Capactor question

[cnc_4_me]

Drools said:
“In my current setup I have 2 of the 430-9001 transformers in series running the G540 with 3 nema23 steppers, XY-axis are using CNCRP ~600oz@3.5 amd stepper and the Z-axis uses a Tormach 280oz@3amp stepper. The G540 current resistor is set at 2.8 amps.
The bridge rectifier I'm using is the MB251 100V@25amp. (I also have 10 of these)

In the new setup I'm upgrading the X-axis to a Kelinginc(automationtechnoligies) Nema34 1200oz@6amps with the G203v. The Y-axis will stay the same and the Z-axis will get the stepper from the X-axis. This leaves me 2 free drivers on the G540 to maybe use the new plastic extruder I seen in this thread. CNC router to 3D printer As well I have always had my mind set on a 4th axis.”

I did not understand if all G540‘s were set to 2.8 amps or just the the Tormach 280oz@3amp stepper. What G540 current setting are you using for the CNCRP ~600oz@3.5 amd steppers,

[Drools]

I thought that might raise an eyebrow, yes the G540 is currently set to 2.8amps for all axis. This was the setup for my original build which used 3 x Tormach 280oz steppers. I just unplugged the control box from that machine and plugged it into the new machine. My intension all along was to add a heat-sink to the G540 and increase the current resistor to 3.5 when I build the new controller for the Nema 34 upgrade. I have been able to cut at 100ipm no problem with the way it is setup now, so I just left it.
I found a good sized heat-sink to be used with the G540 so I’m comfortable upgrading the current to 3.5amps. I’m just finishing up the wiring for the 3 transformers which in testing produced the exact voltages I listed above. I have yet to go hunting for the resistors I need for the caps.
I understand running the ~600oz steppers at 2.8amps effectivly detuned(lowered) the output.

[Drools]

One question I have while building the power supply section is this, the MB251 bridges have a metal casing. Since I'm using 2 of them does the casing have to be isolated from each other or can I bolt them to the same heat-sink?
Note: not the heat-sink used on either the G540 or the G203V, I have a seperate heat-sink for the bridges.

[cnc_4_me]

One question I have while building the power supply section is this, the MB251 bridges have a metal casing. Since I'm using 2 of them does the casing have to be isolated from each other or can I bolt them to the same heat-sink?
Note: not the heat-sink used on either the G540 or the G203V, I have a seperate heat-sink for the bridges.

I am fairly certain the metal case on the heat sink is isolated but it would be easy to check with an ohm meter. Set the meter to 1x or 10 ohms and connect one lead to the case and probe all 4 of the terminals to see if any conduct to the case.

I think you may have a problem wiring up the transformers as you want. Now that i know the current you are using I can run some calculations.

[cnc_4_me]

I thought that might raise an eyebrow, yes the G540 is currently set to 2.8amps for all axis. This was the setup for my original build which used 3 x Tormach 280oz steppers. I just unplugged the control box from that machine and plugged it into the new machine. My intension all along was to add a heat-sink to the G540 and increase the current resistor to 3.5 when I build the new controller for the Nema 34 upgrade. I have been able to cut at 100ipm no problem with the way it is setup now, so I just left it.
I found a good sized heat-sink to be used with the G540 so I’m comfortable upgrading the current to 3.5amps. I’m just finishing up the wiring for the 3 transformers which in testing produced the exact voltages I listed above. I have yet to go hunting for the resistors I need for the caps.
I understand running the ~600oz steppers at 2.8amps effectivly detuned(lowered) the output.

It looks like you will just squeak by with your transformer current rating. As close as can be calculated your bottom two transformers will have to supply 15.6 amps AC to the bridge to run all 3 motors at the same time. Assuming you will be doing that.

[Drools]

Hi cnc_4_me, if I understand correctly the transformers will still only product 15amps even though I have 3 in series. 2 transformers will supply 30VAC to one bridge, running the 2 nema 23 motors. The third transformer will add another 15VAC for 45VAC to run the Nema 34 stepper. If I only use 2/3 of the a motors current then 3.5 + 3.5 + 6 = 15, and 2/3 of 15 is 10. So if I'm correct I will only need 10 amps. Please say I'm right
I hope it will work.
I uploaded a very simple schematic of what I'm wanting to do.

[Drools]

Transformer calculations pending, I found 2 x 5700uf@70VDC in my drawer they are used but still good. Is 70VDC cutting it too close for for the 64 volts we calculated?

[cnc_4_me]

Your schematic looks OK, i assume that the green is DC- and red is DC+. You just have to connect you 2 green wires from the bridges together or you will have a “floating” power supply’s. Best case it would not work, worst case you would start blowing motor drives. The point were the two green wires connect will be called your DC ground, this is the point that will connect to your motor drivers GND connection. From GND to your first bridge is your low voltage output 38VDC and from GND to your 3rd transformer and second bridge is your high voltage output 58VDC.

It might be best if you add that to your schematic with DC GND and + labeled and add your capacitors with polarity marked on them.

64 Volts on a 70 volt capacitor is to close. Remember that there can be variations in electrical power to your house that could raise the transformer voltage up a little and when you decelerate a stepper that loss in mechanical energy goes on to making voltage like a generator and gets put back in the caps, so the voltage on the caps goes up with deceleration. 20 volts is considered the minimum safety margin from power supply voltage to capacitor rating, at these voltage levels. and 30VDC is a good conservative design.

By the way as you are finding out my calculated DC voltage is low compared to your measured values. That is because a transformer has loose regulation. With no load the 14 volt output could be a couple of volts higher or so. The transformer mfg only guarenties that you have 14 volts with a 15 amp load.

One last comment on your schematic, while it is valid, by convention you usually draw a totem pole device like you are making with GND at the bottom and plus at the top. Your top 2 transformers should be at the bottom with the DC- terminal on the bridge labeled as DC GND. Then you have your low voltage plus and your high voltage plus on top of that. That makes for a easy to read schematic.

For the transformer calculations the thing to understand here is that the AC RMS current in to a bridge rectifier with a capacitor on the DC side is 1.8 times the DC output current. So 1 amp of DC costs 1.8 amps of AC.

Your Y and Z motors on the G540 will be set to 3.5 amps each = 7 amps, but they only draw 2/3 that because of the PWM’ing of the gecko drives. 2/3 is .667 so 7 x .667 = 4.67 amps DC from the bridge. 1.8 x 4.67 amps = 8.4 amps AC from transformer. Your transformers are in series so this current flows through the two transformers connected to the bridge across them.

The third transformer is also in series with the other two transformers but it is not in the loop of the bridge on the other transformer so it does not supply any current for them. The other bridge for 58VDC is connected across all three transformers so they will all see this bridges current draw.

The bridge for the 58 volt supply will be connected to a 6 amp motor, .667 x 6 = 4 amps DC from that bridge. 4A DC x 1.8 = 7.12 amps AC from all transformers.

So now add up these currents. If all motors are running the first two transformers will see 8.4 amps AC from Y and Z motors and 7.12 amps AC from the X motor = 15.52 amps AC. The third transformer only sees the 58 volt bridge and only has to supply 7.12 amps AC.
If you truly ran all 3 axis for hours at a time then i would say put a fan on the transformer.

I made a rough hand sketch of the schematic. When my camera is done charging i will take a pic of it and try to post it.

BTW, Mouser electronics has no minimum order. A quick look shows the cheapest cap with just enough UF and a good voltage rating is this one, to use on your 58VDC (measured voltage 63) X axis.

Mouser Part #: 667-ECO-S2AP562FA
Manufacturer Part #: ECO-S2AP562FA
5600uF 100VDC at 5.75 amps max.
$5.11 plus shipping

You need a minimum of 4 amps for the cap. The current is what heats up the cap and causes short life. 5.75 amps is a good de-rating for longer life.

[Al_The_Man]

Just a quick look over the schematic, It appears to me that if you connect the two DC -ve's you will short out the secondaries?
Al.

[cnc_4_me]

Just a quick look over the schematic, It appears to me that if you connect the two DC -ve's you will short out the secondaries?
Al.

OH boy, good catch Al, you are right. We would have fried the transformers and probably the bridges. Thank you for looking this over.

We can not run two floating supplies that will never work. The only way i see to make this work is to have the single transformer isolated from the others and wire the DC supplies in series. This way both DC power supplies are referenced to a single ground. The problem with this is that the total DC current has to flow through the caps off of the two transformer setup meaning none of them have the right specs anymore current wise or uF. But Drools did have 5 of those caps.

Well we need 4 amps DC for X axes and 4.7 amps DC for Y and Z axis. that gives us 8.7 amps DC total that will flow through the two transformer setup. If Drools will add another set of 10,000uF caps in parallel with his other 10,000uF caps he will have 10,000uF at 8.80 amps available because two 10kuF caps in series is 5KuF and two 5KuF caps in parallel is 10KuF. That is just enough current, now lets check the uF that we need.

(80,000 x I, DC amps of motor after 2/3 derating) / V voltage of DC power supply = uF.

(80,000 x 8.7A) / 38VDC = 18,315uF, that is a problem we are way to short, we only have 10KuF.

Lets see what Drools has to say about this. Either he has to buy more caps or if he has more transformers he can build separate power supplies and then tie the grounds together. Two transformers in one and 3 transformers in the other.

[Drools]

wow, good thing I asked before hooking these things up. I just want to say thank you for all the help you have provided in this thread.
I think the easiest thing will be to just make another power supply, since these transformers have been collecting dust for a few years I can afford to employ more of them for the cause.
I might just order the caps from mouser or Digikey as it would eliminate one level of complexity.

[cnc_4_me]

wow, good thing I asked before hooking these things up. I just want to say thank you for all the help you have provided in this thread.
I think the easiest thing will be to just make another power supply, since these transformers have been collecting dust for a few years I can afford to employ more of them for the cause.
I might just order the caps from mouser or Digikey as it would eliminate one level of complexity.

How many of these transformers do you have. From the ad on Ebay it looks like he sells them in lots of 8.