What size fuse?

[NickM]

Hi all, your advice to a definite non-electrician would be much appreciated.

A fellow member of my club took delivery of a fully assembled 7 1/4 battery electric loco just before Christmas. It has two 750W motors and two 12V 90ah batteries connected to give 24V. It was originally fitted with a 50A fuse but this repeatedly blew when the loco was put under load. Some one used a clamp meter on one of the motors which showed it as taking a maximum of about 80A.

Is this 80A about what would be expected and if so, what size fuse should be fitted that will protect what it is supposed to protect but won't pop every time you set off with more than 4 passengers?

thanks in advance and apologies for any mistaken terminology!

[houstonceng]

The size of fuse depends on what you want to try to protect.

If it's an electronic controller then putting a fuse in circuit rated at the maximum current rating of the controller will have very little safety effect. Semi-conductors make wonderful "fuses" in there own right - and are a lot quicker in operation than real fuses so make sure the e-controller is rated well above the maximum 80A measured.

Nor will the fuse protect the motor if it decides to go short-circuit through overheating. The damage is already done.

OK. The primary reason for the fuse is to protect:-
1. The wiring, so make sure that the wires can take at least 100amp if an 80A fuse is used.
2. The batteries from a short circuit.

Assuming that he's got two 750W motors connected in parallel fed by 24vdc. That means at least 70A. However, it would not be unlikely that the current drawn at starting was 100A+. If the motors are connected in series, you'd have at least 35A at rated output, so 80A+ on starting is also probable. It all depends on the type of controller used and the efficiency of the motors.

Soft start circuits (like the 4QD controller) should be able to limit the starting current, whereas on-off control will, definitely, draw a very large initial current.

Assuming that the controller is "man enough" and the wiring is correctly rated, I'd fit a 90A fuse and see how it goes.

[steam4ian]

G'day Nick

Just to add to what Andy has said.

It is a common misconception that a fuse or circuit breaker "blows" the instant the current exceeds its rating, 50 amps in this case. This is not the case because these are thermally operated devices.

Your 50 amp fuse will probably take two and a half minutes to blow if carrying 100 amps. Under 65 amps the fuse will not blow for hours. I base this on Alstom data for type T HRC fuses, this data is typical.

You 1500 watts of motor will draw 62.5 amps so once running at full load the fuse should not blow. HOWEVER, the fuse rating does seem to be marginal.

One thing that can cause fuses to fail is hot joints at the fuse holder, this is worth checking.

Check also the motor field connections in that the tonged current appears higher than what I would expect on the nameplate. Make sure you are getting the full field current if a shunt field and that the field is applied before the armature circuit is energised.

You haven't said what type of controller you have.

I would get advice from whoever fitted the original fuse, they must have a reason for choosing 50 amp.

Regards
Ian

[peterseager]

My 5"G Hymek uses a 50 Amp Miniature Circuit Breaker (MCB). It has 600 Watts of motors running off 24V. I see a peak current at starting of 50 A but soon settles down to 10 - 20 A. The psu is a 100 A Parkside unit. It was originally supplied with a 63 A MCB but they were hard to find locally. But after two seasons no problems. There is even more thermal inertia in an MCB which is why it works for me. Most manufactures info shows MCBs and not fuses, I guess because it does two jobs, a resettable fuse and On/Off switch.

It would therefore seem reasonable that you require a higher rated protection device. Unfortunately the highest rated MCB in the Parkside catalogue is 63 A but it might just work.

Peter

[havoc]

Where is the fuse placed? Just after the battery, after the controller but before both motors, each motor circuit?

[houstonceng]

Just after the battery is best.

[houstonceng]

BTW. Ian should have said that there are "Fast Blow" and "Slow-blow" fuses.

A 50A Fast Blow fuse will definitely blow in "less than two shakes of a lamb's tail" if 65A is flowing through it. Motor circuits are usually protected by Slow-blow fuses.

Similarly, MCBs come in two types. The thermal only types equate, usually, to Slow Blow. the Termal-Electromagnetic type equate to Fast-blow.

[havoc]

I meant where is the fuse that is blowing right now in NickyM's engine. The type and rating of the fuse depends on that.

To really make sure your rating is OK you would need to measure the current with a scope clamp so you can see what the I²t of the inrush current is. Only then can you make a reasonable guess (and even then...).

[peterseager]

I found this useful reference to MCBs when I was designing my loco:

www.tlc-direct.co.uk/Technical/Distribution/Mcb.htm

MCBs are designed for mains operation but their characteristics are very similar for our use.

4QD show the MCB placed between the two batteries which is what I have done. It was convenient to wire up. But note that if you take a 12V feed from the same point it must be connected to the +ve of the 0 - 12V battery and not the -ve of the 12 - 24 V. 4QD got this wrong in their early literature so beware. I use a B6 MCB for the 12V supply to the horns.

Peter

[bigt1677]

Just for info, Wylex make a 63 amp MCB, that will easily absorb a 100 amp surge load on start up - available from screwfix. I undersatnd that 63a is about the largest size you can get in domestic use, if you want to go bigger you are looking at industrial size switchgear - probably 3 phase. Someone stated that 4QD put their breaker in between the battery links and this is true, I have their drawings but in practice, best place to put it is just before the positive wire joins the controller.