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THREE-LEVEL CONTROL IN GENERAL PURPOSE INVERTERS – A SOLUTION TO SOME OF THE PROBLEMS WHICH RESULT IN PREMATURE MOTOR FAILURE By W Law, Applications Engineer, Varispeed, a Division of Hudaco Transmissions (Pty) Ltd, P O Box 4202,Halfway House, 1685, South Africa; e-mail: [email protected] Abstract This paper describes the application of a power control method known as three-level control to the inverter section of general purpose DC-link AC variable speed drives. The method of implementation and the resultant improvements and benefits to motor performance are described Introduction Most AC variable speed drives commercially available today are of the two-level control type and have a number of inherent problems arising from the design of the inverter bridge, The results of these problems include premature breakdown of motor insulation, electrolytic bearing corrosion and the generation of electrical and audible noise. An alternative approach is to use an inverter bridge that is capable of performing three-level control of the output waveform. While the variable speed drive itself may, initially appear to be more expensive, the need for output chokes, filters or motor terminating devices is eliminated and motor failures are substantially reduced. The operation of a three-level inverter is described in simple terms below. The most common type of AC drive available today is the DC link converter with an inverter bridge made up of six power devices. This configuration produces a two-level output waveform. An improvement on this type of design is the three-level inverter which utilises twelve IGBT’s in the inverter bridge. Two-level inverters A DC link converter consists of an input rectifier, which may be passive or active, a DC link section with a capacitor (and sometimes an inductor), and an inverter bridge made up of six power devices; normally IGBT’s or variations thereof. This configuration generates a good approximation of a sine wave with a pulse width modulated two-level output waveform. Fig. 1 below shows the basic power circuit of a two-level inverter. The control mode of the inverter output could be a variable voltage/variable frequency (v/f) mode, a sensorless vector or a flux vector type of control. The waveforms applied to the motor by this type of inverter can have the following disadvantages: Switching spikes with a peak of up to twice the DC bus voltage applied to the motor, i.e. spikes of 1 300 volts on a 400 volt system and 1 800 volts on a 550 volt system. These can cause premature insulation failure on older or poor quality motors. To prevent such a failure the use of output chokes, filters or a termination device at the motor is required. There is some leakage current that flows back to the inverter via the designated earth return path, the motor frame and the motor bearings, shaft and driven machine. That part of the leakage current that flows through the motor bearings can cause damage referred to as electrolytic corrosion and is due to the vibration caused by the waveform on the steel parts. The switched or modulated nature of the output waveform can generate a considerable amount of electromagnetic interference. The waveform will also cause audible noise due vibration of the laminations around which the stator windings of the motor are wound. + Q1 Q3 Q5 + L1 L2 L3 U V W Q2 Q4 M Q6 _ DC LINK CAPACITOR RECTIFIER INVERTER Fig 1 The Power Circuit of a Conventional Two-Level Inverter Three level inverters A three-level inverter differs from a two-level type in the design of the inverter bridge. A three-level output waveform is produced when twelve switching devices are used in the inverter stage. A third voltage level in the DC bus is created by using two capacitors in series for the DC bus capacitor as well as two additional clamping diodes in each phase of the inverter bridge. The third voltage level is created at the centre point between the two capacitors and is referred to as the ‘neutral’. The switching sequence for the power devices is described in detail later. Figure 2 below shows the basic power diagram of a three-level inverter. + Q1a Q3a Q5a Q1b Q3b Q5b + U L1 L2 L3 V W M NEUTRAL Q2a Q4a Q6a Q2b Q4b Q6b + _ RECTIFIER DC LINK CAPACITOR INVERTER Fig 2 The Power Circuit of a Three-Level Inverter Comparison between Two-Level and Three-Level Waveforms. In both two-level and three-level inverters similar forms of pulse width modulation are used. The difference with the three-level inverter is that, with specially controlled switching of the twelve power devices in the inverter bridge an additional voltage level appears in the waveform. The diagrams below in Fig 3 illustrate the difference in appearance between the output voltage waveforms generated by two and three-level inverters. The notable feature is that with three-level control the peak value of the modulated Line Voltage Phase Voltage waveform is half of the DC bus voltage VPN , while in the two-level case the peak value is the full DC bus voltage. Even though the combined three-level waveform is a series of “blocks” of pulse with modulated wave, the magnitude of the voltage pulses that are applied to the motor measure only half of the peak value of the DC bus voltage. VPN VPN/2 VPN/2 VPN Conventional Two Level Control Three Level Control Fig. 3 Comparison of Two-Level and Three-Level Wave Forms Generation of a Three-Level Waveform. The switching sequences of the power devices for one phase of a three-level inverter are shown in Fig 4 below. In the descriptions below it should be understood that when a switching device is said to be ON, it implies that it is under pulse width modulated control from the control section of the inverter. + + Q1a + Q1a + Q1a + + Q1b Q1b I I U-PHASE U-PHASE NEUTRAL Q2a NEUTRAL Q2a + Q2a + Q2b _ I U-PHASE NEUTRAL + Q1b Q2b _ Q2b _ Q1a & Q1b are on Q1b & Q2a are on Q2a & Q2b are on Positive Current to Load Positive Current to Load Positive Current to Load Via Clamping Diode Via flywheel diodes + + Q1a + Q1a + Q1a + + Q1b Q1b I U-PHASE NEUTRAL Q1b I I U-PHASE U-PHASE NEUTRAL Q2a NEUTRAL Q2a + + Q2a + Q2b Q2b _ Q2b _ _ Q1a & Q1b are on Q1b & Q2a are on Q2a & Q2b are on Negative Current from Load Negative Current from Load Negative Current from Load Via flywheel diodes via Clamping Diode Fig.4 Switching Sequence for one Phase of a Three-Level Inverter. Characteristics of a Three-Level Waveform. When any switched waveform such as the pulse width modulated types from inverters are fed to a motor via a cable, the natural capacitance of the cable is charged and discharged by the “DC pulses” in that waveform. Voltage spikes occur which force the peak voltage of the waveform to rise above the DC bus voltage in the inverter. The longer the cable, the higher the capacitance, and therefore the higher the switching spikes will be. Fig 5 below illustrates typical spikes that could occur on a 400 volt inverter with various lengths of cable. Fig.5 The Effect of Cable Capacitance on a Pulse Width Modulated Waveform. Because the “blocks” of pulses in a three-level waveform have a magnitude of only half of the DC bus voltage the peak of the switching pulse is considerably reduced, in fact it will be 1,5 x V pn as opposed to the 2 x Vpn experienced with a two-level system. A diagrammatic representation comparing two pulses in each type of inverter is shown below in Fig.6. Suppresion effect 1,5xVpn Level 3 2xVpn Level 2 Vpn Level 1 Two-Level Switching Three-Level Switching Fig.6 Comparison Between Two-Level and Three-Level Switching Waveforms. It is clear from the diagram above that the switching spikes will be lower in a three-level inverter than in a two-level inverter by half of the DC bus voltage. In a 400 volt system this is equal to about 270 volts and in a 550 volt system the reduction is about 390 volts. Advantages of a three-level inverter. There are several significant advantages in using a three-level inverter. These are summarised below. Motor Spike Protection Devices. When two-level inverters are to be used on motors with long cable runs, i.e. in excess of about 50 metres, output inductors, output filters or some form of motor termination device is normally used, particularly on 550 volt systems and where older motors with perhaps poor insulation are to be used. With a three-level inverter no spike suppression device is needed between the inverter and the motor thus saving on capital cost, space and installation time. Leakage Current and Bearing Current There are three paths via which leakage currents return to the inverter. As shown in Fig. 7 below these are: The installed earth wiring. The motor mountings and earth. The motor bearings (and possibly the bearings of the driven machine) and earth. The bearing current can cause damage to the bearings due to arcing across the bearing surface. The current is concentrated through the contact points of the bearings and wear occurs. Arcing also causes the lubricating grease to loose its properties compounding the problem. A three-level inverter will therefore improve bearing life, due to reduced leakage current. The leakage current is reduced because the three-level waveform is a closer resemblance to a sine wave and thus dv/dt is considerably reduced. Audible Noise. Inverters cause audible noise in a motor due to the dv/dt of the pulse width modulation at carrier frequency. The motor laminations tend to vibrate at this frequency. As the three-level waveform is significantly closer to a pure sine wave with low dv/dt the total noise level can be reduced by up to 5 dB. Selecting a carrier frequency well out of the audible range, say 12-15 kHz will further improve the audible noise. Performance. Depending on the design of the control algorithms in the inverter a three-level type is capable of providing higher motor torque at low speeds, typically 150% at 0,3 Hz in open loop vector mode . Faster torque response is also achieved. These improvements are mainly due to the improved sine wave output. Conclusion. The three-level inverter, although more costly than a conventional two-level type due to the increased number of power devices, can in fact produce overall savings when all of the factors discussed above are considered. References. Information from Yaskawa Electric Corporation.