[howanc7]

Hi all

I am retrofitting a Fanuc 5T controller that used a transformer to provide 60 VAC and 90 VAC for the X & Z drive boards. On the drive boards were SCR's that did the AC to DC, for the Fanuc Brush type DC motors. I am fitting AC/DC Servo Drives (30 Amps), in place of these drive boards, so need to rectify the AC voltages from my original transformer, into DC voltage, for the servo drives. Can anyone help with what quality of DC is required? Does it need to be fully smoothed? Does anyone have a suggestion of diodes and capacitors, that I should use?

It is a 3 phase transformer.

All help much appreciated, regards, Colin

[holbieone]

Ajax CNC Components | CNC Retrofit Control Systems for Milling machines, Routers and Lathes. Runs Mach3 or CENTROID Software

[howanc7]

You missed the fact my transformer is 3 PH.

[holbieone]

use one leg to the center tap

[howanc7]

Thanks Holbieone,
but here in NZ we don't have star configeration, just 3 PH and earth. I would get 230 Volts between phases, but I think I will go down the way of a 3ph bridge rectifier (6 diodes in a single package), with a small capacitor on the output. I think this will achieve my goal, at under NZ$100 per motor.
Cheers, Colin

[cleonard]

Since you have three phase power the ripple after a bridge rectifier is much less than a single phase setup. I think that you will be good with the rectifier and the capacitor. The final voltage depends on the winding configuration. A delta wired secondary will give a lower voltage but more current than the wye connection.. The difference between Wye and Delta is a factor of 1.73 so it is significant.

[howanc7]

The transformer I want to use was previously used with SCR rectifiers, built in to the old motor driver boards. The transformer is clearly marked with the input 3 ph voltage as 230 volts. And the two outputs, one for the Z motor and one for the X motor, 90 volts and 60 volts respectively. On measuring the speed achieved on the motors, with a 12 volt battery at 13.7 volts, and the rated full speed of the motor (2000 rpm), I would need 160 volts to achieve this on the larger motor, but my guess is, that max speed is not required, hence the lower voltage. The problem I have is what type, what size, and how to wire suitable capacitors to the output of the bridge rectifier I will use. The rectifiers I plan using have a 1200 Volt, 30 amp rating. The motors have a max current of 20 amps and 7 amps respectively. I have looked and looked at web pages that give multiple formulas for calculating capacitors, but all I achieve is more confusion. I now think about the inrush current that can happen charging too big capacitors, and there is talk of special starting conditions so that it can only be switched on when the AC is at 0 volts. All very confusing! All I can deal with is very simple instructions, but I can't seem to find any. Help )
Colin

[Al_The_Man]

As already stated, if you are using 3phase, the capacitors required are much less due to the percentage ripple with 3 phase rectification against 1ph (100% ripple),
The capacitor simply go on the output of the 3 phase rectifier, if this is what you are using.
And again your DC is going to be x 1.414 of your AC, it is not necessary to size the P.S. voltage to the motor rated voltage, anywhere between x 110% and 130% is normal as the maximum rpm is governed by the controller and drives.
Another way to reduce the output if you have star secondary transformers is to use the star point as common and use half wave rectification on each phase.
Al.

[howanc7]

So from the + to the - on the output side of the rectifier I wire an electrolytic capacitor with the correct polarity.
Would a 200 volt capacitor be suitable, or could there be 600 volt surges on power up?
Will a 1µF, or a 100µF be required?
Sorry, but I have only replaced existing ones, or used what I have been told to use, up to now.
Colin

[Al_The_Man]

You will need a bit bigger than 100µf, but the final value will depend on whether you are using 3ph and if it is full wave or half wave as I suggested.
The voltage of the Capacitor should be at least 1.5x what the resultant DC is.
Al.

[cleonard]

Take a look at the transformer. It may state if it's delta or wye (also known a Y or star) connected. If it has a model number you might be able to look it up and get the specs.

[howanc7]

We have 3 wires and earth going in, so it must be a delta transformer, we have 3 wires coming out, stateing that there is 90 volts on each wire, so I guess that is delta as well. we are going to attach these three wires to a 1200 volt, 30 amp, full wave rectifier (3 wires in, 2 wires out), and with a suitable capacitor, connect this to our new motor drivers. What I need to know is what size capacitors to put across the output, to the new drivers, when I expect a maximum ampage of 20 amps. Sorry to be so thick, but I obviously am, as I can not seem to formulate the right question.

Cheers, Colin

[howanc7]

Thanks Al, much appreciated. That is what I will attach, and I'm sure all will be well.

Cheers, Colin