MCB for battery loco

[duncan webster 1Participant @duncanwebster1]

I’m making a little loco for the grandkids (honest). It has 2 off 12v MY6812 motors wired in parallel. The only current info I can find is for the 24v version, which quotes ~7A, so I’m guessing at least 15A per motor. Do I have one 20A MCB per motor?. Wiring one 40A  MCB to drive both motors doesn’t feel right

[Roger BParticipant @rogerb61624]

The information on these motors is somewhat limited/variable. They appear to exist in 12V and 24V versions and 100, 120, and 150 W.

12V 100W would have a theoretical full load current of around 8A. In practice I doubt if the efficiency is much better than 75% (maybe worse) so maybe 11A.

A 20A MCB per motor would be appropriate as starting currents will be higher. What sort of controller will you be using?

As you are working at 12V arc supression in the MCB will not be a problem. This generally comes in above 50V.

[duncan webster 1Participant @duncanwebster1]

It’s a 120W motor. Would I be better with fuses, they would fit the restricted space better.

[John HaineParticipant @johnhaine32865]

I don’t think you want an MCB, and certainly not a fuse as you’ll spend your life replacing them.  And you’ll want speed control. Have you looked at a 4QD DNO which is made for the job?  Costs a bit but likely to be much better.

[Roger BParticipant @rogerb61624]

The controller would normally have a current limiting system to protect itself and the motor under normal operation. The fuse/MCB is there to deal with abnormal situations. There should be a fuse in the battery feed to the controller (may be built into the controller).

If you are using one controller to feed two motors there may be some sense in protecting each motor with a fuse or MCB. If a drive belt breaks, for example,  the full current will go to the other motor.

[duncan webster 1Participant @duncanwebster1]

It’s got one control per motor. Cheapo ebay but should be OK for one motor’s current. Our club 08 loco has both MCB and fuse and a 4qd controller. I’ve never known it blow either. Control from one potentiometer so they share the load (near enough). Fuses twixt battery and controllers it is then

[Robert Atkinson 2Participant @robertatkinson2]

If using a MCB make ure it is rated for DC operation. Many are but it’s often not on the “headline” specification. You need to use the detailed datasheet. MCBs certified to UL 1077 will work with DC. An example is the ABB S201 series.
If the MCB is not DC rated the speed of opening and gap when opening may not be enough to extinguish a DC arc. The arc can start a fire. AC arcs are easier because the voltage drops to zero every half cycle.

Robert.

[Roger BParticipant @rogerb61624]

At this voltage level it is not a problem. The voltage needs to be above 50V before it is a real problem. OCV on arc welders is a minimum of 50V. 12 – 24 V no issues.

[Robert Atkinson 2Participant @robertatkinson2]

Roger B,
Can you provide any references or evidence to back up the assertion that “The voltage needs to be above 50V before it is a real problem.”?
Arc welders are not a comparison.Even DC welders use a pulse output that causes the voltage to drop every half cycle. The exception is industrial welders with 3 phase input and some inverter welders.

Robert.

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