Leblond Lathe Brake Voltage/Diode Issues?

[markz528]

We have an early 80's Leblond lathe with a disc brake. We moved the lathe, and when setting it up in new location I did some quick checking of the voltages in the cabinet.

I was very surprised to find only 30 VDC going out to the disc brake (I would expect 50 volts DC on a 120 volt AC single phase half wave rectifier - am I correct?). Its got a selenium diode with 2 parts - one is a half wave rectifier feeding the brake coil and the other half is a free wheeling diode across the coil. There is also a capacitor across the coil. I tried to measure the voltage across the diode with a Fluke diode checker, but both parts just said OL. I don't know much about selenium rectifiers but Google tells me that a typical failure mode is increased forward voltage drop. Is this one going bad?

But the manual says the brake is 90 VDC (can't access the brake nameplate). Would they feed the brake with half wave even though its rated 90 VDC? Can I put a modern silicone diode on it and be done with it? Any concerns?

Thoughts?

 

[CarlBoyd]

Silicon diodes have a voltage drop of about a volt, Selenium diodes are much higher and I believe quite resistive so the fluke meter diode check is not telling you much.
How much DC voltage you get from 1/2 wave rectified 120 AC depends on several things. I suspect you are getting a time average voltage, the voltage will be positive when the rectifier diode is "on" and negative when the free wheeling diode is on in the other half of the cycle. Figuring out the "effective" voltage on the brake coil with a meter is not going to work.
If the disk brake works, I suggest you don't mess with it.

CarlBoyd

 

[CarlBoyd]

Silicon diodes have a voltage drop of about a volt, Selenium diodes are much higher and I believe quite resistive so the fluke meter diode check is not telling you much.
How much DC voltage you get from 1/2 wave rectified 120 AC depends on several things. I suspect you are getting a time average voltage, the voltage will be positive when the rectifier diode is "on" and negative when the free wheeling diode is on in the other half of the cycle. Figuring out the "effective" voltage on the brake coil with a meter is not going to work.
If the disk brake works, I suggest you don't mess with it.

CarlBoyd

Another thought.
What you can do with the meter is measure the current through the brake by putting the meter inline, the current will always flow in the same direction. Then measure the resistance of the brake coil. Then multiple the resistance by the current and you should get a much better estimate of effective voltage.

CarlBoyd

 

[markz528]

Thanks for the response!

Surprisingly I could not find anyone that had a lot of knowledge on selenium diodes but it does appear that they are nasty when they fail. I replaced it yesterday with a modern diode bridge.

Surprisingly the peak voltage drop across the diode was only 10 or so volts. I expected more based on the DC voltages. At 120 AC volts in I expected 54.0 volts DC without a capacitor. With the brake coil in the circuit, the DC voltage measured 35 volts. It has a 0.25 microfarad capacitor across the coil and it measured good at 0.27 microfarads. When I pulled the coil off, the DC voltage with the selenium diode was 135 volts DC. The 135 volts makes sense (see waveforms).

With the new diode bridge the DC voltage was 55 volts which is pretty much where I expected it to be. The peak voltage was 176 volts just like the line voltage (versus 166 volts peak with the selenium rectifier) so the drop across the diode bridge is minimal just as expected. Basically it behaved just as expected.

It even sounds better now............