DC motors vs AC motors

virtualy

I'm befuddled. I don't have a clear understanding of the differences between DC and AC drive motors. I need to haul dirt, gravel, straw, plants, logs, etc up a 20% grade on a short (100 yard) dirt road. I'd also like to take the dogs about a mile up (uphill) a neighborhood street, and return, of course. Is one type of motor clearly better suited to these missions than the other?

BGW

__________________
This advertising will not be shown in this way to registered members.
Register your free account today and become a member on Buggies Gone Wild Golf Cart Forum

JohnnieB

There are many types of AC and DC motors and each has their merits and shortcomings.
Ironically, most DC motors run equally as well on AC, but I'll limit it to the three types commonly found in EZGO golf carts.
1. Series wound motors. (Series Carts) Maximum torque at stall. RPM is limited by applied voltage.
2. Sepex Motors (SEParately EXcited). (PDS and DCS carts) Less low end torque than Series wound motors. Max RPM is voltage limited, but typically designed for higher RPM than Series wound motors and RPM limited by Field excitation level as well as voltage applied to Armature windings.
3. AC Induction motors (RXV carts). Low torque at low end, but the RXV uses a 3-phase AC voltage to increase torque.

A golf cart is designed to carry two players and their clubs over groomed cart trails and to do much more than that, the drive system has to be upgraded. Upgrading a cart with a series wound motor is probably best for hauling stuff up hills, but a properly upgraded Sepex system will do the job almost as well and will have more speed without increasing battery pack voltage.

The RXV (AC motor) is fairly new and while they can be programmed by the dealer for different performance parameters, the aftermarket stuff for them is limited, so your ability to DIY is limited.

virtualy

Excellent reply. And the penny drops.
Forever indebted.

mtpolarbear

JohnnieB, I am doing the frame up resto on my '84 Marathon. I would like speeds between 25-30 and I only need need average torque. Does an AC motor make sense for me?

JohnnieB

I'm more into troubleshooting and theoretical stuff than building things, so there are many around the BGW loony bin far more qualified the answer that question.

In fact, no matter what you think about doing with or to a cart, there is probable somebody around here that has done it, or tried to do it, or at least thought about doing it.
Of course. the trick is to find those people.

I'd try looking through other sections of the form and post you question in likely places. Places like Extreme DC or Modified Carts or Restorations for example.

---------
My thoughts: An AC motor might do the job, but an AC system is going to be more expensive and there will be fewer options in how it is built.

In general terms, an AC motor only has modest torque and the cart gets the torque that is applied where rubber meets road by converting RPM into torque through gearing.
IE: the standard differential gears in carts with Series and Sepex motors is 12.44:1 while the standard gears in a RXV cart with an AC motor is 16.99:1, which means the motor has to spin 37% faster for the same ground speed.

Personally, if I was going to build an on-road cart to run in the 25-30MPH range, I'd use a Sepex motor.
If I was going to build an off-road cart, I'd use a Series wound motor.
If someone gave me a cart with an AC motor, I'd use it for a trade-in.

Of course, there are probably some cart builders than disagree with my opinions.

txbonds

I've got no knowledge to dispute what you are saying, but it does make me wonder why the Polaris Ranger EV and the various Bad Boy Buggies models are running AC setups if there is no torque per se. Those are builders building carts just for off road and farm type use, so how are they making it work? Is it the 72v? The gearing?

I don't own an AC cart and am not buying one, but was curious about the Ranger when it claims speeds into the 30's, range of 50 miles on a charge, and has the low end torque to be an off road usable cart. I'm hearing the same things about the new Bad Boy's too. Those carts had me thinking about an AC setup one day for my own, but the cost has prevented me from seriously considering it.

Just curious.

JohnnieB

No problem.

I have no knowledge about the BBB and Rangers using AC motors, so I don't know what they do or how they do it.
My thinking and comments were more or less limited to off-the-shelf EZGO components.

Induction type motors have low starting torque, that is why the ones that have to be started under load have "starting" capacitors or shaded poles.
Note: The ones with "Run" capacitors are shaded pole motors that add capacitance to the shaded pole once they are at speed to alter the power factor and increase efficiency.

Another issue I have with induction type motors for mobile applications is that they only way to carry electrical power with you is in a battery or something similar and the output of those storage devices is DC and DC choppers are more efficient than the DC to AC inverters needed to run an induction motor.
(The controllers used in Series and Sepex carts are DC choppers - The output to motor is Pulsed DC)

I'm sure there have been improvements in AC induction motors and DC to AC inverters since I last looked into them, but if I was going to build a cart with an AC drive system, I'd use multi-phase power at a high line frequency and a motor designed for high RPMs and gear it down to usable axle speeds.

Maybe, there are some AC systems out there that have overcome or circumvented my fundamental concerns. I don't know.
That's why I first suggested asking around in other areas of BGW before giving my opinion.

trbizwiz

Scottyb's site has some information about the differences. I think it boils down to Heat, pardon the pun.
AC motors generate less heat at low speeds and dc motors generate more heat at low speeds.
There are things you can do to each to make them better suited to what ever you are doing. But I woudl guess you need to decide the primary purpose, and the cost to make it do each purpose. That will help you decide which cart to buy. I would guess DC carts would be cheaper over all to compensate for doing what they were not designed to do. but they may not reach your goals.
I am in the same boat as you. trying to decide which cart to buy to build off a great platform.
I want an off road buggy, but still want some speed sometimes.
I am leaning towardan RXV. It will be more costly to build. But these carts seem to come pretty capable. Also I suspect leases will be ending soon, adn used carts will be starting to flood the market. The tech should follow that and upgrade prices will start to drop. It will just require some patience.

simple man

I may be totally wrong here, but I would think converting DC ( batteries ) to AC ( for an induction motor ) to be rather inefficient! You would also be putting more equipment into a cart to do it.

txbonds

http://www.polaris.com/en-us/ranger-...er-ev/features

http://www.badboybuggies.com/sport/recoilis.html

http://www.badboybuggies.com/sport/instinct.html

Here are the ones I was talking about. Polaris is 48v and bad boy is 72.