I hope this hasn't been covered already at nauseum, a quick search did not reveal any specific results.
In order to save some money, (I believe most newbies and first time builders would probobly agree), and instead of buying a 72 volt 8 HP motor for $500 bucks, what if I picked up one of them used golf cart 36v 3 HP or 48v 4 HP motors on eBay for $100 bucks and just ran them at higher voltage. I read somewhere (and this is where I get dangerous) that increasing the voltage increases the RPMs, and that you can run these motors at twice the volt ratings, but you're still limited on the amperage. However those golf cart motors are rated for under 20 MPH, so could I get them up to 40 MPH. I think what ever motorcycle (like a Ninja 900) I convert it will weigh less than a golf cart.
Assuming I don't care how long the motor lasts me, I'm not thinking years here I am thinking months. Also I just need this for a 9 mile trip to work and back, so roughly 20 mile round trip. That's maybe 20 minutes of run time each way, with stops at red lights etc. Maybe occasionally run an errand, or pass a few gas stations and laugh.
golf cart motors are almost always brushed.
increasing the voltage on a brushed motor is more likely to end badly as you are working against the limitations of the commutator.
the margin of error allowable in brush timing diminishes the higher you go.
step outside of this margin, and you get arcing on the commutator, usually followed by rapid failure.
the ideal brush timing varies with rpm and amps.
golf cart motors are usually fairly heavy for the power they put out.
you are correct, higher voltage means the same torque at higher rpm (if the motor survives)
on the off chance you find a BLDC motor, then yes, you can bump up the voltage safely (though you might need a new controller to accept the higher voltage, though this is the same for the brushed motor)
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
Alot of builds use golf cart and forklift motors... they can safely go to 1.5-2 times their rated voltage. Once you start going much higher than 96V on a 48V motor, you need to get the brushes advanced. Most motor shops can do this.
I use a K91-4003 motor in my conversion, works great at 72V, and I plan on going higher.
____________
Travis Gintz
1986 Honda VFR Conversion
www.evfr.net
I read somewhere... that you can run these motors at twice the volt ratings, but you're still limited on the amperage.
Hold on a second! If you increase voltage at any given RPM's shouldn't that increase amperes as well? After all voltage is likened to "electric presure." If you run twice the "presure" or voltage through the same resistance wouldn't that double the current (amps)? Of course the amperage will drop as RPM's increase due to the back electromotive force produced in the spining motor. What I'm trying to say is that doubling the volts should also double the amps and in effect quadruple the watts and internal heating of the motor.
your controller will explicitly control the actual motor side voltage (and therefore implicitly the motor side current).
you are correct in saying doubling the voltage at a particular rpm will increase (depending on the motor type this can be double, or many multiples) motor current.
when you increase the voltage available to the controller, you increase the rpm at which the controller can apply a high enough voltage to the motor to stay in current limit.
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
another thing to consider is that MOST golf cart motors that I've seen don't have an end cap... its meant to fit into the transmission. Basically there's no bearing or support at one end.
____________
Travis Gintz
1986 Honda VFR Conversion
www.evfr.net
I really don't agree to this suggestion. If you are playing golf, then you must spend some money. Why go for cheap cart parts and accessories. Golf is a game for the privileged.
controllers over 72V are much more expensive. Seriously consider using the 48V motor at 72V and then adding a 2-speed transmission.
I have seen several builds where a motor was placed where the gas engine was, onto a manual transmission and clutch, both a motorcycle and a car. The moto used only 2 gears out of 5, and the car used 3 gears out of 5