Fuse?

[AP]

hi - wondering where best to wire in circuit protection and what amperage fuse to use? I have a 30 amp in-line off the red lead from controller but that's just a best guess based on not much :-]

thanks

[myelectricbike]

The rear motor kit I just got has the standard male (3 female pins) connector and a female (3 male pins) connector with pigtails for connection to the batteries. The positive (red) pigtail has an inline fuse holder with a 100(?) amp fuse so the place for the fuse is  inside the battery casing. I've heard that 35 amp fuses are commonly used but for one thing the controller shuts down if it is overloaded from a shorted output power lead so the fuse in the pigtail is not used for this but used to protect the wiring between a short between the controller and the batteries. Direct shorts can blow the batteries but propably not with 14 AWG before they melt. Even 12 AWG will probably become red hot and melt the insulation first before melting itself or the lead electrodes. Fire is the greatest hazard. You want a fuse large enough to keep you going in traffic under extreme usage conditions but small enough to blow if there is a direct short. So I'd select a fuse to match the maximum amperage allowed for whatever AWG is being used between the batteries and the controller; which is most likely either 12 AWG or 14. Hope this helps.

[29a]

you should size your fuse based on the max discharge the battery allows without damage.
With SLA this would be no more than 1C or the same as your amp/hour rating

[myelectricbike]

Unless you are using batteries with less amperage carrying capacity than the wiring always use a fuse that will blow below the maximum amperage capacity of the wiring.

[29a]

lol lol
OK !
 The max fuse size I was promoting related to SLA which was sold with this kit and I was being generous.
Most SLA are rated at 0.05 C but max discharge 0.2 C so the max fuse size not to damage your SLA battery pack should be one fifth of the Ah rating.
So a 20Ah SLA  should have no more than a 4 amp fuse.
If you are lucky enougth to have not purchased the SLAs with kit then you may have a battery pack that will exceed the wire rateing or the more likely amp rating of the motor controller and your fuse size should be based on the motor controller max amps, which I can not comment on as this is still unknown to me.

[myelectricbike]

DC amperage is calculated by dividing voltage into wattage. Fuse sizes unless stated otherwise are rated on AC line voltage (either 120 or 240 volts) as is hookup wire size.

Amperage, wire and fuse size are inversely proportional to voltage for the same wattage so physical constraint on stator winding wire size limits overall amperage which requires voltage adaptation to wattage increases instead.

Since the controller limits amperage to around 20 and shuts down internally from over current loads, fuses between the controller and the battery protect the supply wiring from the battery pack to the controller and by so doing protect the battery pack as well. However, unless the fuse you use is rated at the voltage you are using you must do the calculations to determine appropriate fuse size based on watts.

[Dalecv]

To answer the original question, the best place to locate a fuse is on the positive lead as soon after the battery pack as reasonable, preferably at the battery post. As stated earlier, the main purpose of a fuse on the battery power supply is to prevent fire or battery destruction in the event of a short circuit, not to prevent circuit failure from over use.

As to what size of fuse to use. A 700 Watt motor at 36 Volts can use 19.44 Amps of current plus the controller circuitry will also use some current as well. Fuses should be sized at 125% of current demand so a 25 Amp fuse would be about the smallest size you should use unless you want to be stranded when under a heavy use, I think your 30 Amp fuse is about right.

Dale

[Tobyjug]

HI, in my experience you need a 30 amp fuse. I purchased a Front Hub kit a couple of months ago with the Golden Motor SLA 36V battery pack from the US distributor and it blew the 20 amp fuse in the battery box first time I applied full power. I fitted a Watts Up meter between the Battery and the Controller and during it has recorded peak amp draw of  29.62 amps with max wattage input readings of 1023 watts, it regularly shows amp draw above 29 amps under full power load but never above 30 amps, I am sure that the controller that came with my kit has a 30amp cut off not 20amps.
David b

[myelectricbike]

I've tried a 15 amp circuit breaker like you find in surge protector strips but they will pop at the slightest acceleration. I've never tried to use either fuses or breakers in parallel but it might very well work. Once the first one goes though you'll have to bare the cost of the second one unless you use the circuit breaker mod.

[myelectricbike]

... I am sure that the controller that came with my kit has a 30amp cut off not 20amps.

Have you measured the current going through the power leads between the controller and the motor?

[Tobyjug]

Have you measured the current going through the power leads between the controller and the motor?

Nope have not tried that yet, are you suggesting the controller draws 29+ amps but only supplies 20?

[myelectricbike]

MOSFET switching is usually very efficient even with six ICs and I've never measured the temperature of my controller enclosure although the hub covers get to hot to touch for very long following prolonged acceleration or a hill climb so a few amps are being dissipated as heat. My guess would be ...err ten?