DC Supply! Industrial Electrical Ezine

CBC Design (tm) - Issue 17, 2002/2003. ISSN 1475-3464
Email: cbc_design@btconnect.com
http://www.cbcdesign.co.uk

"...Maintaining a reliable DC supply."

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IN THIS ISSUE

- Introduction
Fault Finding (Article)
- Readers Questions
- Subscriber Ads

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Editorial.
 
Welcome to issue 17 and a big thank you for continuing to subscribe to our
publication, we appreciate your support.
 
First off all, we would like to appologise for the length of time between this and
the previous issue of our publication. We have been working on a number of new
projects and simply didn't have enough time to complete this ezine until now.
 
As I have mentioned in previous issues, it is becoming increasingly difficult to
create articles that avoid covering old ground. As a result, we have decided to
reduce the number of articles from 2 to 1. We continue to post each issue to
our web site and the information available is now considerable. Please feel free
to copy or print as many articles as you like.
 
Please note that we will not accept submissions from free email addreses. We
were inundated with failed delivery messages when sending out our previous
issue and spent some hours trying to re-send the publication. Private email
adress subscription only from now on please. 
 
Enjoy the content and hope you will continue to subscribe to our publication.
 
Happy Reading!  

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                           Editor: Alan Fidler.

Alan is the owner and manager of CBC Design, a leading battery management company                            based in the UK. He has worked in the industry for over Eighteen years and has designed                         charging equipment and battery monitors for some of the world largest companies.
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ARTICLE: Fault Finding. Author: Alan Fidler.
 
One of the most difficult aspects of battery and charger maintenance is actually
diagnosing what has happened when something eventually goes wrong. With enough
experience, it is relatively simple but even I find it tricky in some circumstances.
 
The best way to start, it to examine the complete installation, stage by stage, keeping
notes on what is found and where. This is the best way to ensure that there are enough
clues upon which to piece together a failure sequence. In this respect, the job of fault
finding is rather like a criminal investigation.
 
The first thing to check is fuses. Are they intact or have they ruptured?
Use a digital continuity meter set to ohms to confirm the status of each link. A healthy
fuse will show zero ohms, a failed fuse will show open circuit. Remember to write down
the status of each fuse link on paper.
 
Next, examine the batteries. Measure the voltage across each battery and make sure
they are within a voltage band of 1.5 to 2.2 volts per cell for lead acid and 0.7-1.41 volts
per cell for nicad cells. Many industrial chargers incorporate short circuit protection
and will not deliver charging current into batteries which fall outside the above limits.
 
Check the battery casing for signs of heat where they show an abnormal result in the
above tests. A deeply discharged battery may show a reverse voltage and this will
certainly mean replacing it before proceeding.
 
Having confirmed that the batteries are OK or following replacement, the next stage
involves the charger components themselves. Most transformer based systems
incorporate full wave bridge rectifier or half control diode/thyristor blocks. In either
case, the diodes should exhibit a volts drop of between 0.55 and 0.8 volts DC using
a diode tester. The positive probe is connected to the anode side of the diode which
would be rectifier negative in either configuration or either AC terminal in full wave
bridge rectifiers to test the forward diodes. Connect the negative probe to each AC
termination in both configurations or rectifier positive to test the forward diodes in
full wave bridge rectifiers.
 
Look at the transformer now by examining the windings where they are visible for
obvious signs of heat. A burnt out transformer will have black, heat damaged
insulation and fused windings. If in doubt, take the transformer to a local motor
rewind specialist and ask them to check it for you. Burnt out transformers
are usually the result of over rated fuses or a total absence of one in the mains
supply. Rarely do industrial grade transformers suffer this kind of damage when
properly fused.
 
Lastly, check any large capacitors fitted to the system for obvious signs of
heat damage. A capacitor can be checked by taking it out of circuit and testing
it with a digital meter set to ohms. A healthy electrolytic capacitor will charge
up via the test probes and the ohms reading will steadily increase as it does so.
Any capacitors which show zero ohms should be regarded as suspicious and
will need to be tested using a current limited bench type power supply.
 
General heat damage on cables and crimps needs to be examined closely. A
loose connections can result in localised arcing and therefore heat which cracks
crimp insulation and damages cable insulation. Heat damage which is evenly
distributed across the whole length of a cable indicates excessive current flow.
This may have been caused by a short circuit condition or uncontrolled current
flow, normally the result of a short circuit diode or thyristor or reversed battery
connections or battery polarity through deep discharge.
 
Use all of the notes you have kept to try and ascertain the failure sequence. We
know that fuses fail because of excessive current. If we can determine why the
excessive current flowed in the first place, we will know what failed first and how
the installation was compromised.
 
Damaged mains fuses may be the result of a switch-on surge, excessive charging
current caused by a failed component or transformer primary failure. Check the
components first then examine the transformer. If all is ok, replace fuse with
type "T" Anti-surge type and test the charger.
 
Buckled batteries are the result of excessive heat. If the charger is functioning
normally upon replacement of the cells, it is likely that the original batteries were
exposed to a large discharge, perhaps as the result of a short circuit or load
failure. Consider fitting a fuse in series with the charger supply to the batteries
where component failure has resulted in excessive charging current and
consequential heat damage to the cells.
 
Remember: Look after your batteries and your batteries will look after you!
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CLASSIFIED ADS: 

NEW: AUTOMATIC NICAD CONDITIONER 
Our latest Nicad charger automatically conditions Nicad batteries by discharging the
cells to a specified potential before applying a 3 stage charging cycle to recharge
the cells safely and completely.
 
Features:
Automatic discharge as required before charging cycle commences!
3 stage charging, Constant Current (Boost) Float Charge and Trickle charge!
Led indications for Battery Connected, Discharging, Charging and Battery Charged!
Available with 110/120 or 220/230 Mains input!
RFI Suppression!
Designed for fixed recharge applications!
Ideal for Switch Tripping applications!
Can be used to recharge Lithium Ion Cells!

Go to
http://www.cbcdesign.co.uk/ and click the "New Products!" link for more information.

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CLASSIFIED ADS: 

12 & 24V DC DIMMER SWITCHES
 
*Adjust 12 or 24V lamps from a dim glow to bright light with a CBC Dimmer Assembly*
*Extends bulb life and reduces battery power consumption*
*Works just like the dimmers in your house*
*Easy to install with integral connections*
*20 year industrial design life*
 
Visit our website http://www.cbcdesign.co.uk/index3.html for details
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READERS QUESTIONS.

Questions from in-house 
1. What causes batteries to buckle?
 
This is usually the result of heat causes by over charging or over discharging the cells.
2. What is the maximum charging rate for sealed lead acid batteries?
 
Normally 1/20th of the battery capacity. Check the manufacturers data sheets for
confirmation.

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