IMPEDENCE - SOURCE INVERTER FOR MOTOR DRIVES

OBJECTIVE To implement an Impedance Source Inverter for motor drives. Which can be used as Buck-Boost the input voltage, minimizes the Harmonics, increases the efficiency and reduces the cost.

Traditional Source Inverters Voltage Source Inverter. Current Source Inverter.

Voltage Source Inverter Problems This inverter can be used only for Buck operation. For application where the required output voltage is high, an additional dc-dc boost converter is needed to obtain a desired ac output. This inverter does not have ability to provide ride-through during voltage sags . additional power converter stage increases system cost and lowers efficiency. It is immune to EMI noise. M2 M1 V1 50v D4 1 2 D3 M6 M5 M4 M3 D6 D5 C3 B A TO LOAD C

Current Source Inverter Problems This Inverter can be used for only Boost operation. The dc link Filter Inductor is large, costly and Contributes to losses The open circuit by EMI noise is the major concern Can cause high voltage spikes on motor terminals. Usually not possible to use with more than one motor. Lower efficiency due to high power loss M2 M1 V1 50v D4 1 2 D3 M6 M5 M4 M3 D6 D5 L6 B A TO LOAD C

FEATURES OF Z – SOURCE INVERTER Z - SOURCE INVERTER FOR MOTOR DRIVES FEATURES OF Z – SOURCE INVERTER The impedance source inverter can provide Buck-Boost operation. The output voltage range is not limited. It is less effected by EMI noise. This Inverter provide ride-through during voltage sags It reduces the harmonics therefore increase the power factor and efficiency. The impedance source inverter cost is low. The impedance source inverter concept can be applied in all ac-ac , dc-dc, ac-dc, dc-ac power conversion

Proposed Z-Source Inverter Unit

Block Diagram –Impedance Source Inverter for A.C Drives Three Phase Inverter IMPEDANCE NETWORK Motor AC To DC Rectifier AC Supply PWM Technique PWM Thecni

CONSTRUCTION OF Z - SOURCE INVERTER Z – Source Inverter mainly consist of three parts. They are 3 – Φ Diode Rectifier Impedance Network 3 – Φ Inverter

1) 3 - PHASE DIODE RECTIFIER When an ac supply is given to the Diode Rectifier, it converts the ac voltage to dc voltage. The diodes are numbered in order of conduction sequences and each diode conduct for 120 degrees. The conduction sequence of diode is 16, 63, 32, 25, 54, and 41. The pair of diodes which are connected between that pair of supply lines having the highest amount of instantaneous line-to-line will conduct . It is commonly used in high power applications. A 3 – Φ Diode Rectifier is shown in Fig 1. The waveform of 3 – Φ Diode Rectifier is shown In fig 2. Fig 1

Fig 2

2) IMPEDANCE NETWORK Fig 3 Fig 3 shows a Two Port Impedance Network. These networks are used in filter sections and are also used as attenuators and they are sometimes used in preference to ladder structure in some special applications. This lattice network, L1 and L2 are series arms inductances,C1 and C2 are diagonal capacitances and it consists of split inductors L1 and L2 and capacitors C1 and C2 connected in X-shape. This network is coupled with the main circuits and the source. Fig 3

Inductor and capacitor requirements: The Impedance source network is a combination of two inductors and two capacitors. The impedance source network is the energy storage or filtering element for the Impedance source inverter. This impedance source network provides a second order filter. During Buck operation the two inductors (L1 and L2) approaches to zero and the Impedance source network reduces to two capacitors (C1 andC2) in parallel and becomes traditional voltage source. Similarly, during Buck operation the two capacitors (C1 and C2) approaches to zero, the Impedance source network reduces to two inductors (L1 and L2) in series and becomes a traditional current source. The equivalent circuit of Impedance Network is shown in Fig 4. Fig 4

3) 3 - PHASE INVERTER When the obtainable dc voltage is given to the inverter it converts the dc voltage to ac voltage. Basically, there are two possible schemes of gating the switches. In one scheme, each switch conducts for 180o and in othe scheme, each switch conducts for 120 degrees. Fig 5 shows a 3 – phase inverter. Fig 5

In both these schemes, gating signals are applied and removed at 60 degree intervals of the output voltage waveform. The operation of three phase inverter can be understand by obeserving the waveforms shown in Fig 6 and Fig 7. Switching sequence of inverter is 561 (V1)  612 (V2)  123 (V3)  234 (V4)  345 (V5)  456 (V6)  561 (V1) Fig 6

Fig 7 The equations of 3 – phase inverter are Line to line voltages. 1. Vab = VaN - VbN 2. Vbc = VbN - VcN 3. Vca = VcN - VaN Phase voltages. Van = 2/3VaN – 1/3VbN – 1/3VcN Vbn = -1/3VaN + 2/3VbN - 1/3VcN Vcn = -1/3VaN - 1/3VbN + 2/3VcN Fig 7

OPERATION AND CONTROL OF Z - SOURCE INVERTER When an ac supply is given to the Diode rectifier. The Diode rectifier operate in six possible conduction intervals as shown in Fig 1. In the operation of the Diode rectifier at any instant of time , only two phase having the largest potential difference may conduct , carrying current from ac to dc line. When the potential difference between the two phases “a” and “b” is the largest , diodes Dpa and Dnb conduct as a pair in series with capacitor Ca. Fig 1

To explain the further operation of Z – Source inverter one interval of Diode rectifier is taken as an example as shown in Fig 2. Each interval consist three modes of operation. They are a) MODE 1 b) MODE 2 c) MODE 3 Fig 2

MODE 1 In this mode the Inverter bridge operates in one of the six traditional active vectors , thus acting as a current source (Ii ) . The Z-Source circuit always forces diodes (Dpa and Dnb) to conduct and carry the inductor current (ILd ) and Inverter dc current. (2ILd – Ii) as shown in Fig 3 Fig 3

MODE 2 Fig 4 The inverter bridge is operating in one of the two traditional zero vectors and shorting through either the upper or lower three devices, thus acting as a open circuit viewed from the Z-source circuit. The diodes ( Dpa and Dnb ) conduct and carry currents. Fig 4 shows the circuit for this mode. Again, under this mode, the two diodes (Dpa and Dnb ) have to conduct and carry the inductor current, which contributes to the line current’s harmonic reduction. Fig 4

MODE 3 Fig 5 The inverter bridge is operating in one of the seven shoot – through states. During this mode, both diodes are off, separating the dc link from the ac line. The line current flows to the capacitor (Ca). Fig 5 shows the resultant circuit. This is the shoot-through mode to be used every switching cycle during the traditional zero vector period generated by the PWM control. Depending on how much a voltage boost is needed, the shoot – through interval or its duty cycle is determined. Fig 5

Pulse-Width Modulation (PWM):

Inverter output voltage When vcontrol > vtri, VA0 = Vdc/2

Application Variable Speed Drives For Fuel Cells in motor vehicles. Uninterruptible power supplies. Induction heating. High voltage dc transmission lines The impedance source concept can be applied in all ac-ac , dc-dc, ac-dc, dc-ac power conversion

CONCLUSION This project has presented a new ASD system based on the Z-source inverter. The operating principle and analysis have been given . Simulation and experimental results verified the operational and demonstrated the promising features. In summary, the Z-source inverter ASD system has several unique advantages that are very desirable for many ASD applications. it can produce any desired output ac voltage, even greater than the line voltage. provides ride –through during voltage sags without any additional circuits and energy storage. minimizes the motor ratings to deliver a required power. reduces in-rush and harmonic current and improves the power factor. unique drives features include buck-boost inversion by single power-conversion stage, improved reliability, and low EMI. the Impedance source technology can be applied to the entire spectrum of power conversion.

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