A little understanding will help you to extend the life of electric motors in equipment such as winches and air compressors. Here’s how…
Warning! The following Unsealed 4X4 story contains mathematical equations… but it’s not too hard to understand, and it will help you extend the life of the electric motors in 4×4 equipment such as winches and air compressors.
Save your 12-volt motor: A guide to amp draw.
Chances are you’ve got a 12-volt motor somewhere in your four-wheel drive, whether it be in the winch hanging off your bull bar or your compressor for airing back up after a day on the tracks… nearly all of us have one. But when was the last time you gave a thought to its lifespan, or what you can do to extend it? And, interestingly, I’m not talking about stripping it down and rebuilding it. I’m talking about giving it good, quality power.
In this yarn, I’m going to walk through the maths around 12-volt systems, amp draw and how this affects your 12-volt gear.
Let’s get the maths out of the way first. The calculation, or equation, you need to do is very simple, and can be easily knocked over with the calculator on your phone (even if you still own a 3310!). You need two pieces of information to arrive at the third. The equation looks like this:
I = P/V
Makes bugger-all sense, right? What if I write it like this:
Amps = Watts ÷ Volts
It’s basically the same thing; where I is current in amps, P is power in Watts and V is voltage in volts. Pretty simple, right? So that first one, to get amps, you just divide the Watts by the number of volts. What about if you want to find the Watts? You just need to move the equation around:
P = V x I
Or, more simply (if you weren’t paying attention above):
Watts = volts x amps.
If you want to get carried away and confirm the specified voltage of a bit of gear, you can work out volts too by:
V = P/I
But we know we’re working with a 12-volt system, so we don’t need to be too concerned with this equation.
So why would you want to know this?
We’re glad you asked! Let’s just say you’re trying to work out what amp rating you need for your wiring for something. And let’s also assume you know its power rating in Watts. As an example, car stereo amplifiers are often rated in Watts, and we’re talking RMS Watts here, not PMPO (if you’ve got a 3000W PMPO stereo, it’s actually probably closer to 300W RMS).
Anyway, before I get carried away on a ‘fully-sick stezza’ tangent, we know that the amplifier we have will draw 300 Watts, and we also know our four-wheel drive is 12-volts (-ish). So, to work out what size wiring and fuse we need, use this equation:
I? = 300w/12v
300 Watts divided by 12-volts is 25 amps (when it’s at absolute max power draw or max volume). So, we need wire that will happily carry 25 amps, and a fuse below our wire rating. So if it were me, I’d be going for some 8AWG wire that’s rated at 50 amps and putting a 30 amp fuse next to the battery. This means that the wire will carry up to 50 amps without melting, and the fuse should be able to hold the amp draw without an issue. But we’ve got a 30 amp fuse because if the wiring does happen to short out, or get crushed, the fuse will blow before the wire (and in turn your four-wheel drive) starts to burn.
Mind you, I’m not getting into wire run lengths and voltage drop at this stage; I’m trying to keep it simple. That’s a yarn for another day, I reckon.
Ok, so what’s that got to do with compressors and winches?
It’s the other side of the equation we want to think about now; amps. The single biggest killer of electric motors is heat. The term ‘burnt-out motor’ isn’t just a tricky name. First off, you should know with an increase in amp draw comes an increase in heat generation, thus dead or burnt-out electric motors.
There are two things that will increase amp draw in anything electric; first off, load. An electric motor will run at just about any voltage, quite happily, with no load on it (keeping it simple). Add a load to the motor, whether it be turning a winch drum, pulling a four-wheel drive out of the mud or pumping a piston up and down to generate air pressure, you’re loading it up. In turn, you’re increasing the amps drawn, and thus increasing the heat generated.
The second thing that will increase amp draw is the voltage (number of volts) you’re supplying. “But Wes, my four-wheel drive has a 12-volt system,” you might say. You’d be right… and wrong. Hear me out…
When your four-wheel drive is running, your alternator is charging the battery and supplying anywhere between 13.8 volts and 14.4 volts. Turn it off and the battery will still sit around the 12.6 and 12.8 volts. Still more than 12, right? Ok, now run your winch or your compressor with your fourby turned off and all of a sudden you’re down to 11.4 volts to 10-ish volts. When you stop winching or turn off the compressor, the battery will generally recover back up to around 12.6 volts, but while you’re drawing the current from the battery, the voltage drops considerably.
Lets go back to the maths (groan!)
Let’s say we have a compressor (like the ARB twin compressor I used for the video in this yarn) and it’s got a stated amp draw of 40 amps while under load. This amp draw rating is expressed at 12 volts (unless otherwise stated… and it sometimes is at 13.8 volts).
So from our maths above, we know the following:
@ 12 volts, we’re drawing 40 amps which we can calculate as 480 Watts (Watts = 12v x 40a). The motor will run at this generating a bit of heat, but within spec.
@ 14 volts, we’re drawing 34.2 amps (amps = 480w ÷ 14v), so there’s less amp draw, less heat and a happy compressor.
@10 volts, we’re drawing 48 amps (amps = 480w ÷ 10v) , so there’s greater amp draw, greater heat and an angry compressor.
Now, an extra 8 amps of power draw doesn’t sound like much, but when you think about that, over the space of 10 minutes pumping up four 33-inch tyres, and then your mates tyres, or the camper, it gets up there. Especially so when you’re pumping up to 40psi. There’s bugger-all load when your compressor is venting to atmosphere but when it’s trying to build pressure, it gets right up there (mind you, apparently those little ARB twin compressors are good for up to 140psi – I’ve gotta find a proper truck tyre and test that!)
Where this gets rather interesting is when you exchange an air compressor for a winch. The average winch will draw upwards of 400 amps at 12 volts. So add an extra 80 amp draw to that and watch how quick it gets hot and melts the motor!
So you said something about saving money?
If you take nothing else from this yarn (and maths aren’t fun… so I understand if you glossed over that bit), make sure you run your bloody four-wheel drive engine when you’re running any electric motor. Even if it’s for 10 seconds, but especially if you’re going to pump a tyre up or try to recover a bogged vehicle.
Despite shortening the lifespan of your electric motor’s brushes and commutator, you can have a massive failure of the motor, melt the windings up, and your motor won’t work anymore. If you happen to have bought said compressor or winch recently and it’s still within warranty, I’m sorry to tell you, chances are, wherever you bought it from won’t warrant it. That’s because you’ll have done the wrong thing, which caused the failure.
By knowing all this, hopefully you’ll run your engine when you’re airing up or winching, not just put the ‘reds’ on. And chances are this will save you the cost have having to buy a replacement compressor or winch!
Anything else I should know?
This same principle affects all electrics but motors are one of the more impacted bits of kit you’ll have that will suffer from low voltage/high amperage. And it applies to other electrical items, not just those in your vehicle, such as the fridge in your house (though that’s at 240-ish volts) to the phone in your hand. This is one of the greatest reasons lithium batteries have advanced so much, and have been such an improvement for cordless drills, for example, because they give a constant voltage right up until they’re flat. Old Ni-Cad batteries didn’t, and many a motor and set of brushes were burnt out sooner than they do these days.
Another pay to maximise the life of electric motors is to have a bit of mechanical (and electrical) sympathy. Unless you’re in a mad rush, take your time with vehicle recoveries and give your compressor a minute or two to rest between tyres. As well as your electrical components thanking you for years to come, you’ll also get more chin-wagging time with your mates.
One last thing…
If you happen to be an electrical engineer, and need to point out how I’ve skimmed over the details and over-simplified stuff in this story, or you feel the need to educate me on Ohm’s Law, drop us a line… but yes, I know.
Story too long? Check out Wes’ video: