Thread: silicon rectifiers

Hi everyone,
I have a lathe that runs on 600V 3 phase, with a 200V dc motor for the apron drive. The main motor and the phase converter motor are both rated at 3hp each.

I want to replace the old selenium rectifiers with silicon ones. My question is: Can I buy the small silicon ones rated at 1000V-50A., or should I stick to something as close to the 600V of the lathe; on the same thought, can I use a 400V-35A silicon rectifier.
Thanks a lot

Originally Posted by tbirds

Hi everyone,
I have a lathe that runs on 600V 3 phase, with a 200V dc motor for the apron drive. The main motor and the phase converter motor are both rated at 3hp each.

I want to replace the old selenium rectifiers with silicon ones. My question is: Can I buy the small silicon ones rated at 1000V-50A., or should I stick to something as close to the 600V of the lathe; on the same thought, can I use a 400V-35A silicon rectifier.
Thanks a lot

Tbirds:
There are several, it depends in the answer.

Rectifying 600V will get you much higher voltage than 200VDC.

I would "guess" that there is a transformer in there somewhere that drops the 600V down to a much lower voltage. Can you measure the voltage at the input to the selenium rectifiers?

It is good to use rectifiers (and most other electrical part, one exception would be zener diodes) with a higher voltage and current rating than "needed". Using a rectifier near the limit of it's rating results in short life. Using a rectifier at half its rated voltage and half it's rated current results in long life.

CarlBoyd

The 600 Vac will have a rectified output of about 850 peak volts. That is at the nominal voltage, and normal line variance will sometimes produce an even higher output. For 600 V I would use 1000V rectifiers.

There is a lot more to control than the recs, better ask for help from someone that can look at your machine...Phil

I think the answer to the question is higher rated rectifiers will be fine. When you are building 10000 of something the difference in price matters, when you are doing one, not so much. There are some situations where a giant device will not work adequately in place of a tiny device, but in normal constraints you will be fine with a higher rated device

in addition to what Gustafson sys, the silicon rectifiers will give a slightly higher output voltage than the existing selenium rectifiers That's why they don't need fins all over them, because they have less voltage drop. But the difference won't be enough to do any damage to the motor.

Use the higher voltage and current rated ones if you can get them at the right price, it won't do any harm at all.

Originally Posted by gustafson

I think the answer to the question is higher rated rectifiers will be fine. When you are building 10000 of something the difference in price matters, when you are doing one, not so much. There are some situations where a giant device will not work adequately in place of a tiny device, but in normal constraints you will be fine with a higher rated device

In consumer electronics they design things for savings of a few cents.

Agree that a higher rated device is no problem.

Originally Posted by rons

In consumer electronics they design things for savings of a few cents.

Agree that a higher rated device is no problem.

Hi everyone;
I got some excellent answers from you guys; although I had a hunch from reading on the Internet that a high rectifier would do the job, there's nothing like talking to the people in the know.

The lathe is all apart for the time being; I'm doing a reconditioning job on it, so it won't be getting power anytime soon.
I'm no machinist nor an electrician, but I will hire an industrial electrician when the time comes.

About the transformer: yes, there is one; it was inside the cabinet originally, and there was a burnt spot just above it, and someone repositioned it outside the cabinet, but it looks like a sore thumb, and the wires inside had been taped together!! This will be modified.

Appreciate all inputs,
Thanks a lot