Only post here if you are in expert in an area and are sure that this is something we MUST know before starting a project!

Moderators: robnewyork, bassiclyLouDog, gameboy

User avatar
By gameboy
originally posted by
eZiggy Posted - 08/21/2009 : 03:48:13

I am an electrician in the USAF and safety should always be of the utmost concern. I have read many posts about motor controllers catching fire due to overheating from to much amperage.

DC Motors by design will pull as much amperage as is required to reach full potential. The rating for motors Amperage is based on what the motor will pull when the rotor is locked, so the slower the motor is rotating the higher the amperage will be. If the amperage is too high for the controller then it will overheat and could catch fire. That is why you need a breaker which will open the circuit in a high amperage situation.

Voltage and Amperage are directly related too, so if you double the voltage you will halve the amperage, so you can use a breaker or fuse that is rated at a different voltage to suit your amperage requirements in your custom build.

For example I am in the process of customizing my e300. I will be running 36v LiIon batteries in parallel (actually two 9.2aH in parallel (18.4aH total) which are capable of 80A continuous draw. My motor controller is 36v 40A and I am installing a 750 Watt 27A FLC motor (Full Load Current/Amperage Locked rotor will draw).

Also FYI 746 Watts equals 1HP.

If anyone has looked for breakers/fuses you will see they are not readily available. So I found a 32v 50A breaker at Monster Scooter Parts for $25. Running at 36v the breaker will blow at 44.44A. This is about 10% higher than my controllers rating but I figure this is something I am comfortable with, if it catches fire I will let you know.

Also Blue Sea breakers are good too, you just have to get one that will work for your setup. AC and DC fuses/breakers are different, so make sure your using a DC Rated breaker or fuse! DC Current is much more strenuous, So there can be a amperage rating difference of 200% to 400%.
ie: 10A @ 250V AC = 10A @ 62.5V DC

To figure the rating for a breaker divide the breakers voltage by your battery voltage and multiply by the breakers rated amperage.

Breaker rated 32v DC 50A
Battery voltage 36v


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