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Rectifier test with switch hooked up to it.

A

Awleaphart

Aluminum
Joined
Mar 16, 2019
Hello,
Set up-Step down transformer 220V to 110V on one side the other side 30V. The 30V goes to a rectifier and then comes out as 24Vcc that goes to a brake on my lathe. When i turn the brake on it blows the 3 amp 250V fuse. So I checked the rectifier and it checks good until I turn the brake switch on. It has about .54 resistance in all directions. With the switch off it checks out good. Is this correct?
Thanks
A Leaphart
 
Questions

Awleaphart said:
Hello,
Set up-Step down transformer 220V to 110V on one side the other side 30V. The 30V goes to a rectifier and then comes out as 24Vcc that goes to a brake on my lathe. When i turn the brake on it blows the 3 amp 250V fuse. So I checked the rectifier and it checks good until I turn the brake switch on. It has about .54 resistance in all directions. With the switch off it checks out good. Is this correct?
Thanks
A Leaphart
Click to expand...

Not enough information
1) are you running the primary at 120 or 220?
2) With no load, the rectified voltage (DC measurement) should be higher that the unrectified voltage (AC measurement), what is Vcc
3) please provide a schematic or wiring diagram of the switch
4) what is the resistance of the brake?
5) the rectifier should not have .54 resistance, a schematic or wiring diagram will help, what is you measurement method.

CarlBoyd
 
Check resistance from point 10 to ground. Minimum should be 24V/3A=8 Ohms. If any less, find the cause and it will most likely be the source of your problem.
 
Yea,
Im confused too. e8 is a fuse on the machine. In the manual it calls out e8 to be a direct current valve? Also the e8 fuse is on one of the 30V legs before the rectifier. Manual shows if after. With the fuse out Im getting 20AcV/120dcv on the + and - on the rectifier. Also i could not find point #10 to check the ohms.
When I turn the brake switch on the 20acv on rectifier drops to zero?
A Leaphart
 
Is the rectifier changing AC to Dc here or changing AC to Vcc? So you check Vcc on AC on your volt meter?
A Leaphart
 
Awleaphart said:
Is the rectifier changing AC to Dc here or changing AC to Vcc? So you check Vcc on AC on your volt meter?
A Leaphart
Click to expand...

It's changing AC to peaks of current in one direction. There is a average DC result but the waveform will look like the humps on a Camel's back.
Your DC meter will display a value, but it is not true DC. The load will behave as if it is DC.
 
I agree with Rons. The bridge is wired wrong. On one half cycle the diodes are a direct short and completely open on the next. Drafting mistake.

E (dc) = .9 x E (rms)

Tom
 
I think its wired right. The brake was working some what when I got the lathe. I think the dia. drawn wrong.
This is the reading Im getting on the + and - of the rectifier with the volt meter on dcv.
power on motor off 106 dcv
Brake switch on power on motor off 000 dcv
motor running 100 dcv
motor running switch on 000 dcv
Now Spindle engaged and turning 100 dcv
brake switch on spindle turning 104 dcv. All this with the e8 fuse not in the machine.
A Leaphart
 
First: I agree bridge rectifier is drawn wrong but thats an error on the drawing, not the machine. I notice that (r) is a brake protective device. It is shown in parallel with the brake (S). My guess is that this is shorted. In fact, how could it protect the brake other than to blow the fuse unless (e8) is a current limiting device. Perhaps that is what they mean by direct current valve. Google did not help with the term "direct current valve".
 
Yea,
I looked it up also. I agree. The fuse dont blow unless you switch the brake on and the brake doesn't work at all.. And you have to switch it on about two times. I think all the above are in the head. (r) That is and the brake its self. Do you think it would be in the Oil in the gear head?
A Leaphart
 
As I look further I see that (ls1) is an oil pump micro switch. Maybe another safety device for the brake. So is there only breaking when there is oil pressure or when there is not oil pressure?

Sorry, I have no idea where the brake is or even how it functions. What is the other symbol (h)? It is not in the legend.

I think you will have to try and follow the wires from 9 through LS1 and to 10.
 
H-s-r must be inside the head. I cant find them any where. It only blows the fuse if I try to use the brake. It has a dome site glass on top of the head so you can tell it is pumping oil per the manual. Basically it has a small tube inside the dome site glass and oil comes out the end of the tube and hits the dome shape site glass. like a visual check.
A Leaphart
 
When you close the switch LS1, the brake is conected across the output, so there is another path for the ohmmeter to read, in parallel with the diodes.

So leave LS1 OPEN, and read across the brake itself. If you read a higher resistance, then the problem is elsewhere, but if you again read the low resistance, then there is an issue with the brake or the "protector", which may be an MOV type device.

Those MOVs degrade a little each time they are "hit" with an overvoltage, which woul be each time the brake is operated. So they can become conductive at the normal circuit voltage after some number of "hits". That will blow the fuse at some point, when conduction through the MOV is too much. Significantly before that, the MOV may begin to heat up, and it may fail due to heating also.

With large available current, the MOV may literally disappear, but in this sort of circuit, it is likely to just become a short or rather low resistance.

You need to find that "protector" if it appears to be the problem/
 
I'm going to have to take the top off the gear head. LS1 is another item that must be inside there. Now you say MOV-there is a limit type switch hooked to the apron lever that starts the spindle.It don't show on any of the ELE. diagrams. Its an AEG GTg 11. Im going to look in the head for those items i cant find.
A Leaphart
 
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