controller fuse

[devybob]

I've shorted out 2 controllers,1 by being stupid,and the other one by being stupid.
The new controller (altrex400PDS)I ordered comes with a fuse in the system. Would the other shorted controllers i killed have possibly been helped with a fuse of this type and if so, why don't the factory install them with stock controllers.

[JohnnieB]

In this application, the fuse is more of a fire prevention device rather than an equipment protection device.

Basically, the controller will fail before the fuse blows, but should the controller's fail mode continue to allow high current flow, the fuse will open, hopefully before anything catches on fire and burns up the cart and burns down the building the cart was sitting in.

As to why fuses aren't installed on carts by the manufacturers, their lawyers and accountants must have reached the conclusion it is less expensive to replace a few carts during the warranty period and maybe a garage or two, than it is to install fuses in each cart that rolls out the door.

Would a fuse have kept the two controller from being damage? Probably not.
An electrical event significant enough to take out a fuse would probably take out the controller before the fuse opened.

[kgsc]

jb, on this note... What do you think about using like a 60 amp fuse just to test the circuit ? Not sure if it would be enough to spin the wheels but at least would tell you if something major was wrong.. Or still to big to save the controller..

[devybob]

In your opinion,is there a way to fuse the controller?

[ab46501]

I guess in theory you could put fuses on each connection to the controller. I am not sure the would be practical though?

[JohnnieB]

Originally Posted by kgsc

jb, on this note... What do you think about using like a 60 amp fuse just to test the circuit ? Not sure if it would be enough to spin the wheels but at least would tell you if something major was wrong.. Or still to big to save the controller..

Originally Posted by devybob

In your opinion,is there a way to fuse the controller?

Originally Posted by ab46501

I guess in theory you could put fuses on each connection to the controller. I am not sure the would be practical though?

I'll try to answer all three questions in one shot.

First of all, what is a fuse. It is a resistor that heats up as current passes through it. If too much current passes through it, it gets hot enough to melt, opening the circuit it is in.

How fast a fuse melts open depends on the type of material the resistor is made of and how it is mounted mechanically and some other factors.
Standard fuses will stay intact for 1 second with 200% of their rated current passing through them, a fast-acting fuse will take 200% for about 0.1 seconds and a slo-blo will hang in there for tens of seconds.
Of course, the higher the current, the faster they blow.

On the other hand, solid state devices like diodes, transistors, ICs and the like, will only tolerate their current rating being exceeded by 50% for a few milliseconds (0.001 Second) and for some PN junctions, it is down in the microsecond range. (0.000001 Second)

Basically, to protect a solid state device with a fuse, you would have to use a fuse whose current ratting was so far below the current rating of the solid state device that the system would not function at its rated capacity.

Since we are primarily concerned about protecting a controller, lets look at what the controller does. It limits the amount of current the motor is allowed to draw.

At rest, an electric motor is very near being a dead short across the controller output and it will draw all available amps the battery pack can produce. It is only after the motor starts spinning that an electromotive force is generated that impedes the amount of current the motor can draw.

When the control element in the controller (MOSFETs) are conducting, a voltage proportional to current passing through them is developed and that voltage is fed back to the drivers that turn the MOSFETs on and off, limiting the current flow to the max current rating of the control element. That is what keep the controller from a melt-down when it starts a motor spinning, and what shuts down the controller when the motor stays stalled.

Granted, the MOSFETS are are turned on and off about 15,000 times a second and how long the stay on each time they are turned on, determines the average current flow, but each time they are turned on, peak current flows through them, but by design, the max current rating of the control element (MOSFETs) exceeds the max current rating of the controller.

Ultra-fast fusing devices do exist, but they are expensive and when the cost of the protection exceeds the cost of the device being protected it isn't normally considered a practical solution.

As for using a 60A fuse for testing, it might be better to bypass the controller entirely and use a lower voltage to test the motor and the rest of the high current circuit.

I'll be using a 250A fuse in conjunction with a 400A controller, but to protect my cart from fire since a ANN250 (Very Fast Acting type) will pass 1,500 Amps for about 0.1 Seconds before it burns open.

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In a related topic, the 50A or 60A fuse in the charging loop is another fire protection device since the 30A rated diodes in the charger will blow before the fuse will.

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Hope this clears some of the fog.

[devybob]

Well,I got my answer and a degree in electrical engineering to boot.I guess the best fuse to use is to be careful and remember to turn off the tow switch before doing anything on the cart.

[scottyb]

The short answer is no.
I have smoked a controller programming it on the bench with 2 small 9v batteries in series.