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Old 10-30-2011, 10:41 AM   #31
72volt_ezgo
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Default Re: DC motor voltages.

And for anyone reading this:
I was purely making a point with my example of illustrating how drastically amps drop when voltage is doubled for more or less the same usage on a regular cart with everything else being stock (minus high voltage battery pack and controller)!

I said it in post #7 and I'll say it again.. I'm all for good solid battery cables of larger diameter!!! It will do nothing but good for the system! Go get it if you want it/need it!

My case is just an illustration/example! I do not recommend it as standard permanent hookup! It just works for me in my application as a utility cart and when I park my cart at the end of the day,I can quickly and safely disconnect the main battery power from the cart to leave it there until next usage.

Barna
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Old 10-30-2011, 11:05 AM   #32
simple man
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Default Re: DC motor voltages.

Quote:
Originally Posted by 72volt_ezgo View Post
And for anyone reading this:
I was purely making a point with my example of illustrating how drastically amps drop when voltage is doubled for more or less the same usage on a regular cart with everything else being stock (minus high voltage battery pack and controller)!

I said it in post #7 and I'll say it again.. I'm all for good solid battery cables of larger diameter!!! It will do nothing but good for the system! Go get it if you want it/need it!

My case is just an illustration/example! I do not recommend it as standard permanent hookup! It just works for me in my application as a utility cart and when I park my cart at the end of the day,I can quickly and safely disconnect the main battery power from the cart to leave it there until next usage.

Barna
You wouldn't believe what I have used for jumpers! Temporarily, of course!
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Old 10-30-2011, 11:25 AM   #33
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Default Re: DC motor voltages.

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Originally Posted by simple man View Post
................The voltages on the various controllers confuse me too. Some are dedicated 1 operating voltage, some 2, some a great range! If you wouldn't mind, could you explain how this works, and is a dedicated voltage model better than one that can take various voltages?

@ " bbriggs " and " scottyb " I'm going to take both of your suggestions to go with the 48V pack. After consideration of cost and reliability, I really think this is the way to go!
The advertized voltage is just that, advertized. Probable determined by accountants and/or lawyers rather than engineers.

The output stage to the motor consists of several (a dozen or more) MOSFETs in parallel. The max voltage is determined by the max voltage rating of the individual MOSFETs. It is the same whether there is only one or dozens in parallel. The same is true of min voltage.

On the other hand, current capacity is determined by the current rating of the individual MOSFETs times the number in parallel. Example: If a 200 Amp controller had a dozen MOSFETs in parallel, a 400 Amp controller would have two dozen. (or use more expensive MOSFETs with a higher current rating)

In ether case, they will work at a voltage or current less than max. How much less, I don't know.
(NOTE: That is max rated - not max adevertised. Most likely the max advertized is already below max rated or they are going to be replacing a bunch under warranty)

There is another consideration on voltage. The control circuits within the controller will have a max/min voltage range over which they will work.
This was pointed out by the suggestions to up the motor to 72V and run the interlocks and throttle at 36V.

I believe the best thing is to select a controller with the highest current rating that is affordable and the highest voltage rate you foresee in the future.
Since speed isn't your goal, current is King.
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Old 10-30-2011, 11:37 AM   #34
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Default Re: DC motor voltages.

@JohnnieB!
Thanks for the excellent post on back EMF!

I think of it as doubling the voltage will not going to increase motor torque but greatly widen the rpm range in which the motor can still provide usefull torque!

As the input voltage is being countered by the generated voltage by the motor when the motor rpm reaches a point where these to voltages are equal,(running on a slight downgrade for exampe)there is zero torque left/no acceleration because current flow diminished. If we can double the input voltage at this point, the difference between input voltage and generated counter voltage becomes greater therefore allowing current to flow again and accelerate the motor to a new higher rpm where the new equilibrium is established and no more acceleration. (not counting wind resistance,rolling resistance etc).

A similar effect when charging batteries,the difference between battery voltage and the voltage that the charger is trying to provide (but clamped to battery voltage) promotes current flow toward the battery. As battery voltage is increasing, the difference between battery voltage and charger voltage is decreasing wich is proportional to the current flow hence we see the ammeter decline toward the end of a charge!
This is just the way I see it.

Thanks!
Barna
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Old 10-30-2011, 11:46 AM   #35
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Default Re: DC motor voltages.

Thank you, " JohnnieB "! That was a great explanation! I do suppose these ratings take into account the higher voltages during charging?
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Old 10-30-2011, 11:58 AM   #36
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Default Re: DC motor voltages.

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Originally Posted by scottyb View Post
JohnnieB, thank you for sharing your electrical knowledge about this electrical 'theory' that 72v requires little concern for competent conductors.
I'm not entirely sure if that is a or a , but it doesn't matter how much or little voltage you use, you've got to use compatible cable and components for the task.

Fortunately (or unfortunately depending on your point of view) higher voltage systems are more tolerant of poor and/or inadequate wiring than lower voltage systems. A prime example of this is the Power Grid.

About the highest voltage commonly used is 440VAC, but the voltage on the power lines coming from the generating plant is measured in KV. Reason, wires have resistance and the more current that passes through that resistance the more heat is generated. By increasing the voltage on the transmission lines to KV and then stepping it down at the user end, the power companies don't loose as much energy in heat ($).

Personally, I don't use 2Ga cables on an otherwise stock cart because I have plans to increase its speed or performance, I use them because they are more efficient.
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Old 10-30-2011, 01:46 PM   #37
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Default Re: DC motor voltages.

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Originally Posted by 72volt_ezgo View Post
................I think of it as doubling the voltage will not going to increase motor torque but greatly widen the rpm range in which the motor can still provide usefull torque!...........
That's close enough to win the cigar.
Combine it with the way a battery charger's current drops as the battery pack voltage rises and you've really got it nailed.

Back-EMF is the same for a given RPM regardless of the battery pack voltage.
That means the voltage differential between Back-EMF and battery pack voltage is greater with a 72V system for a given RPM than it is with a 36V system and the 72V system will push more current through the motor than the 36V system. Viola - More torque over a wider RPM band.

Example: If you are moseying along at 10 MPH and encountered a hill that requires full throttle to maintain your speed with a 36V system, you could accelerate from 10MPH while going up that same hill with a 72V system, or go up a much steeper hill without loosing speed.

Technically, the heat generated by a 72V system rolling along at a quarter, half or full throttle is about the same as a 36v system. (Rolling resistance/drag converted to heat) The heat problem occurs during acceleration.

Simply stated, Electrical energy not converted to mechanical energy (motion) by the motor is converted to heat and a 72V system is capable of sending more current through a slow turning motor than a 36V system. Since Newton;s Laws of motion apply equally regardless of pack voltage, more heat will be generated by the 72V system.

Of course, there are controllers and motors designed for 72 volts. And there are system used in lower voltage system that are capable of running at higher voltages.

Bottom line: Be aware of the capabilities and limitations of your system.
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Old 10-30-2011, 01:51 PM   #38
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Default Re: DC motor voltages.

Quote:
Originally Posted by JohnnieB View Post
Simply stated, Electrical energy not converted to mechanical energy (motion) by the motor is converted to heat and a 72V system is capable of sending more current through a slow turning motor than a 36V system. Since Newton;s Laws of motion apply equally regardless of pack voltage, more heat will be generated by the 72V system.
Add voltage drop across the pack into this equation and the amperage raises even more for the same amount of work done. 500a systems NEED 2g cables, regardless of system voltage.
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