monitor or set RPM limit

[Capt. Lenny]

I have a 2003 TXT series cart that I'm doing an upgrade to 48 volts on. So far I've installed 2 gauge wires, Alltrax 48500 500 amp controller, 400 amp HD solenoid, HD F&R switch (soon going to a HD reversing contactor when Scotty gets them in stock) And a Bandit motor from Plum Quick. Robbie advises not to exceed 6500 RPM with that motor so I'm wondering, how do I monitor the RPM? Since this motor doesn't have a speed sensor wire can I use the Alltrax tool kit to set an RPM limit when I install the 48 volt battery pack? Also, does anyone have any recommendations for initial settings for the Alltrax controller? I have not accessed the toolkit since Scotty B programed it for 36 volts when I bought it.

[JohnnieB]

Since series drive motors do not have speed sensors like sepex drive motors do, series drive controllers cannot limit or monitor the motor's RPM.

If you know the exact height of the rear tires and the exact gear ratio of the transaxle/differential, then you can calculate motor RPM from cart speed.

For example, the stock EXGO TXT differential gears are 12.44:1 and the stock tire height is 18", so with stock tires and gears the motor will be spinning at 6500RPM when the cart speed is 28.0MPH. Change either the gear ratio or tire height and the cart speed for 6500RPM changes. IE: With 20" tires, 31.1MPH is 6500RPM at the motor.

Since the differential gears are most likely stock, measure your tire height and I can tell you how fast the motor spins per MPH of cart speed.

[Capt. Lenny]

Thanks JohnnieB! The gears are stock and the tires are replacements that measure 18-1/4" so I'm pretty close to your calculation. What I wasn't sure of is, if by going to 48 volts if the motor would be spinning faster exceeding 6500 RPM.

[cgtech]

They seem to do fine on 48v.

I ran my factory "freedom speed" (18.5mph stock) motor on 48v with no problem at all.

[JohnnieB]

Originally Posted by Capt. Lenny

Thanks JohnnieB! The gears are stock and the tires are replacements that measure 18-1/4" so I'm pretty close to your calculation. What I wasn't sure of is, if by going to 48 volts if the motor would be spinning faster exceeding 6500 RPM.

With 18.25" tire and stock 12.44:1 gears, 6500RPM at the motor is a cart speed of 28.4MPH.

Not sure what the top speed on level ground for your cart with a PQ Bandit installed is, but whatever it is going from 36V to 48V will increase the top speed on level ground by 33.3%

Another place to watch out for is how fast the cart goes downhill.

[Capt. Lenny]

Thanks for your reply JohnnieB! I have read about your experience with over-revving going down hill and that's what led me to asking the question about rev limiting. Fortunately in Florida we don't have very many hills , it's mostly flat ground. I guess the best rev limiter for my setup is my right foot!

[JohnnieB]

Originally Posted by Capt. Lenny

Thanks for your reply JohnnieB! I have read about your experience with over-revving going down hill and that's what led me to asking the question about rev limiting. Fortunately in Florida we don't have very many hills , it's mostly flat ground. I guess the best rev limiter for my setup is my right foot!

I've been told highway overpasses are the highest and steepest hills in FL.

Add some type of speedometer to your cart if a 48V battery pack will put you close to the max recommended motor RPM on level ground.

Exploding my motor was a combination of high RPM, high heat and high regen braking. It probably would have survived the revs alone.

With proper banding and balancing, a DC traction motor can safely spin quite a bit faster than 6500RPM.