Cycloconverter VSI and CSI

Cyclo converter fed synchronous motor drive

Cyclo converter is a device which perform the action of fixed AC frequency to variable. The below block diagram shows self controlled synchronous motor fed from a three phase cyclo converter.

The three phase supply is given to cyclo converter and the output is fed to synchronous motor. The rotor position sensor is an integral part of the self controlled synchronous motor drive.

Applications of synchronous motor drives:

*Compressors, cement plants

*fan steel rolling mills

*electro optical sensors

*hall effect sensors

*digital encoders.

There are two modes when cyclo converter feeding a synchronous motor

1.Line commutated cyclo converter (VSI):

It is a single stage frequency converter. By using cyclo converter variable frequency of synchronous motor is derived. It is a phase controlled and line commutated.

*At high output frequency the distortion occurs in waveform. So to maintain the limited output frequency they allowed frequency us about one third of the supply frequency which is suitable for low speed and low frequency operations.

*Cyclo converter allows power flow in both the directions, four quadrant drive with regeneration facility can be developed. The harmonic content become minimum by making output voltage waveform into segments.

Advantages:

1.Machine inductance smoothens the current waveforms.

2.The significance of harmonics are low

3.Torque pulsations are minimized

4.Better efficiency

5. Machine power factor improved by decreasing line power factor

Disadvantages:

1.The drive is accepted only at low speeds

2.It requires rotor position sensor.

2. Load commutated ( CSI):

Cyclo converter is a single stage frequency conversion equipment. It accomplish four quadrant operation. The converter operates on trapezoidal excitation as a current source to the motor drive load commutation

*There is a necessity of commutation assistance at low speed because at low speed mode the cyclo converter is in voltage source operation. The Field winding of machine rated for large current.