# 20A CB not activating

Discussion in 'General Electronics Discussion' started by Marrion, Aug 19, 2015.

1. ### Marrion

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0
Aug 19, 2015
According to the formula P=VI, if I use a 300W device that draws 25A at 120 volts. Why does the 20A CB not pop when the formula says it should? The reason I am asking is that I am running a 62A 3000 Watt motor on a 120 volt generator and the 20A CB does not pop. The motor is a 48VDC so it should be drawing 62A but the 20A CB does not pop and I am assuming that it is not using the full 62A. I am going to see what the motor is actually drawing today.

Last edited by a moderator: Aug 19, 2015
2. ### Marrion

2
0
Aug 19, 2015
I need a way to drop 66VDC to 48VDC at around 3000W. I need all of the current I can, but can't find the best way to get from 66 to 48 without losing any current. I looked at resisters but don't like the efficiency. I also looked at zener diodes but they are not rated very high in the current department. Any help is apreciated.

3. ### Minder

3,149
680
Apr 24, 2015
What is the source? If the present source is a linear supply from a transformer, the most efficient way would be to look at the possibility of removing turns, the secondary is usually that top winding so although tricky, it can be done, done it more times than I like to remember.
What is the KVa of the supply?
M.

4. ### Minder

3,149
680
Apr 24, 2015
You would need to monitor the actual current, CB's are fairly imprecise devices and also there is a time delay built in, usually.
M.

5. ### davennModerator

13,903
1,971
Sep 5, 2009
According to the formula P=VI, if I use a 300W device that draws 25A at 120 volts.

not according to my maths

check again

6. ### davennModerator

13,903
1,971
Sep 5, 2009
and that doesn't make sense

a 48V DC motor on a 120V (DC or AC ) generator

you really need to get your figures sorted out

7. ### davennModerator

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Sep 5, 2009
I moved that other thread to this one ... they are on the same subject

I personally don't see any easy way to do that

NO you cannot use resistors, they will limit your current plus finding resistors of that sort of wattage rating
would be impossible

All through these merged threads you seem to have your facts very poorly defined
get some real figured and explain exactly what you are up to

Dave

8. ### Merlin3189

250
69
Aug 4, 2011
Can I ask you to clarify what you have and what you want?
So far you seem to have a 120V generator (petrol/diesel?) capable of supplying 20A (ie. 2.4kW) and having a 20A CB.
From this you want to power a 3kW motor rated at 48V 62A .
Is the generator AC? Is the motor DC or single phase AC?

Obviously you are not going to fully power a 3kW motor from a 2.4kW generator whatever you do, but you may be able to use a transformer to step the 120V down to 40 - 48V or so to run the motor at lower power.

I don't see where the 66V comes in at all.

9. ### Kiwi

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Jan 28, 2013
Sorry Marrion, but I am totally confused by your posts.
Please clarify exactly what you have and what you are trying to do.

10. ### Y2KEDDIE

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Sep 23, 2012
You say you are using a 120V generator: is this true or is it an alternator? A generator produces Direct Current. If using an alternator you could use a transformer to reduce the voltage.

If you have a generator feeding a DC motor and it is running, your probably not drawing max current because your not fully loaded.

11. ### davennModerator

13,903
1,971
Sep 5, 2009
Thread closed ... No OP response .... wasting everyone's time

@Marrion, if you do decide to return and answer all the questions,
message me and I will reopen the thread

Dave