Dc cnc retrofit help needed

Thread Starter

Cell 55

Joined Jan 1, 2024
7
Hi All,
I have a 380 volt 3 phase machine I am trying to retrofit using the original 80 volt dc servo motors. I removed the old drives and got new components for the 80 volt system. Problem is I get 113 volts out from the original transformer. I did not connect the bridge rectifiers yet that I kept separated for each motor. Everyone say the voltage will remain the same when rectified. I see there is three resistors not connected but they were not part of the old drive system. I was thinking of putting them in series with the three 113 volt wires. My motor and drives is for 20 amps. Or is there a better option?
Cell 55
 

LadySpark

Joined Feb 7, 2024
188
Reading 113v from a 80V bare transformer is going to read high because its not loaded. But the other layer of it is what the rectifiers are. Especially if they are some sort of industrial selenium rectifiers.Are you fallowing the wiring diagram?
 

Thread Starter

Cell 55

Joined Jan 1, 2024
7
Reading 113v from a 80V bare transformer is going to read high because its not loaded. But the other layer of it is what the rectifiers are. Especially if they are some sort of industrial selenium rectifiers.Are you fallowing the wiring diagram?
 

Thread Starter

Cell 55

Joined Jan 1, 2024
7
Hi LadySpark
The old drives had large heat sinks, I am using them with 25 amp stud diodes. I replaced all the old stuff so a wiring diagram is not the problem they must have used other ways inside the old drives that reduced the voltage. I will connect the rectifiers and caps next week and run one motor direct and see what the voltage will be under load, its just that 80 to 113 seem very high.
Thanks for the reply.
 

LadySpark

Joined Feb 7, 2024
188
Hi LadySpark
The old drives had large heat sinks, I am using them with 25 amp stud diodes. I replaced all the old stuff so a wiring diagram is not the problem they must have used other ways inside the old drives that reduced the voltage. I will connect the rectifiers and caps next week and run one motor direct and see what the voltage will be under load, its just that 80 to 113 seem very high.
Thanks for the reply.
Its hard to say its not correct at all considering transformers read higher voltage when not loaded. You can always check what the transformer voltage is by loading it 25-50% . My 120V isolation transformer reads 160V not loaded. Besides that, a DC servo amp is also inline to all of this in a CNC. That is why I asked if you had diagrams.
 

MisterBill2

Joined Jan 23, 2018
18,986
With many variable speed drives the mains input voltage is rather different from what is fed to the motors at the output of the drive package. The last servo drive I designed into a machine used the 480 volt3-phase mains for the input connection but all of the motors were 24 volt devices. It loaded sintered connecting rods into a multi-ton trim press, 15 times a minute. That was the most well loved machine I ever created, because the operator was 15 feet away loading the connecting rods onto a conveyor, and the servos were loading the press..And every four seconds there was the soft "boom" as the press cycled.
My point here is that the motor voltage and the AC power into the drive are usually not the same. So the TS needs to read the book that came with the new servo drive.
 

Thread Starter

Cell 55

Joined Jan 1, 2024
7
What make of drives were they?
Hi Max,
The old drive was DC Siemens drives. I removed everything I will just use the 3 phase transformer. I isolated each motors rectifier from each other, so if i have to use resistors on the DC side each motors load will be separate. I was thinking of using a hand 220 volt drill for a load to see how much the voltage will drop.
 

MisterBill2

Joined Jan 23, 2018
18,986
Hi Max,
The old drive was DC Siemens drives. I removed everything I will just use the 3 phase transformer. I isolated each motors rectifier from each other, so if i have to use resistors on the DC side each motors load will be separate. I was thinking of using a hand 220 volt drill for a load to see how much the voltage will drop.
That type of test may reveal any issues, and it makes sense. Let us know the results. But be careful concerning shock hazards. I suggest latching the drill motor "ON" , with hands-off for the actual test. Do the switching at the AC mains power control. Just in case something is connected someplace not intended, that might cause a shock.
 

Thread Starter

Cell 55

Joined Jan 1, 2024
7
Its now even worse after I rectified it and connected the capacitor, the now DC voltage jumped to 157 volts. the drill only dropped the voltage by about 2 volts. The old Siemens drives had lots of transistors on its heat sinks that must have lowered the voltage. The new drives is made for 80 volt max I don't want to use a higher voltage than 80 on them and rely on the motor loads to get to the 80 volts. Has anyone ever just used 2 of the 3 phases from a transformer? What if I not connect one of the phases going to the rectifiers making it a 2 phase supply.
 

LadySpark

Joined Feb 7, 2024
188
The old Siemens drives had lots of transistors on its heat sinks that must have lowered the voltage
So this says to me the servo amp was on the motor.
So you need a servo drive amp. That is the device that normally goes between that transformer and motor. Which is not a big deal to get. So what is your cnc controller?
 

LadySpark

Joined Feb 7, 2024
188
Hi LadySpark.
Now I have a 3 phase transformer output goes to 3 phase rectifier and then it must go to this http://leadshineusa.com/UploadFile/Down/DCS810V1m.pdf
ok this explains a lot. You are trying to refit an old analog servo system with a digital drive. The old analog amp boards ran off rectified DC voltage from a 3 phase transformer (140-180VDC) with an H bridge transistor drive. Mostfet drive types require a regulated supply. Which that Chinese amp card needs a regulated supply.

So for each servo axis module, you will need to add a 80V 25A switching power supply before it, and power them from the existing isolation transformer (fused individually). Which is a standard power supply size.

A 75A supply to power three 20A motors with cards is going to cost a lot more and it will not be as robust in harsh environments. Plus replacements might be limited to a few manufacturers compared to the 25A supplies if you ever need to replace them.
 
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