Powering a dc motor

Thread Starter

Chunderpants

Joined Feb 28, 2020
2
Hello all.

New to the group, unknown to all things electrical, kindly forgive my ignorance in advance of the question I'm about to ask.

I have an old car, and my intention is to use an old dc motor from a forklift truck, connected to the gearbox.

Ordinarily, this would be powered by a series of batteries, which would limit it's range, so I've been thinking about using either a 50cc or 100cc moped engine to power up a couple of alternators.

My question is, would I need to send the alternator power to a series of batteries, with the battery power going to the motor... or, could I send the power direct from the alternator to the dc motor?

Thanks in advance

Gav
 

MaxHeadRoom

Joined Jul 18, 2013
20,023
If automotive alternators, they are not rated high enough to direct drive a large DC motor.
In the charge mode, the batteries would normally store sufficient energy in order to run the motor.
You often see large power ratings for auto alternators, but this is only for over Very short periods at that power rating.
Max.
 

Alec_t

Joined Sep 17, 2013
10,909
In the long term you can't get more energy out of the DC motor than the 100cc engine provides, unless you have some additional way of charging the batteries.
 

Thread Starter

Chunderpants

Joined Feb 28, 2020
2
I get the inertia thing, but this is using the drive shaft or chain on the moped engine to spin the alternators, so holding it at about 2000rpm, which is the average drive spin for a car on the motorway / highway, you'd have a decent output from the heavy duty alternators.

You'd only need a 36 or 72 volt motor, so nothing major to power it... but if an alternator gives out the power, in theory, would it not stay on a level playing field with output equalling input?

You'd effectively get moped mileage out of a larger vehicle as the petrol engine would generate the power for the electric motor.

The gearbox would allow for increased speed
 

AlbertHall

Joined Jun 4, 2014
9,310
You cannot get out more power than you put in however you arrange it - except for the initial use of a charged battery.
After that the maximum power from the electric motor will be less than that provided by the IC engine (less than because there will be losses.
 

shortbus

Joined Sep 30, 2009
7,461
I get the inertia thing, but this is using the drive shaft or chain on the moped engine to spin the alternators, so holding it at about 2000rpm, which is the average drive spin for a car on the motorway / highway, you'd have a decent output from the heavy duty alternators
The 2000 RPM sound close to the car engine RPM. But your not figuring in the ratio between the crankshaft pulley(big) and the alternator pulley (small). Usually this is around 4:1.
 
Last edited:

dendad

Joined Feb 20, 2016
3,322
Just think. Would the 50cc (or100cc) motor drive the car directly? If it would, why do we have larger motors in the cars?
Running the moped motor to drive an alternator to drive en electric motor will in fact deliver less power still.
Having the batteries there will buffer the higher currents needed to accelerate.
I have a Prius, and that runs a 1500cc motor. That size motor is quite ok for highway driving, and the electric motors assist in the starting and getting up to speed. The electric motors also charge the batteries when excess power is available.
You could run a small motor to charge the batteries but that size motor will not drive the car at any great speed.
 
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