CSI vs VSI? 1

[sparkyMV]

Can anyone help me explain in laymans terms the difference between a current source and a voltage source drive. I know basically the difference(diode front end compared to scr-active front end) but find it hard to explain. Even if there is link to go to would be great.

 

[CJCPE]

The power circuit topology of an SCR type current source input (CSI) drive consists of a controlled rectifier input section, a DC current filter inductor and a six-pulse SCR inverter bridge. The output current of the rectifier/filter section is regulated such that the minimum current is sufficient to supply the magnetizing current of the motor and the maximum current is sufficient for the motor to produce a maximum design-point torque at design-point voltage and slip. The inductance of the filter choke is large enough to assure that the input rectifier does not reach discontinuous current at minimum output current. The reference for the DC current regulator is derived from the error signal of an outer control loop that regulates the motor voltage or speed.

The inverter bridge is switched in a sequence to produce a stepped current waveform that is like the voltage waveform of a VVI drive. The output voltage waveform is like the output current waveform of a VVI drive. With an output voltage regulated CSI drive, the inverter is switched at the desired operating frequency and the voltage regulator receives a reference to produce the proper V/Hz. The error signal from the voltage regulator causes the DC current regulator to produce a current that results in the DC bus voltage needed for the required V/Hz.

 

[sparkyMV]

Thanks CJPE. We have both types of drives here and I had to stop one of the guys from shorting the output on the VSI drive like you can do for a CSI drive when doing start up etc. One thing I tried to explain is that with a CSI drive the output voltage is depended on the current and load ( current can remain relatively constant)and the voltage will vary accordingly and with a VSI drive the output current will vary depending on the volatge and load (the output voltage can remain relatively constant). Just having a hard time explaining how a CSI drive regulates current and why you can short the output and not cause failure like a VSI

 

[CJCPE]

With a CSI drive, there is only a large inductor in the DC link, no capacitor. The controlled rectifier can relatively easily control the current in the inductor as its only load. That means that it is relatively easy for the CSI drive to control the current into a short circuit. That is essentially what a DC drive is doing when starting a DC motor.