Controller Amperage Question

[scottyb]

Originally Posted by Frizbgolf

I agree with both of you, if voltage is the water in the hose, and rpms are determined by the voltage available, well then it only makes sense that the rpms of a motor are directly proportional to how fast the voltage arrives at the motor, ie amperage is the water pressure, therefore the more ampacity that your controller can deliver to the motor the faster the voltage will arrive there and the faster the motor will reach maximum potential rpm. Basic electrical theory says that a joule (measurement of electrical power) is equal to one coulomb passing a single point in a single second therefore the faster the coulomb reaches that point ( amperage) the more power. Man I thought my theory teacher was full of crap when he said this stuff would be usefull someday lol.

I challenge you to find 5 people who understand what you just posted

[sunking]

Originally Posted by Gorhad

so would a smaller amp controller limiting the motor in rpm due to the current it was pulling

Nope rpm and voltage are proportional. Current supplies the torque.

DC series wound motors do not work in the way most would think. Maximum torque is delivered at Stall RPM (0 rpm) and decays to 0 proportionally as RPM reaches max RPM.

Maximum power is supplied at half the rated RPM. The graph below is a classic Torque, RPM, and Power curves.



So the take away for you is if you change out the controller to a higher amperage, and leave everything else the same. You only gain torque on the low end, but rpm's or top end speed stays the same. To get more speed you need either need more voltage, or a higher rpm motor.

[scottyb]

Well said SunKing

[Sam Pryor]

Voltage goes up Amperage comes down.
Voltage comes down Amperage goes up

[scottginfla]

Let me add my .02, I have always been under the impression that a 6 battery 48v pack can only produce a maximum of 500 amps at any one time, so the need for a controller bigger than 500 amps is a waste of money, is this right?

[impalass88]

Originally Posted by scottginfla

Let me add my .02, I have always been under the impression that a 6 battery 48v pack can only produce a maximum of 500 amps at any one time, so the need for a controller bigger than 500 amps is a waste of money, is this right?

Not totally correct. I spoke with an engineer at Crown battery and asked this same question. His reply was that typically they suggest only a 500 amp load BUT if your controller, motor, wires, and solenoid will handle and pull more, the batteries will deliver it. I am running a 650 amp AllTrax AXE controller and it will draw 650 amps under extreme load. They just don't usually suggest it.

[sunking]

Originally Posted by scottginfla

Let me add my .02, I have always been under the impression that a 6 battery 48v pack can only produce a maximum of 500 amps at any one time, so the need for a controller bigger than 500 amps is a waste of money, is this right?

Pretty much but it depends on the batteries internal resistance.

I will try to explain. If you look at an automotive battery you will see something called Cold Cranking Amps or CCA. What this tells you is the maximum current the battery can deliver for 30 seconds if fully charged at 0 degrees and maintain a voltage of 7.2 volts for a 12 volt battery. So let's say the CCA = 500 amps. That tells you the battery internal resistance is 7.2 volts / 500 amps = .0144 ohms, or 70 moh's.

You will not see a CCA rating (or MCA either) for a true deep cycle battery. If you do is a dead give away it is not a true deep cycle battery. Example you will here some claim a Marine deep cycle battery is a deep cycle. IF it has a MCA rating don't believe it, it is a hybrid cross which you do not want in your golf cart.

Ok here is the deal, a true deep cycle battery has higher internal resistance than a comparable cranking battery. For a Trojan T-105 they are secretive about the internal resistance, but after running some test I came up with a ball park figure by putting some various loads in a lab. I came up with around .015 ohms. So what this means is for 6 volt Trojan T-105 will deliver a maximum of 400 amps with a terminal voltage of 3 VOLTS.

Note what is happening here that 6 volt battery drops to 3 volts at maximum current capacity. That is a function of Ohm's Law and the internal resistance of the battery. You cannot get around it other than add another parallel string of batteries to get more current, or a much larger AH rated battery with a lower internal resistance.

So in my opinion anything more than 400 amps is a waste of money. Keep in mind even though you may install a 400 amp controller does not mean the motor can handle that much current. But that is another thread.

[gornoman]

Originally Posted by sunking

So in my opinion anything more than 400 amps is a waste of money. Keep in mind even though you may install a 400 amp controller does not mean the motor can handle that much current. But that is another thread.

Let me add that the statement applies to conventional golf cart style batteries. Advanced design cells could take advantage of much higher current draws. However, now we have crossed the line into the racing zone.

[sunking]

Originally Posted by gornoman

Let me add that the statement applies to conventional golf cart style batteries. Advanced design cells could take advantage of much higher current draws. However, now we have crossed the line into the racing zone.

I would be interested in what product line you are referring too out of curiosity.

[gornoman]

Check out the units GarageBuilt is running in his drag cart. They are tiny little things that discharge very fast @ high current AND with less voltage drop than traditional cells. I do not have the brand name or specs. If I remember correctly he can only get 2 drag runs on a charge. Not real practicle, but FUN TO WATCH!