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Old 08-11-2009, 03:36 AM   #1
Easyelectric
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Default Benefits of higher voltage with proper gearing.

Hello, I am new member here. I read through some of these posts and I decided to help you guys out with DC motor understanding and gearing.

Today I want to concentrate on series-wound motors as they still make a up large market in electric golf cart sales. All DC brushed motors have the following characteristics: speed is controlled by voltage and their torque by current. Knowing this we can predict the motors behavior for max torque at zero RPM or any torque figure beyond 0 rpm up to full load rating and the speed at various voltage levels when provided with the motor's various data curves.

So lets do an example that may be mind blowing to some of you. Lets say our golf cart will use a Alltrax AXE7245 for the 72 volt setup with current set at 400 amps limit and the 36 volt set up using Alltrax AXE4845 which is set at 400 amps current limit. All else will be equal such as same motor, gear ratio, tire size. When know the speed will be higher for the 72 volt setup and we know that since everything else is equal except for voltage that both will have the same peak torque at the wheels at 0 RPM.

So what is so amazing about this other than more speed? Well pulling power is one of them if we change one of the variables, which will be gearing. Let's say the 36 volt setup goes 27 MPH. Both motors will have the same peak torque as both of them have the current limit set at 400 amps. Here is where it changes; the 72 volt system when geared to go at the same speed as the 36 volt system will have more pulling ability. That's right, you heard me correctly. Why is this? Because the gear ratio has multiplied the motor's torque by a higher ratio, thus resulting in more torque at the wheels.

The 72 volt motor has developed more horsepower or more specifically the motor at 72 volts has a speed torque curve that has shifted up by a factor of 2! Doubling the voltage has resulted in the motor producing double the RPM's for various load points. So if a load for example needs 20 ft*lbs of torque that causes the motor to be loaded down to 1500 RPM, the motor will spin at 3000 RPM for the same torque value when the voltage is doubled!

Gear selection is easily accomplished if the same speed is desired as it was when it was at a lowered voltage by simply multiplying be the factor you have increased the voltage of the motor at. So a stock golf cart with a 6:1 gear ratio at 36 volts would be increased to 12:1 at 72 volts. As stated earlier you get better pulling power, but as well as this, the current draw will be less for the same speed resulting in a cooler running motor and cooler electrical connections! This means less power losses as heat resulting in better run times. Keep in mind that in order take advantage of this YOU MUST increase the gear ratio by a factor of however much you increased the voltage to.

There are some negatives though. At some voltage point the motor will not be able to tolerate the higher voltage because armature reaction starts to come in to play. You will know if your motor cannot handle the extra voltage by looking for any excessive sparking where the brushes touch the commutator. If you see visible large yellowish to white sparks then the voltage is too high and must be reduced or commutator pitting damage will result.


I hope I was helpful!
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Old 08-11-2009, 07:14 AM   #2
galloway
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Default Re: Benefits of higher voltage with proper gearing.

First off welcome to the boards.

so you are saying. . . .
if you have a 36v system running 6:1 gears and you put 72v in the cart it will run like it has 12:1 gears?

but instead of running 3000 rpm @ 36v your motor should be able to turn 6000 rpm @ 72v?

and if this post is in reference to my posts about motor acceleration etc, You are missing my point. I am trying to figure out the gear/tire combo for a drag cart to get the most out of it in an 1/8th mile without having to buy 10 sets of wheels and tires and 4 sets of gears.
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Old 08-11-2009, 07:41 AM   #3
GarageBuilt
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Default Re: Benefits of higher voltage with proper gearing.

There are some negatives though. At some voltage point the motor will not be able to tolerate the higher voltage because armature reaction starts to come in to play. You will know if your motor cannot handle the extra voltage by looking for any excessive sparking where the brushes touch the commutator. If you see visible large yellowish to white sparks then the voltage is too high and must be reduced or commutator pitting damage will result.

Welcome to Buggies Gone Wild.

Home of some of the world’s quickest high voltage Electric Golf Carts in the world. This is a great first post, & I think you for it. But here at BGW we have no lamentation. We work thru our problems. High voltage carts are a big deal right know & still getting bigger. With the support that we are getting from the after market guys. As in motors that can live at 7,000 to 8.000 RPM"S & even more. controllers that don’t liment the amount of volts going to the motor. And all of the sick guys that are here building & racing these things.

What is considering high voltage by today’s standers? Less then 3-years ago, it was any thing over 48-volts. Today I have a 108-volt cart I don’t consider that high voltage anymore. Not when we have guys that are very capable of pushing 200 plus volts thru a DC motor.

So take all of your knowledge, go buy & build a 1/8 mile drag cart & come have fun with us. Once again, Welcome to BGW.
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Old 08-11-2009, 08:40 AM   #4
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Default Re: Benefits of higher voltage with proper gearing.

Welcome to BGW, you are on the right track. Keep in mind there is no free ride and the price for this setup is double the wattage. If both the 72V system and 36V system are at the same amperage the 72V system will be pulling twice the number of watts out of your battery pack.
I am with Garagebuilt, build a high voltage cart and race for fun! You seem to be in to this kind of stuff and thes folks are great people to hang out with.
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Old 08-11-2009, 12:28 PM   #5
Easyelectric
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Default Re: Benefits of higher voltage with proper gearing.

Quote:
First off welcome to the boards.

so you are saying. . . .
if you have a 36v system running 6:1 gears and you put 72v in the cart it will run like it has 12:1 gears?

but instead of running 3000 rpm @ 36v your motor should be able to turn 6000 rpm @ 72v?

and if this post is in reference to my posts about motor acceleration etc, You are missing my point. I am trying to figure out the gear/tire combo for a drag cart to get the most out of it in an 1/8th mile without having to buy 10 sets of wheels and tires and 4 sets of gears.
No, you must physically gear the the motor down to 12:1 for the same top speed as it would if you ran it at the lower voltage with the lower ratio. If the motor slowed down to 3000 RPM from some X torque and you double the voltage and the same X torque figure is needed then speed will be doubled of what it was versus at a voltage level that was half the value. My post is not in reference to yours.


Quote:
Welcome to Buggies Gone Wild.

Home of some of the world’s quickest high voltage Electric Golf Carts in the world. This is a great first post, & I think you for it. But here at BGW we have no lamentation. We work thru our problems. High voltage carts are a big deal right know & still getting bigger. With the support that we are getting from the after market guys. As in motors that can live at 7,000 to 8.000 RPM"S & even more. controllers that don’t liment the amount of volts going to the motor. And all of the sick guys that are here building & racing these things.

What is considering high voltage by today’s standers? Less then 3-years ago, it was any thing over 48-volts. Today I have a 108-volt cart I don’t consider that high voltage anymore. Not when we have guys that are very capable of pushing 200 plus volts thru a DC motor.

So take all of your knowledge, go buy & build a 1/8 mile drag cart & come have fun with us. Once again, Welcome to BGW.
The problem is as I stated you will have severe sparking that will put deep pit marks in the commutator bars and the brushes. What I mean by high voltage is in reference to what the motor can handle without showing the signs of armature reaction (i.e. excessive arcing). There is a way around this by advancing the timing on the motor by shifting the end bell that has the brush assembly a few degrees from the neutral point. I had to have my 36 volt A00-4009 ADC motor on my electric go kart advanced 5 degrees in order to reduce the sparking for 72 volts. I did run it at stock before I had this done and the result was that I had to get new brushes and the commutator shaved till the pit marks where gone. So to save the headache I advise to keep it on the safe side unless you get the timing advanced or if the motor has interpoles in which case it can be left as is or if money is no object then simply run the motor till it dies.

Quote:
Welcome to BGW, you are on the right track. Keep in mind there is no free ride and the price for this setup is double the wattage. If both the 72V system and 36V system are at the same amperage the 72V system will be pulling twice the number of watts out of your battery pack.
I am with Garagebuilt, build a high voltage cart and race for fun! You seem to be in to this kind of stuff and thes folks are great people to hang out with.
That is not a "price to pay" as you say. If the batteries are the same amp hours, and if the same current level is drawn, then the run time will be equivalent. The price you pay is can the low voltage motor handle the higher voltage without damage to commutator and brushes.

Also I cannot think of anyway for the new voltage to cause the motor to draw the same amps unless it was a constant torque load. A load that comes to mind that would cause this is hoist type loads where objects are lifted against gravity and the gear ratios where the same as before as with the old voltage. So you increase your feet per minute of lift rate but the same torque is required by the motor for the new voltage as well. I rather gear it down till the FPM was the same as it was for the lower voltage thereby taking advantage of going with higher voltage which reduces the current drawn which will reduce heat losses in wiring or buy motor designed for higher voltage which requires no gear ratio change at all and the current will be less as well, but changing out a gear is cheaper than buying a new motor for the voltage change.
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Old 08-11-2009, 12:35 PM   #6
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Default Re: Benefits of higher voltage with proper gearing.

To BGW
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Old 08-11-2009, 12:44 PM   #7
galloway
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Default Re: Benefits of higher voltage with proper gearing.

Hey Easy,

The people you check out this section of the forums are people that are pushing the limits of the equipment. They are buying specialized motors that are balanced up too 7 to 8000 RPMs.

The price you pay that Nate was talking about was the cost of the bigger controllers and the price of the 6 12v batteries and such.

as for your mention of going with 12:1 gears @ 72v. If I am not mistaken, don't carts come stock with 12:1 gears?

I think your post has alot of good info, but I think you are talking about apples and oranges.

Are you talking about how to get the most of your voltage on a lifted cart or a drag cart?

If you were going 72v and wanted speed why would you go from a set of 6:1 gear to a 12:1 gears that would be undoing the voltage upgrade.

now if you were going 72v and spin a set of 22" off road tires. going from a 6:1 to a 8:1 gear or 12:1 makes more sense to make up for the motor having to spin the extra weight of the tire, but with the extra height of the tire you would gain some speed back.
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Old 08-11-2009, 03:36 PM   #8
Easyelectric
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Default Re: Benefits of higher voltage with proper gearing.

Quote:
If you were going 72v and wanted speed why would you go from a set of 6:1 gear to a 12:1 gears that would be undoing the voltage upgrade.

Better pulling than the one with stock voltage and gearing, but same top speed and less losses as heat. This was mentioned in my very first post...I find pulling to be extreme. I'd be willing to bet you that a 72 volt fitted golf cart with gear ratio change and extra weight of the added batteries would drag a stock 36 volt one because as I stated the torque at the wheels is greater for the 72 volt one with it's 12:1 gearing. Everyone has their own definition of extreme. I think pulling is.

If speed is extreme in your view then it would not make sense doing what I am doing.
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Old 08-11-2009, 04:04 PM   #9
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Default Re: Benefits of higher voltage with proper gearing.

Most of you guys are talking about speed. I think easyelectric is saying that with everything constant except voltage you can expect more available torque, but you would have to correct the gearing to have the same speed. A jump from 36 to 72v would cut your current in half, and probably come close to double the run time. Like he said it all depends on the the strength and condition of your commutator . I dont think hes trying to win any races.

James
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Old 08-11-2009, 04:14 PM   #10
jahaiap
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Default Re: Benefits of higher voltage with proper gearing.

Quote:
Originally Posted by Nate View Post
Keep in mind there is no free ride and the price for this setup is double the wattage. If both the 72V system and 36V system are at the same amperage the 72V system will be pulling twice the number of watts out of your battery pack.
How do you figure your current will be constant if your voltage is your variable? Im really curious. I dont have an electric cart and Im not sure how the controllers work. Im an electrician and I work on AC motors often, mainly pumps, just not familiar with DC. Thanks

James
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