Fuse Amp Ratings for 6 to 36 Volt Storage Battery Test Supply

[dArsonval]

In testing low voltage D.C. Electric Motors rated at numerous high current ratings under "no load" conditions,
I've opted to build a test station using the old fashioned high current battery
capacity of lead acid storage batteries for their "pure" Direct Current attributes.

I realize I could have gone the route of feeding a high current welding transformer (for example) via a suitable variable auto-transformer
with a rectifier, etc. for a smooth "ramping up" of the required voltage.

I also understand the test batteries will need to be alternated from time-to-time to equalize their time under demand,
and for equally recharging.

Sometimes one has to go with their last idea, as being their best idea so,
I have to finish what I started.

The attached pencil drawings of the control circuit, and heavy current circuit are illustrative, and are not finished documents.

The sequence of battery connections in each "stepped" configuration are as follows:

Batteries 1, 2, and 3 are paralleled for providing 6 volts.

Batteries 1, 2, and 3 are paralleled, and connected in series with paralleled batteries 6, 7, and 8
for providing 12 volts.

Batteries 1, 2, and 3 are connected in series to provide 18 volts and so on... up to 36 volts.


Here's what I need help with:

If the maximum capacity of the final output contact(s) illustrated in the attached
drawing(s) are rated at 200 amps, how should I approach sizing the individual fuse ratings
for each 6 volt battery at their respective terminal post to protect them in the event of relay contact failure,
excessive current draw during a test, and so on?

Thanks to all, in advance.

John

http://files.engineering.com/getfile.aspx?folder=86b6f71b-94b1-429e-bc90-3c45162a62c1&file=1B.jpg

http://files.engineering.com/getfile.aspx?folder=5ac29c61-84f9-416c-be1a-8b87c97bdd7e&file=1A.jpg

http://files.engineering.com/getfile.aspx?folder=fc57aa2d-af9f-40c5-872b-778188edb249&file=6.JPG

http://files.engineering.com/getfile.aspx?folder=caccd6d5-5971-47de-9183-d575fc437e68&file=7.JPG

YouTube URL:

http://www.youtube.com/user/ElectroMechApparatus

 

[rhatcher]

If you intend to have 200 amps going through each output contactor, meaning 200A output at each voltage 6V-36V, then I am not sure about the rating of the other, interconnecting contactors.

For example, at 6V you have batteries 1-3 in parallel with the final output through the 200A contactor 15. For a 200A output, each battery will output 67A. Wouldn't this mean that contactors 2 and 5, rated for 100A each, will have to carry 134A.

For 12V, batteries 1-3 are in parallel and also in series with the paralleled batteries 4-6 with the final output through the 200A contactor 16. For a 200A output, each battery would output 67A. Contactors 2, 5, 8, and 12, rated for 100A, will have to carry 134A and contactor 7, also rated for 100A, will have to carry 200A.

For 18V, batteries 1-3 are in series with the final output through the 200A contactor 15. For a 200A output, each battery will output 200A, meaning that contactors 3 and 4, rated for 100A, will have to carry 200A.

For 24V, batteries 1-4 are in series with the final output through the 200A contactor 18. For a 200A output, each battery will output 200A, meaning that contactors 3, 4, and 7, rated at 100A, will have to carry 200A.

For 36V, batteries 1-6 are in series. One problem here is that your control circuit omitted contactor 7 that is necessary to tie batteries 1-3 in series with batteries 4-6. Assuming that contactor 7 was added, for a 200A output, each battery will output 200A. Contactors 3, 4, (7), 10, and 11, rated for 100A, will have to carry 200A.

Beyond the problems with contactor ratings, a 200A output at each voltage with 200A fuses on each battery will mean that for the parallel connections for 6V-12V, the paralleled fuses will be greatly oversized compared to the contactor ratings and the anticipated load and will not offer proper protection against overcurrents.

----

If we assume 200A output for 6V-12V and 70A output for 18V-36V, then 70A fuses can be used for each battery. This still leaves the problem with the ratings of contactors 2, 5, 7, 8,and 12 when used in the 6V and 12V connections. However, this does eliminate the problems with the ratings of the rest of the contactors as used in the 18V-36V series connections. Of course, the problem of missing contactor 7 for the 36V connection remains.

Finally, all of the above analysis is based on continuous current ratings. There will be an inrush current as each voltage is initially applied. This would imply that the fuses require a time delay element.

I hope this answer helps you. Let me know what you think.

 

[rhatcher]

Correction to my previous post: For the 12V connection and 200A output, contactors 2, 5, 9 (not 8), and 12 are overloaded at 134A and contactor 7 is overloaded at 200A.

The maximum output that can be achieved using the existing design and equipment without overloading any contactors follows.

6V = 150A output : 50A per battery in parallel with greater output limited by contactors 2 & 5.

12V = 100A output : 33A per battery in parallel with greater output limited by contactor 7. Changing contactor 7 to a rating of 150A (or 200A) allows an output of 150A, 50A per battery in parallel, with greater output limited by contactors 2, 5, 9, & 12.

18V - 36V = 100A output : 100A per battery in series with greater output limited by all of the contactors in series except the 200A output contactors.

There is no single fuse rating for the batteries that will allow maximum output at each voltage while still protecting the contactors, and the motor being tested, from overload or short circuit.

Use of 50A fuses will reduce the 18-36V output to 50A. Use of 100A fuses will leave the 6-12V output unprotected.

If you used a shunt overload on the device output terminals, you could achieve 100A rating for all voltages without overloading the contactors. In this case, you could use a fast acting fuse on each battery to protect against short circuits within the device. Of course, the fast acting fuse would need to be large enough to allow for the inrush as each higher voltage is selected.

If you want 200A output at every voltage, the only way to achieve this is to change all of the contactors to 200A rating except 1, 6, 8, and 13. Of course, you will still have a problem selecting a fuse that will protect both the 6-12V parallel battery connections and the 18-36V series battery connections. A shunt overload on the output terminals for overload protection and fast acting fuses on the batteries for short circuit protection would still be required.

I'll finish by adding that for a device designed to be a testing power supply, an output voltmeter and ammeter would be a nice feature.

I hope this helps.

 

[iop995]

Fuse must have nominal current higher or at least egual with maxim normal operating current. Operating current is done by load; you know load type / testing conditions of motors. Fuse protect devices and avoid fire / explosion. Check motor with maxim rotor locked current and scale with voltage to see maxim current through a battery leg. This maxim value must be below nominal fuse current.

 

[dArsonval]

rhatcher (Electrical)

A "Belated Reply" in this [day-n-age]. Thank You for all the attention to detail you put forth in calculating the very primitive circuit as posted.
We all believe, or disbelieve when we're too lazy to think... and your reply certainly provided me with plenty to "think" about.
I'm more an "artist", than an "engineer", and Love it All.

Thank You... again. You've helped me a lot!

John

 

[rhatcher]

dArsonval,

Thanks for the feedback. I'm glad that you found my work helpful.