VSI and CSI fed Induction Motor drives

Voltage source inverter (VSI)

Most of  the applications preferred VSI fed to 3 phase Induction motor due to its self starting nature. A normal VSI with 180° conducting mode required only forces commutation.

  

     The circuit for VSI fed synchronous motor is shown  above. It consists of voltage source Inverter i.e. the combination of transistors and diodes which provide variable frequency control to the Induction motor. The terminal voltage kept constant when the load is varying. The output voltage is square wave hence the inverter is named as variable frequency or square wave inverter.

* The frequency is varied by changing the triggering pulses switching times across the transistor.

* The magnitude of output voltage is controlled by controlling DC supply and frequency is controlled by Input wave time period.

* The above figure shows the two different operations one with three phase AC supply and other with DC supply

There are two techniques to get desirable outputs

1.Controlling the inverter voltage by PWM technique

2.Connecting the DC chopper in between rectifier and inverter.

A VSI is a self or separate control feeding Induction motor. Open loop and closed loop operation is possible but VSI provides instability and hunting problems.

      The stator current drawn by the motors when fed to VSI has sharp peaks and is rich in harmonic content which causes additional losses and heating. This technique is used for high and medium speed applications.

PWM technique:

           This technique is used to reduce harmonic, the voltage is controlled with in the inverter itself. The stator currents are less peaking and harmonics reduced, all the effects become minimal and commutation is also possible at low speeds. Dynamic braking can be employed.

When PWM inverter is used two cases may arise

* When inverter is fed to a constant DC source regeneration is straight forward.

* The DC supply to inverter may also be obtained from a diode rectifier which requires additional controlled converter on line side.

        The phase control decides the line power factor. The line power factor improves to unity if a diode rectifier is sufficient in PWM inverter. The field control improves the machine power factor by minimizing the losses. So it is fetching to operate motor under Unity power factor (UPF).

Advantages:

1.VSI drive provides good efficiency

2.Multi-motor operation is possible

3.Commutater less motor (CLM) mode is very stable

4.PWM drive has better dynamic response

5.VSI used for high and medium power applications, PWM used for low and medium power applications.

Disadvantages:

1.The converter cost is high

2.By using VSI there is instability and harmonic content in output

3.VSI is not applicable for low speed operations

4. The speed range of motor is limited to 40% due to harmonics

5. Efficiency inversly proportional toLosses

6. Smooth operation is not possible, troubles occur in torque pulsations

7. harmonics directly proportional to Losses

CSI (Current Source Inverter)

CSI is used in many applications due to its flexibility, reliability and commutation. CSI operates on closed loop and capable of generation, by using CSI variable frequency is obtained.

The variable  frequency control  of an induction motor can also  be obtained using CSI. The inductor is connected in series with the input.

* By varying the thyristors conduction periods and Inverter, the DC current is converted into a three phase current source.

* The stator current is a function of rotor frequency, by keeping flux constant the magnitude of stator is controlled by the rotor frequency.

* When supply is AC the controlled rectifier converts it into variable DC, if the supply is DC  the chopper manages the circuit.

* From the circuit diagram for commutation of  six thyristors the circuit is provided with six diodes and six capacitors.

* The six thyristors are triggered with a phase difference of 180°. The diodes are used for preventing discharge of capacitors through load.

Characteristics:

*For below rated speed, motor operates at constant flux mode.

*For above rated speed, motor operates at field weakening mode.

*For speed equal to rated speed, the voltage reach its rated value and no further increase of speed.

The characteristics are shown below on taking percentage slip on x-axis and percentage torque on y-axis.

Advantages:

1. CSI is robust and simple

2. Four quadrant operation is possible

3. Controlling the speed in simple way

4. Better controlled performance.

Disadvantages:

1. Cost is expensive with PWM technique

2. Unsupported for multi-motor operation

3. CSI not used in open loop control drives

4. Undesirable dynamic performance

5. At no-load condition it is difficult to operate.

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