Powersupply questions (took some measurements)...

[radioactive]

In the process of retrofitting my milling machine. I gutted the cabinets on the machine last night. I left the large transformer, rectifier, and capacitor installed and did some testing.

I thought originally the transformer had 240 VAC powering it, you can see in the picture which coils were energized. X1 & X 2 are the output side and go to a 25A bridge rectifier and then on to a 8700 MFD capacitor rated for 150 VDC w/ a bleed resistor.

To test it I just energized the transformer with 120 VAC and measured X1 & X2 at 80 VAC. Across X2 & X3 I get 6.5 VAC. Then I measured across the capacitor and I came up with 108 VDC?

So the rectifier takes the 80 VAC (which is 113 VAC peak-to-peak) and rectifies it to come up with 108 VDC. What I don't understand though, is that I thought this transformer had 240 wired to it, which would imply that the cap sees 216 VDC??? This can't be correct as the drives are only rated for 140 VDC.

I suppose the transformer could have only been wired for 120 VAC originally and this is the voltage it ran the drives at... 108 VDC vs. 140 VDC)

The reason I'm unsure on the transformer voltage is that some of this was disconnected when removed from the owners shop. The hot line was tied into one of the three phase lines and the neutral and ground wires were disconnected when I got it.

My hope was to reuse this transformer for my new Gecko drives which can take up to but not over 80 VDC. I don't think this one is going to work... Ideas?

[vger]

The transformer has 2 primary windings. In parallel they are probably 120V and in series they would do 240V. Looks as though they are wired in parallel so 120V input it is. This is fairly common for multi input voltage transformers.

One thing you could do it rewire it for 240V AC input and connect it to 120V. That would cut your output voltage in half giving you about 54V DC at the cap. The only drawback there is that your available current would also be cut in half.

Steve

edited

[salzburg]

If you are careful undoing the insulation on this transformer you could take
some windings off the primary side till you are down to 69 volts AC. When
you rectify this you get about 80 volts DC at 120 volts AC input.
I just rewound a micro-wave oven transformer to run my Gecko's.
It works.

[Al_The_Man]

Originally Posted by salzburg If you are careful undoing the insulation on this transformer you could take some windings off the primary side till you are down to 69 volts AC. When you rectify this you get about 80 volts DC at 120 volts AC input. I just rewound a micro-wave oven transformer to run my Gecko's. It works.

I assume you meant secondary as usually this impracticable to modify the primary as it is generally wound on first.
On that size transformer you will probably get between 2 and 3 turns/volt.
Al.

[salzburg]

Yes, you are right, I meant secondary. I wound a new one with 12 gauge
transformer wire which I have plenty of.

[radioactive]

Originally Posted by salzburg If you are careful undoing the insulation on this transformer you could take some windings off the secondary side till you are down to 69 volts AC. When you rectify this you get about 80 volts DC at 120 volts AC input. I just rewound a micro-wave oven transformer to run my Gecko's. It works.

Thanks for the advice, I may try this as it would save me $150 if it works out. I guess I'm not out much if I screw it up. What do you use to reinsulate it when you are finished?

My Gecko's arrived on Friday, I can't believe how small they are!!!

[Al_The_Man]

I usually use electrical tape, it is a bit tricky to thread around on a square section TFMR.
The Toroidal are a bit easier.
If you want to find out ahead of time how many turns to take off, Wind on 10 turns of small gauge wire, power up and see what the open circuit voltage is of the winding, the turns/volt can be calculated from this.
Al.

[radioactive]

Thanks for the tip...

[radioactive]

Well, I looked at the number of turns on it for the X2 & X3 connection and it was 5 turns. This gave 6.5 V AC on the output of the transformer. So using this ratio, I removed 18 turns from the X1 & X2 connection and dropped the voltage from 80 VAC to 56.3 VAC. This figures to 79.6 VAC peak to peak and figuring a 4% loss during the rectifying portion (which is what it lost before), it should put me right at 76.4 VDC. This will leave me some margin from the 80 VDC max the Gecko's will accept.

Now the question is why did the existing servo's have smaller transformers connected to their power wiring between the servo and the servo driver cards? One small transformer for each drive...

[vger]

radioactive,

are you shure they are transformers and not reactors to reduce electrical noise emmision in the motor leads ?

Steve

[Al_The_Man]

If they were the supply transformers for each drive, it is sometimes done because of cost and availability at the time of build.
Al.

[radioactive]

The main large transformer is shown above, it dropped the voltage, then it went thru the rectifier and the cap. From there it feed all three servo driver cards with seperate power wiring. From each driver card, the power connections (two wires) went to a small transformer (2.5" square), then two wires left the transformer and headed to the servo.

I haven't measured them to see what the turn ratio is, but vger is probably right. Isn't a reactor just a transformer with a 1:1 turn ratio? What exactly is the purpose, to isolate the driver card from noise or as vger mentioned above to reduce noise emissions from the motor leads?