive is an important research and development in power electronics. In the past 20 years, there are many different techniques proposed and they include the variable speed variable voltage drive, voltage control fixed frequency drive and the power factor correction method. All of them are based on the methods to optimize the efficiency of the motor, reduce the magnetizing loss or conduction loss. Fig 2: Typical inverter for motor drive For lower power inverter, the 3-phase rectifier is used without power factor correction front-end converter. It just uses simple bridge rectifier. For bridge rectifier, the power factor is still reasonable, but its power factor cannot be easily improved by simple capacitor. This is because the power factor problem is because of the low order harmonics rather than by the reactive components or the motor. It can be seen that the DC link has provided a decoupling between the motor-drive and the rectifier. The power factor variation in the motor due to the motor load cannot affect the AC input. The DC link capacitor provides a decoupling. 2.2 VARIABLE FREQUENCY VARIABLE VOLTAGE (VVVF) In this scheme of control, basically the motor is needed to run at its optimized flux level in order to have the highest efficient region. Therefore the ratio of the voltage of the magnetizing flux to the frequency should be maintained constant. This is also

2 called the constant V/F control. In the simplified system, only the applied voltage to frequency is maintained constant and the voltage drops in the stator winding and the leakage inductance are ignored, therefore such simple V/F control cannot always provide good efficiency under all regions of operation especially when it is operated near the low speed region or as a servo drives. A good variable speed drive is to always provide constant V/F control even during transient and low speed region. Motor parameters are needed to program into the motor drive controller or certain parameter estimation is needed. Typical popular method is the vector control or field orientation control [5]. It is a decoupling technique to control the field and torque of the motor separately and therefore the controllability of the motor is good. It automatically provides constant V/F control in the magnetizing flux of motor and therefore vector control has a good efficiency in motor drive system. Fig 3 shows a vector control schematic diagram. Other control methods such as direct torque control are very famous for motor drives. Fig 3: A vector control diagram for induction motor drive The motor torque T under the VVVF control is: T 2 E 1 ω2r2 = ω (1) 1 R2 + ω2l2 It is seen that with constant V/F control, the ratio of the stator emf E 1 to stator frequency ω 1 is constant. T only depends on rotor resistant R 2, rotor leakage inductance l 2 and slip frequency ω 2. The torque-speed characteristic of the VVVF is shown in Fig 4. Different curves represent different synchronous frequencies of operation. Using VVVF method, the maximum torque T m can always provide a maximum torque level Tm Motor speed (rad/s) Fig 4: Normalized maximum torque-speed characteristic Fig 5 shows the general characteristics of induction motor. It is clear that the maximum torque, efficiency and power factor are all around the same motor speed or

3 slip frequency. Therefore it is necessary to operate the motor near this region which is around a small slip frequency. Fig 5: General characteristics of induction motor The basic principal of induction motor drive is to control it to under small slip. Therefore the supply power frequency to the motor is very close to the motor rotor equivalent frequency. The name of VSD means when the motor speed is needed to change, the supply power frequency is also changed accordingly. It is noted that the efficiency of this operation give poor efficiency at low speed. Therefore, without VSD, the motor is not preferred to work under low speed. 2.3 WHY SMALL SLIP MEANS HIGHER EFFICIENCY For the operation of induction motor, the smaller slip has another meaning in constant flux. Most motor is designed at constant flux of optimal flux. The flux is usually proportional to the ratio of flux and frequency: Ndφ/dt=V (2) φ ~=V/(Nf) (3) i.e. the flux is proportional to V/f. If the optimal flux is needed to be maintained, when the motor frequency is reduced, the voltage is also needed to be reduced in the same manner. For example, For 50Hz 220V operation motor, when the frequency is reduced to 25Hz, the motor voltage should be reduced to 110V in order to maintain constant flux. 2.4 DISADVANTAGE OF VSD VSD has been used in most of the compressor for energy saving. However, when the motor is running at light load and mains frequency, the motor is usually operated at 50Hz. The VSD does not performance well. This is for example the transportation belt and escalator. During that case, the operational voltage is unchanged and the associated loss in the magnetizing is still high. This causes the overall light load efficiency is lower.

4 Another consideration is the power quality, the input side of the VSD does not guarantee of high power factor or low harmonics. As it can be seen in Fig 2, the input rectifier and filter does not give high power factor and the associated current harmonicas could be a consideration in the international regulation. Table 1: Maximum Total Harmonics Distortion of current Designed circuit current (A) Max THD I < 40A 20% 40A I < 400A 15% 400A I < 800A 12% 800A I < 2000A 8% I 2000A 5% (Source: EMSD Code of Practice for Energy efficiency of Building Services Installation) Simple bridge rectifier may be used in some low to medium power or lower cost VSD, the implied THD may not meet the standard as needed by EMSD or international standards. 3. IMPROVED VARIABLE VOLTAGE CONTROL (VVC) The common inverter motor drive requires 3 pairs of IGBTs and the control method is usually based on constant V/F control or vector control. Some simple voltage mode control has been found in the market. They are basically to regulate the voltage applied to the motor by using phase-delay turn-on. Therefore the voltage applied to motor is reduced. Fig 6 shows the typical circuit. Fig 6: Schematic diagram of the induction motor drive with controlled voltage A. Principles In general, an induction motor is designed to maintain high efficiency when it is driven in the region of 75 percent to the full load [6]. Fig. 7 shows the approximate equivalent circuit of a per-phase for induction motors where the rotor variables and parameters are referred to the stator, V 1 is the stator phase voltage vector, I 1 the stator phase current vector, I m the magnetizing current vector, E 2 the rotor emf vector referred to the stator, I 2 the rotor current vector referred to the stator, r 1 denotes the stator phase resistance, r 2 the rotor resistance referred to the stator, x l the leakage reactance of the stator and rotor referred to the stator, and s denotes the slip. According to Fig. 6, one obtains V1 = E 2 (5) I = I m + (6) 1 I 2

5 Consequently, the vector diagram of the voltage and current is illustrated in Fig. 8. Clearly, I m does not change with the load. Whereby, I 1 changes with I 2 (the load). When an induction motor runs at a large load (near its rated load), the rotor current is I 2 and the corresponding phase delay angle is ϕ. If the induction motor operates at a light load, the rotor current is I 2 and the corresponding phase delay angle is ϕ. It is clear that I 2 is smaller than I 2 and ϕ is larger than ϕ. Thus, if the load factor (real output power / rated output power) of an induction motor becomes low, the power factor also becomes small. Fig. 7: Approximate equivalent circuit Fig. 8: Vector diagram of the current From the approximate equivalent circuit in Fig. 7, the power factor can be computed approximately from cos r I 2 1 ϕ = ( r 1 + ) (7) s V1 It can be seen that the power factor is directly proportional to the stator current and inversely proportional to the stator voltage when the slip is constant. Hence, the stator voltage must be reduced when an induction motor runs at a light load such that it operates with a high power factor. The efficiency of an induction motor is widely varied with power factor [1]. Thus, the efficiency can be improved if the power factor is enhanced. Reference [1] has validated that the power factor and efficiency of an induction motor can be substantially improved by reducing the terminal voltage at a light load. Fig. 6 illustrates the schematic diagram of the controlling the stator voltage of an induction motor. Fig. 9 depicts the typical waveforms of the source voltage and current where ϕ is the phase delay angle and α is the firing angle. Fig. 9: Typical voltage and current waveforms

6 B. Measurement data on energy savings (the rated power of the prototype is 1.5 kw) The method is validated by using a 1.5kW induction motor drive system. The power factor of the motor is detected and the phase-shift controlled SCR is varied to reduce the applied voltage. The voltage is adjusted on-line with the loading condition. Fig 10 shows the percentage of the power consumed with the original full-voltage and the proposed VVC (reduced voltage) method. It can be seen that the power reduction is significant during the light load Input Power Full voltage Reduced voltage Load Factor Fig. 10: Change of the input power with the load factor for the prototype Fig 11 shows the measured power factor of the original full-voltage scheme and the proposed VVC method. There is a significant improvement in the power factor. Fig 12 shows the percentage of improvement in power factor and the energy saving using the VVC method Power Factor Full voltage Reduced voltage Load Factor Fig.11: Change of the power factor with the load factor 30 Percent Improved PF Energy savings Load Factor Fig. 12 Improvement in PF and energy savings using the proposed VVC method.

7 C. Discussion The method of VVC is different from the classical thyristor phase control. It online adjusts the voltage with loading condition in order to reduce the energy consumption. The method is simple as compared with VVVF method. It is especially useful for the condition of the speed relatively unchanged and only the loading is varied. Therefore the achievement by VVC cannot be fulfilled by VVVF. VVVF cannot provide energy saving when motor is running at top speed but load varies widely. Under such condition, VVC should be used for energy saving. 4. POWER FACTOR AND HARMONICS OF VVC As seen in Fig 9, the current harmonics are not very good and the THD is usually higher the simple rectifier, and the associated power factor is also not high as well. Therefore it does have a concern in compiling the THD standard in Hong Kong or other cities as compared with Table 1 which is also an international standard. For the simple system which no inverter or VVC is used and the motor is directly connected to mains, the power factor is useful not good enough and even during full load, the power factor is still less than 0.9 and less than 0.5 in the light load. There are suggestions to use local capacitor compensation which improves the power factor locally and hence the conduction loss between the motor and the source or the switching board with large capacitor banks. Active capacitor bank should be used because the variation of motor power factor with load. The estimated reduction of loss is around 5%-1% that varies with the motors, wiring and loading condition. However with the inverter connected motor system, there are a large portion of inverter not using power factor correction rectifier but only simple bridge connected rectifier. Simple capacitor compensation is not able to provide power factor correction. Active power factor correction or matrix filter is needed. Both harmonics and power factor can be improved and the associated conduction or harmonics loss can be reduced in the wiring. 5. MOTOR EFFICIENCY The motor efficiency is usually higher for high power motor and vice verse. This is mainly before some overhead loss becomes less significant at high power operation. Table 2 shows the typical motor operational efficiency for your reference. Table 2: Minimum nominal full-load motor efficiency for single-speed 3-phase totally enclosed motor Motor rated output power Minimum rated efficiency (%) (P in kw) 2-pole 4-pole 0.75 P < % 79.6% 3 P < % 85.5% 5.5 P < 7,5 85.8% 86.6% 7.5 P < P < P < 37 92% 92.3%

8 110 P < % 94.5% P % 95.1% (Source: EMSD Code of Practice for Energy efficiency of Building Services Installation) Fig 13: The min efficiency of the motor The above shows you the minimum efficiency. When a motor is used with a VSD or VVC, the overall efficiency will be lower. The full table of Table 2 can be referred to the standard. CONCLUSIONS This article discusses the general method for energy saving in machines. This includes the energy management, variable speed drives, variable voltage control and the power factor correction. The variable speed drive gives good saving when the motor has a wide variation of speed whereas the VVC is for relative fixed speed but with wide variation of load. It is also noted that VSD does not ensure good input power quality unless an active power factor correction is used. VVC usually has lower power factor and higher harmonics. Therefore both of them need filter to reduce the harmonics and improve the power factor. It is also noted that capacitor type power factor correction is not effective in such cases because the power factor is mainly caused by harmonics. Reference [1] Code of Practice for Energy Efficiency of Building Services Installation, EMSD Jan