3 phase rectifiers.

[najnielkp]

hi all,

i have a question about 3 phase rectifiers with rutex drives. do they require a smoothing capacitor , or is the 3 phase dc ripple low enough?

[Karl_T]

All the DC supplies I've ever seen have the capacitor. Its a cheap item, why take the chance?

Karl

[NC Cams]

My Fanuc's use 3 phase drives. They do NOT have a capcitor in the power side.

However, they DO have an inductor IN SERIES with the output which would help smooth out the ripple.

AND there is a resistor across the DC output to absorb any feedback voltage induced by the DC servos.

[Al_The_Man]

Capacitor size can be reduced with a three phase supply, but should always be smoothed to reduce ripple. The rectified AC never goes to zero as in a 1ph supply
But use three phase full wave rectifier.
Al.

[Al_The_Man]

Originally Posted by NC Cams My Fanuc's use 3 phase drives. They do NOT have a capcitor in the power side. .

NC, That is because the drives you have actually switch the 3phase, rather than rectify and use PWM for switching, They operate direct across the line, and are prone to be 'Noisier' electrically than the PWM type.
Al.

[najnielkp]

thahks all for the replies.
can the fanuc rectifiers be used to power the rutex 2040 drives?

anyone done this?

[Al_The_Man]

Usually these drives are an SCR bank set up as three phase rectifier configuration and the Phase angle of the drive signal determines the energy sent to the motor, the result is a varying DC out. So they are in of themselves a drive,as is the Rutex.
Al.

[najnielkp]

thank you al,

what would happen if all scrs had signal , all switched on?

[Al_The_Man]

Then you would have a three phase bridge, but for a DC supply you would still need caps.
Al.

[NC Cams]

The Fanuc's work as follows (perhaps oversimplified yet understandable):

There are 12 SCR's that are sequentially triggered to generate a full wave rectified but un-capacitive filtered DC - the only "filter" is the afore mentioned inductor.

They basically sequentially "fire" the 12 SCR's in batches of 6 (3 pairs). Thus they sequentially fire 6 biased one way to generate CW rotation and the other 6 to generate CCW rotation. This sequencing is done via the a triggering that is generated by the positive side of the incoming AC wave itself.

When they sequentially fire the CW SCR', they are selectively rectifying the "+" section of the AC wave and vice versa to rectify the "-" section of the AC wave.

If you switche ALL SCR's on, you'd effectively be dumping a crowbar across the phases - probably an explosion and surely a lot of smoke. The length of time the SCR's stay on recifies MORE of the + or - section of the wave and thus generates more voltage/current. Essentially, it is a combination of rectifcation and PWM'ing of the signal all at the same time.

Today, we tend to rectify via bridge, store resultant current in caps and then switch in PWM mode via high freq fets. Same effect, slightly different process...

A 3 phase bride would have ripple but NOT as much as a single phase - a simple look at the phase relationships that are in my Fanuc and SCR spindle motor drives shows that. The + and - sides of the AC waves are much more completely overlapping and thus although there are more current peaks, they are closer together with more net area under the curves.

Keep in mind that the inductor does serve as a "filtration" device to further recover energy inducted therein as a result of DC current fluction on the output side.

The Fanuc drives are merely very early generation switchmode drives wherein SCR's were used to rectify and switch DC simultaneously. The development of better diodes and mosfets pretty much obsoletes the use of such drives anymore, especially in small drives as in Geckos, Rutex, et al.

EDIT: the Fanuc drives could NOT power the Rutex or Geckos without careful rework - they're worth more un-molested more to somebody who needs them for service of an older machine.

Check out the International Rectifier section of diode arrays in the DigiKey catalog, specifically illustration D63 for bunches of 3 phase AC to DC diode conversion arrays that are QUITE affordable.

Memo to AL: the documetation for the servo drive and the DC motor drives for the lathe spindle are all the same type - SCR' converting 3 phase AC to unfiltered DC (asside from inductor).

The documentation shows that speed reculation is definitely achieved by regulating the on time of the SCR firing duration - ergo PWM as I understand it but definitely not a "square wave, filtered DC output".

Today, filtered DC is PWM'd by a varying duty cycle of the on time, usually by a square wave (ala the PWM fet based H bridge switchers in my 1996 CNC mill).

However, in 1979, SCR's, properly biased achieved rectification and PWM'ing of the resultant DC (albeit rippled as it was) as best they could with the technology of the day.

An explanation of PWM type drive for servo's and steppers can be found in the app note for UC3637 servo controler IC.

[Al_The_Man]

By indicating that all SCR's could be on I was referring to per bridge, as of course to get reverse polarity full wave, requires two bridges.
All you need to get a bridge output is permanently turn all on of one bridge,
the SCR drive was replaced with the appearance of IGB transistors IGBT's ( Mosfets came later).
SCR's though, cannot be PWM controlled, once turned on they stay on until power is removed or reverse commutated, (complicated).
They are fired by turning them on early or late in the duty cycle and they remain on for the duration of that half cycle until it reaches zero.
You only need 6 diodes for a FW 3ph bridge and the ripple freq. is 6x input frequency so the ripple is far less than 1ph.
Al.

[najnielkp]

thanks for the info nc, al. i have a semikron 400v 100 amp 3 phase bridge i was planning on using. rutex recommends 1000 uf per amp.with less ripple because of the 3 phase, how should i size the capacitor for 30 amps per drive?