What power is needed to run a 48V electric motor?

[Beefcaked]

I have an 48v Electric motor with no details of the power required to drive it. This was previously powered with 8 x 6v batteries wired in series details as below:

8 off 6v 225AH Batteries
Capacity in minutes: 447 @ 25amps / 115 @ 75amps
5hr Rate - 185AH
20hr Rate - 225AH

This then passes to the motor via a speed controller through a speed controller which reads 48v 370A.

I would like to power this by transforming down a 240v supply - Obviously I need to transform the voltage down to 48v but what power output (watts or VA) would be required from the transformer to run the motor successfully.

This information I hope will allow me to then determine the correct supply and transformer to run this motor.

Any help would be much appreciated.

Many Thanks

 

[sophiecentaur]

Hi and welcome.
You've got a pretty meaty motor there, if the speed controller is anything to go by (it seems to be rated at 17kVA and is unlikely to have been vastly over-designed). That's not suitable for supplying from a 13A mains socket, which gives you around 3kW (haha). Was this motor used for traction? It wouldn't have run for long on a charge from that battery bank. Do you know its history?

 

[Beefcaked]

It is actually the motor for a golf buggy - I am trying to covert the electric motor to run via a small generator and transformer - I would guess that the batteries are designed to run for about 4 - 5 hours before recharging.

 

[russ_watters]

Googling, it looks like golf cart motors top out at about 10 hp (about 8 kW), which is still quite a bit for a household-sized supply, but not impossible. You'd need about a 60A circuit breaker at 240V.

In any case, I got to ask: you want to run a generator from a motor? You know that'll just be an overly complicated electric heater, right?

 

[jim hardy]

I am trying to covert the electric motor to run via a small generator and transformer -

Are you replacing batteries with an engine & generator?
If I've misunderstood, just skip remainder of post.

The golf cart I owned had 6 batteries and a 1.5 hp motor. Sounds like yours is somewhat larger...
Mine had the electric motor bolted directly to differential. So to go to pure mechanical engine drive would have required sophisticated machine work. I'm guessing yours is same.

I would not mess with AC at all. I'd belt a surplus alternator or generator to the engine and make DC directly. If you use a transformer it will need to have about as much iron in it as as your generator does. (Of course that's probably ok since you're replacing lead batteries...)

http://www.govliquidation.com/auction/view?auctionId=6312019&convertTo=USD
you could probably boost it up to 36 volts easily, maybe even 48.

http://www.prestolite.com/pgs_products/specs.php?item_detail_id=274&item=A0014417AA&product=Alternator

Be aware that the batteries can briefly overload that DC motor severalfold to start the cart. You will notice reduced acceleration with a gas generator that's sized equal to the golfcart's electric motor.

but what power output (watts or VA) would be required from the transformer to run the motor successfully.

What is its nameplate horsepower?
One hp is 746 watts, and one watt is one volt X one amp.
So,
To make one horsepower with 48 volts at 100% efficiency requires 15.5 amperes.
The formula is amps = HP X 746(watts per hp) /48(volts)
But to accelerate that cart from standstill may take 5X that current
so i'd say you need a generator and transformer that's capable of at least 75 amps per horsepower of golfcart motor.

I'd look for a surplus aircraft starter. Saw one in my local junkyard that's capable of 1000 amps.


my two cents

old jim

 

[sophiecentaur]

It is actually the motor for a golf buggy - I am trying to covert the electric motor to run via a small generator and transformer - I would guess that the batteries are designed to run for about 4 - 5 hours before recharging.

I have to feel sorry for you, Beefcaked. You write in with a perfectly reasonable question and we all leap in and tell you you're approaching things the wrong way. That's the problem with enthusiasts, I'm afraid
But it may be worth while taking on board some of these basic responses, step back and consider whether your idea is 'optimal'. If you just want to use the motor, as a matter of principle, then that's fine; we'd all support it. If you really want an engineering solution for a real-world problem then perhaps you could get a 'better' result without the motor.

Either way, you're probably talking in terms of quite a substantial Power requirement for your job - more than yer average radio controlled car or hand drill.

 

[jim hardy]

I was guessing that he is a DIY'er who has a small 240 volt generator on hand

Big transformers are expensive unless you can get them at a scrap metal price, else it'd be cheaper to buy the batteries.

If he could find a few golf cart battery chargers they would have transformers with correct voltage that could be paralleled to accept all the current his generator can produce. Suitable rectifiers, too.

Myself, I'd look for another golf cart with a gas motor.

 

[sophiecentaur]

Putting nominally similar transformers in parallel can cause real problems with circulating currents. They have to be matched.

 

[jim hardy]

Putting nominally similar transformers in parallel can cause real problems with circulating currents. They have to be matched.

Ahhh, yes, the tinkerer's dilemma.
Every time i begin tinkering with a design, each step i take teaches me another thing that the designers thought of and I didn't.
But one learns a lot of designers' "clever little tricks" . And comes to really appreciate them.

Better chargers use ferroresonant transformers with magnetic design that makes them pretty well self current limiting. Still, the outputs ought to be paralleled after the rectifiers on DC side not ahead of them..

I have never done Beefcake's experiment, I replaced my batteries at hideous expense.
I wonder what other lessons are in store... ferroresonants are quite frequency sensitive...

So much to learn, so little time.

old jim

 

[MikeGonemissing]

How did this project turn out? I am doing the same thing to a 36 volt electric golf cart in order to increase the range. I have no desire to size the generator for peak power. To do so would be heavy, expensive and enormous overkill. My system uses several lead acid batteries to deal with peak loads like startup or soft sand. My first version used a 1000 watt generator, which put out about 600 watts continiously. The philosophy being to leave this generator running so the batteries could catch up while I was stopped. This worked passibly well. However, carrying a load up hill quickly depleted the batteries. The generator roared to max rpms and stayed that way for a long time afterwards. My next version will use a 3000 watt AC generator. I think I can get 1500 watts continuously to the batteries/motor, which is about two horse power. I'm using a 50 amp battery charger to make 36 volts dc.

 

[rainman6511]

I have a golf cart that I run with a 24-40 vdc diesel generator and no batteries.

 

[sophiecentaur]

I have a golf cart that I run with a 24-40 vdc diesel generator and no batteries.

No complaints about the engine noise on the course?

 

[Keith H]

I have a 1992 Club Car golf cart. It runs on six 6 volt batteries. I put three 12 volt solar panels on it. My range is awesome. I get about 10 hours of run time. I start out at 39 volts. After 10 hours the voltage drops to 34.8. I put the panels on several months ago, and I have never had to plug it into since. I recommend solar.