How To Improve A Variable Frequency Drive

Transcription

1 VARIABLE FREQUENCY DRIVES ROLF LINDEBORG ROLF LINDEBORG ITT Flygt AB, Sweden Rolf Lindeborg, 52, grew up in Älvdalen, a small city in the county of Dalecarlia, Sweden. In 1963, he moved to Stockholm. From 1974 to 1977, he studied at the Stockholm Technical Institute. In 1977, he graduated as an electro-technical engineer. He then continued to study industrial electronics majoring in drive systems at the Royal Institute of Technology in Stockholm. In 1985, he worked as a teacher in electro-technology at the Solna high school. Mr. Lindeborg began his employment with ITT Flygt in 1986, as head of the motor test laboratory within the motor design department. Mr. Lindeborg is currently working in the motor department with motor testing, measurement methods, and technical support for other departments. Abstract To achieve good manageability, efficiency, and energy economy in industrial processes, it is necessary to resort to controllable drive systems. The typical drive system nowadays is a squirrel-cage induction motor fed from a Variable Frequency Drive, or VFD. The more recent generations of these variable speed drives perform very well and have few complications. One complication of importance, however, is caused by the non-sinusoidal output voltage. This circumstance has led to a number of undesirable consequences. Increased motor losses, noise and vibrations, detrimental impact on the motor insulation system, and bearing failure are examples of VFD-related problems. Increased motor losses indicate a derating of the motor output power to prevent overheating. Tests in the Flygt laboratory show that temperature rises may be as much as % higher with VFD compared with conventional sinusoidal power grids. Ongoing intense research and improvement of VFDs have solved many of the problems. Unfortunately, it seems that solving one problem has accented another. Reducing the motor and VFD losses tends to increase the detrimental impact on the insulation. The motor manufacturers are, of course, aware of this. New motor designs (inverter-resistant motors) are beginning to appear on the market. Better statorwinding insulation and other structural improvements promise motors that will be better adapted for VFD applications. 1. INTRODUCTION One of the most serious objections to the squirrel-cage motor has been the difficulty of adapting it to speed control. The synchronous speed of an induction motor is determined by the following equation. 1 f n s = p (rpm) (1.1) n s = synchronous speed f = power grid frequency p = pole number The only way to change the speed, for a given pole number is to vary the frequency. 2. NOMENCLATURE The Variable Speed Drive is known under several different names. One is Variable Speed Drive (VSD). Others are Adjustable Speed Drive (ASD), and Variable Frequency Drive (VFD). In this article, VFD is consistently used. 3. THE BASIC PRINCIPLE In theory, the basic idea is simple, the process of transforming the stable power-line frequency into a variable frequency is basically done in two steps: 1. The sinusoidal voltage is rectified into a DC voltage. 2. The DC voltage is chopped up into an AC voltage of the desired frequency

2 A VFD basically consists of three blocks: the rectifier, the DC-link, and the inverter. tional disadvantage of the CSI. The transients can reach nearly twice the nominal voltage in the worst cases. There is also a risk that the winding insulation will be worn out prematurely, if this VFD is used. This effect is most serious when the load does not match the VFD properly. This can happen when running at part load. This kind of VFD is losing its popularity more and more. Conventional CSI Figure 1. Basic VFD configuration. 4. DIFFERENT TYPES OF VFDS 4.1. PWM Voltage Source Inverter (VSI) The PWM is widely used and dominates the VFD market. They are available from a few hundred watts up to megawatts. A PWM does not have to match the load exactly, it need only ensure that the load does not consume more current than the PWM is rated for. It is quite possible to run a 1-kW motor with a 5-kW PWM. This is a great advantage that makes operation easier for the user. Nowadays, the PWM s inverter circuit is designed using fast power transistors (IGBT). Modern PWMs perform very well, and are not far behind designs using a sinusoidal power grid at least not in the power range up to 1 kw or so. Figure 2. VSI circuit (PWM) VSI PW M 4.2. The Current Source Inverter (CSI) The CSI is a rough and rather simple design compared with the PWM. It uses simple thyristors or SCRs in the power circuits, which makes it less expensive. It has also been judged to be very reliable. The design makes it short-circuit proof because of the large inductors in the DC link. It is bulkier than the PWM. Earlier, the CSI was the best choice for big loads. A disadvantage with the CSI is the need of matching to the load. The VFD has to be designed for the motor used. In fact, the motor itself is a part of the inverted circuit. The CSI supplies the motor with a square-shaped current. At low speeds, the motor produces a cogging torque. This type of VFD will generate more noise on the supply grid compared to the PWM. Filtering is necessary. Heavy voltage transients in the output voltage are an addi- M 3 Figure 3. CSI circuit Flux Vector Control (FVC) A FVC is a more sophisticated type of VFD that is used in applications having extreme control demands. In paper mills for example, it is necessary to control speed and stretching forces very precisely. A flux-vector-controlled VFD always has some kind of feedback loop. This kind of VFD is generally of minor interest in pump applications. It is expensive, and its benefits cannot be taken advantage of. 5. EFFECT ON THE MOTOR An induction motor operates best when supplied with a pure sinusoidal-voltage source. This is mostly the case when connected to a robust utility grid. When a motor is connected to a VFD, it will be supplied with a non-sinusoidal voltage more like a chopped square voltage. If we supply a three-phase motor with a symmetrical three-phase square voltage, all the harmonics that are multiples of three, as well as the even numbers, will be eliminated because of symmetry. But, still left are the numbers 5 ;7 and 11;13 and 17;19 and 23;25 and so on. For each pair of harmonics, the lower number is reverse rotating and the higher number is forward rotating. The speed of the motor is determined by the fundamental number, or number 1, because of its strong dominance. Now what happens to the harmonics? From the point of view of harmonics, the motor seems to have the rotor blocked, which means that the slip is approximately 1 for the harmonics. These provide no useful work. The result is mostly rotor losses and extra heating. In our application in particular, this is a serious outcome. With modern technology, however, it is possible to eliminate much of the harmonic content in the motor current, thereby reducing the extra losses Early VFDs The earliest VFDs often used a simple square voltage (see Figure 1) to supply the motor. They caused heating-up problems and the M

3 Figure 4. Shown are the output voltage and the current in one phase from a 6-pulse inverter. Switch frequency = fbase. This type of inverter provides a motor current far from sinusoidal-shaped. motors ran with a typical noise caused by torque ripple. Much better performance was achieved by simply eliminating the fifth and the seventh. That was done through some extra switching of the voltage signal. Switching frequencies up to khz are available for VFDs in the medium-power range (up to some tens of kw). The motor current with this type of VFD will be nearly sinus shaped. Figure 6 shows the motor current at 5 Hz for a pump fed from an ABB ACS VFD. At a high switching frequency, motor losses are kept low, but losses in the VFD will increase. The total losses will become higher at excessively high switching frequencies. 6. SOME MOTOR THEORY The torque production in an induction motor may be expressed as T = V Γ B [Nm] V = Active rotor volume [m 3 ] Γ = Current per meter stator bore circumference B = Flux density in the air gap E B = proportional to ω = E 2 π f ω = angular frequency of the stator voltage E = induced stator voltage (6.1) (6.2) Voltage % Voltage/Frequency Ratio for Constant Torque and Square Torque Field Weakening Range Constant Torque Square Torque Figure 5. The output voltage and current from a 18-pulse inverter. Switch frequency, about Hz. This inverter provides a considerably better motor current and thus smoother torque VFD Today Nowadays, the technique is more sophisticated and most of the disadvantages are history. Development of fast power semiconductors and the micro-processor has made it possible to tailor the switching pattern in such a way that most of the harmful harmonics are eliminated Figure 6. Motor current at 5 Hz, from an ABB ACS VFD, f switch = 11 khz. Output Current from VFD Type ABB ACS Switching Frequency 11 khz Current (A) Time (ms) Figure 7. Various torque characteristics. To obtain the best performance at various speeds, it becomes necessary to maintain an appropriate magnetization level for the motor for each speed. A range of various torque characteristics is shown in Figure 7. For the constant torque load, the V/F ratio must be constant. For the square torque load, a constant V/F ratio will result in excessively high magnetization at lower speed. This will generate unnecessarily high iron losses and resistance losses (I2R). It is better to use a square V/F ratio. The iron losses and I2Rlosses are thus reduced to a level more acceptable for the actual load torque. If we look at Figure 7, we find that the voltage has reached its maximum and cannot be increased above the 5-Hz base frequency. The range above the base frequency is called the fieldweakening range. A consequence of this is that it is no longer possible to maintain the necessary torque without increasing the current. This will result in heating-up problems of the same kind as with normal undervoltages run from a sinusoidal power grid. The VFD s rated current will likely be exceeded

4 7. RUNNING IN THE FIELD-WEAKENING RANGE Sometimes, there is a temptation to run the pump at frequencies above the commercial power grid frequency in order to reach a duty point that would otherwise be impossible. Doing so calls for extra awareness. The shaft power for a pump will increase with the cube of speed. An overspeed of 1% will require 33% more output power. Roughly speaking, we can expect that the temperature rise will increase by about 75%. There is, nonetheless, a limit to what we can squeeze out of the motor at overspeed. The maximum torque of the motor will drop as a function of 1/F in the field-weakening range. Maximum Torque % Figure 8. Maximum torque drop in field-weakening range. It is obvious that the motor will drop out if the VFD cannot support it with a voltage that corresponds to that needed by the torque. 8. DERATING In many cases, the motor is run at maximum capacity from a sinusoidal power grid and any extra heating cannot be tolerated. If such a motor is powered from a VFD of some kind, it most probably must be run at lower output power in order to avoid overheating. It is not unusual that a VFD for big pumps above 3 kw will add extra motor losses of 25 3%. In the upper power range, only a few of the VFDs have a high switching frequency: 5 to 1 Hz is usual for the former generation of VFDs. To compensate for the extra losses, it is necessary to reduce the output power. Flygt recommends a general derating of 1 15% for large pumps. Since the VFD pollutes the supply grid with harmonics, an input filter sometimes is prescribed by the power company. This filter will decrease the available voltage by typically 5 1%. The motor will consequently run at 9 95% of nominal voltage. The consequence is additional heating. Derating might be necessary. Example Assume that the output power for the actual pump motor is 3 kw at 5 Hz and the temperature rise is 8 C using a sinusoidal power grid. Extra losses of 3% will result in a motor that is 3% warmer. A conservative assumption is that the temperature rise varies with the square of shaft power. Maximum Torque as Function of Speed Torque % 5 Field Weakening Range 7 T p = 1/F In order not to exceed 8 C, we have to reduce the shaft power to P 1 reduced = 3 = 263kW 1.3 The reduction can be achieved either by reducing impeller diameter or by speeding down. 9. VFD Losses When the total efficiency of a drive system is determined, the internal losses of the VFDs must be included. These VFD losses are not constant and not easy to determine. They consist of a constant part and a load dependent part. Constant losses: Cooling losses (cooling fan) losses in the electronic circuits and so on. Load dependent losses: Switching losses and lead losses in the power semiconductors. ETA VFD % Figure 9. VFD efficiency curve. VFD Losses as Function of Speed, at Cubic Load. PWM with IGBT Switching Frequency= 3 khz ETA 45 kw ETA 9 kw % ETA 2 kw % Figure 9 shows the VFD efficiency as function of the frequency at a cubic load for units rated at 45, 9, and 2 kw. The curves are representative for VFDs in the power range of 5 3 kw; with the switching frequency equaling about 3 khz and with an IGBT of the second generation. 1. EFFECTS ON MOTOR INSULATION The output voltages from modern VFDs have a very short voltage rise time. du = 5V/µs is a common value. dt Such steep voltage slopes will cause undue stress in the insulation materials of the motor winding. With short rise times, voltage in the stator winding is not uniformly distributed. With a sinusoidal power supply, the turn-turn voltage in a motor winding is normally equally distributed. With a VFD on the other hand, up to 8% of the voltage will drop across the first and the second turn. Since the insulation between the wires constitutes a weak point, this may prove to be hazardous for the motor. A short rise time also causes voltage reflection in the motor cable. In the worst case, this phenomena will double the voltage across the motor terminals. A motor fed from a 69-volt VFD might be

5 exposed to up to 1 9 volts between phases. The voltage amplitude depends on the length of the motor cable and the rise time. With very short rise times, full reflection occurs in a cable 1 to meters in length. To ensure function and ample motor life time, it is absolutely necessary that a winding be adapted for use with a VFD. Motors for voltages above 5 volts must have some form of reinforced insulation. The stator winding must be impregnated with a resin that ensures an insulation free of bubbles or cavities. Glow discharges often start around cavities. This phenomena will eventually destroy the insulation. There are ways to protect a motor. Over and above a reinforced insulation system, it might be necessary to insert a filter between the VFD and the motor. Such filters are available from most well-known VFD suppliers. A filter will typically slow down the voltage rise time from Might it be necessary to use insulated bearings in order to prevent a zero-sequence current from finding its way to the bearings? Only when we have found all the answers, will we be able to make intelligent decisions about the use of a VFD. Gunnar Henriksson/ITT Flygt du = 5V/µs to 5 V/µs dt 11. BEARING FAILURE Breakdown of rotating machinery can often be related to bearing failure. In addition to excessive heating, insufficient lubrication or metal fatigue, electric current through the bearings may be the cause behind many mysterious bearing breakdowns, especially with large motors. This phenomenon is generally caused by nonsymmetry in the magnetic circuit, which induces a small voltage in the stator structure, or by a zero sequence current. If the potential between the stator structure and the shaft unit becomes high enough, a discharge will take place through the bearing. Small electric discharges between the rolling elements and the bearing raceway will eventually damage the bearing. The use of VFDs will increase the probability of this type of bearing failure occurring. The switching technique of a modern VFD causes a zero-sequence current that, under certain circumstances, finds its way through the bearings. The easiest way to cure this problem is to raise an obstacle for the current. The usual method is to use a bearing with an insulating coating on the outer ring. 12. CONCLUSIONS The use of VFDs is not totally troublefree. Careful planning must be done. There are a number of questions that must be sorted out and solved during the design work. Will it be necessary, for example, to limit the available shaft power to prevent excessive heating? It may prove necessary to run at lower output power to avoid this problem. Will the motor insulation resist effects from the inverter? Is filtering necessary? Modern, efficient inverters have detrimental impact on the insulation due to high switching frequency and short voltage-rise time. Which maximum cable length can be used without producing full voltage reflection? The voltage amplitude depends on both the cable length and the rise time. With very short rise times, full reflection will occur in cables 1 to meters long. REFERENCES Thorborg Kjeld, Power Electronics, ISBN Persson Erik, Transient Effects in Application of PWM Inverter to Induction Motors, IEEE Trans. IAS, vol. 28 no. 5, september/oktober Lindeborg Rolf, Variable Frequency Drives, ITT Flygt,