what controls motor speed..

[racoutlaw]

might be a dumb question but as I have never had a cart I don't know..

seen this on the motor of my cart: Voltage: 36 Volts / RPM 2800 - max safe 8000 RPM

so if the max safe is 8000 then is there a way to speed it up rather than running at the stock 2800rpm?

Thanks for the input

[sportcoupe]

Voltage controls motor speed. You won't be able to turn the motor under power to 8000 rpm unless you converted from 36v to 120v. The 8000 could possibly be reached costing down a big hill though. I'm sure the stock motor won't like even 6000 rpm before it may let go in that scenario.

[racoutlaw]

lol ok was just wondering on that. So in that same aspect then replacing the stock controller with a bigger one does what for the performance of the motor stock or bigger one..?? Thanks

[JohnnieB]

What motor do you have that says the max safe RPM is 8k?
(I exploded a stock PDS motor at about 7,600 RPM.)

In general terms, for a series drive system: Volts=Speed and Amps=Torque
More correctly, Volts equals Motor RPM, since Speed is determined by the Motor RPM and final drive ratio. (Differential gear ratio and Tire diameter = final drove ratio)

Regardless of voltage, a series wound motor will draw all the amps the power supply (battery pack) can supply and a speed controller limits the number of amps the motor is allowed to draw.

When the final drive ratio is changed to one that gives the cart more speed for a given motor RPM, a proportional amount of torque is lost. To regain the lost torque or to increase torque for whatever reason, a controller that allows the motor to draw more amps is needed.

However, cart performance is the sum of Volts, Amps, Torque, Speed, Gear ratio, Tire height and some other factors.

[e-z-go1988]

Originally Posted by sportcoupe

Voltage controls motor speed. You won't be able to turn the motor under power to 8000 rpm unless you converted from 36v to 120v. The 8000 could possibly be reached costing down a big hill though. I'm sure the stock motor won't like even 6000 rpm before it may let go in that scenario.

If your top speed with that motor at 2300 rpm is say 20 mph and doubling that rpm (4600 rpm coasting down a big hill) you get 40 mph, that speed is way unstable for the normal cart! I was concerned about over revving my stock motor with the 24v boost and a big hill but you have put my fears to rest (no way will I entertain 40 mph with my cart)
Dennis

[mlittle]

Just wondering...
the older DC railroad locomotives used steps of "transition" to get more speed and pulling power out of the series DC traction motors. One of the steps the designers used was reduced field, this was accomplished by electrically inserting a high wattage low value resistor in parallel with the field, this caused the field to weaken and caused the counter EMF on the armature to be reduced thus allowing the generator (for our application battery) to push more current through the motor and thereby more speed .

[sportcoupe]

Originally Posted by e-z-go1988

If your top speed with that motor at 2300 rpm is say 20 mph and doubling that rpm (4600 rpm coasting down a big hill) you get 40 mph, that speed is way unstable for the normal cart! I was concerned about over revving my stock motor with the 24v boost and a big hill but you have put my fears to rest (no way will I entertain 40 mph with my cart)
Dennis

20 mph on stock 17.4" tires is 4806 rpm's. 30 mph is 7210 rpm's which is almost the rpm JohnnieB saw when his motor blew apart and locked up the rear tires. You don't have to do 40 mph to kill a motor (on stock tires and gears).

[sportcoupe]

Originally Posted by mlittle

Just wondering...
the older DC railroad locomotives used steps of "transition" to get more speed and pulling power out of the series DC traction motors. One of the steps the designers used was reduced field, this was accomplished by electrically inserting a high wattage low value resistor in parallel with the field, this caused the field to weaken and caused the counter EMF on the armature to be reduced thus allowing the generator (for our application battery) to push more current through the motor and thereby more speed .

Interesting but current doesn't equal speed (rpm) in a series motor, voltage does.

[e-z-go1988]

Originally Posted by sportcoupe

20 mph on stock 17.4" tires is 4806 rpm's. 30 mph is 7210 rpm's which is almost the rpm JohnnieB saw when his motor blew apart and locked up the rear tires. You don't have to do 40 mph to kill a motor (on stock tires and gears).

Thank you for the clarification.
Dennis

[JohnnieB]

Originally Posted by mlittle

Just wondering...
the older DC railroad locomotives used steps of "transition" to get more speed and pulling power out of the series DC traction motors. One of the steps the designers used was reduced field, this was accomplished by electrically inserting a high wattage low value resistor in parallel with the field, this caused the field to weaken and caused the counter EMF on the armature to be reduced thus allowing the generator (for our application battery) to push more current through the motor and thereby more speed .

A little engineering trickery. Electrical type, rather than Locomotive type engineer.

You've got the cause and effect skewed slightly, but you're on the right track. (No pun intended)

The torque curve of an electric motor is determined by the relative strengths of the magnetic fields produced by stator (Field) and rotor (Armature) windings. Within certain limits, the higher the field strength, the greater the low RPM torque at the expense of high RPM torque.

In a general sense, Voltage does determine motor RPM, but what voltage actually does is establish the RPM at which forward and back EMF are equal and if the motor does not have the high end torque to overcome the drag, it will not reach that RPM. For example, carts won't go as fast with a windshield as they will without one, or they don't go as fast with four souls onboard as the do with one, or they don't go as fast uphill as they do on level ground.

Shifting the torque curve towards the high end (Amp draw at higher RPM) will offset each of these conditions with the same voltage applied to the motor.

Altering the magnetic strength ratio on the fly is done with Sepex cart motors to enhance their low end torque and increase their top speed. It is called Field Mapping. What is being done in the Locomotive's series traction motor is shunting a portion of the stator current around the windings, reducing the strength of the magnetic field and shifting the torque curve upwards so the EMF and BEMF equal each other at a higher RPM for the same applied voltage.

The net effect is the same, greater amp flow and higher RPM with the same voltage and drag.

If the stator windings stayed shunted for some reason and the train stopped, you might have to push start it because its low end torque would be gone.