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How do I limit voltage?

 
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conmgt



Joined: 08 Apr 2008
Posts: 1
Location: Philadelphia, PA USA

PostPosted: Tue Apr 08, 2008 4:09 pm    Post subject: How do I limit voltage? Reply with quote

Hello,
First, I am new to this forum and am not savvy in electronics but I am an electrician.
I have a commercial grade wood planer that is 2 years old. There is a large flat metal table that moves up and down by way of 4 spiral shafts chain driven by a 24VDC motor made by Elra. The machine itself is fed by 240VAC 3 phase. There is a small transformer for the DC motor which has an output of 30VAC +/- 4V. Attached to it is a simple PC board with a rectifier (4 posts) and a 50V 10,000mF capacitor. That's it.
The problem is that at one point the 30VAC into the PC board produced 50+ VDC and burned out that 24VDC motor...$400! Now the output is down to 30VDC again. The question is, how did this happen? More important, how do I prevent it? Can I add something between the rectifier and the motor to prevent anything over 30VDC from passing through? I'm thinking some sort of a diode. Should I limit the VDC to 24?
Let's keep it in laymans terms. Thanks
p.s. how did I add an image to this?
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Thu Apr 10, 2008 4:54 pm    Post subject: Reply with quote

I suppose that it is a single phase transformer ? Then, one possible way that it has delivered a too high voltage is that the primary is intended for connection between one phase and the neutral conductor, but then by mistake was connected between two phases. In this case, the secondary voltage will raise to 1,73 times the rated voltage. A such overvoltage will also burn out the transformer within a short time, unless its core is designed with very large margins to saturation.

A good overvoltage protection is a so-called crowbar circuit, with a SCR connected across the output of the bridge rectifier, and its gate connected to the anode via a suitable resistor and zener diode.
Recommendations regarding the design of such circuits can be found in application notes at the major semiconductor manufacturer's (ST Microelectronics, NXP, SanRex and so on) web sites.
Make sure that you have a fuse at the primary side of the transformer and that the transformer's short circuit current is high enough to blow the fuse within one second.
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buljko



Joined: 15 Apr 2008
Posts: 4

PostPosted: Mon Apr 21, 2008 9:35 pm    Post subject: Reply with quote

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buljko



Joined: 15 Apr 2008
Posts: 4

PostPosted: Mon Apr 21, 2008 9:51 pm    Post subject: Reply with quote

http://www.voguehost.com/ims/u/buljko/buljko/protection.jpg
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buljko



Joined: 15 Apr 2008
Posts: 4

PostPosted: Mon Apr 21, 2008 9:55 pm    Post subject: Reply with quote

BTW, How I can edit my post or attach picture? I do not see this options.
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Mon1018



Joined: 22 Apr 2008
Posts: 2

PostPosted: Tue Apr 22, 2008 2:24 pm    Post subject: Reply with quote

You can edit in the Profile.
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Tue Apr 22, 2008 7:40 pm    Post subject: Reply with quote

use "img" tags

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