Powering a dc motor

Thread Starter


Joined Feb 28, 2020
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



Joined Jul 18, 2013
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.


Joined Sep 17, 2013
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


Joined Feb 28, 2020
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


Joined Jun 4, 2014
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.


Joined Sep 30, 2009
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.
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Joined Feb 20, 2016
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.


Joined Feb 2, 2019
GMT800 used the Delphi AD244 alternator in 104 & 144 amps. They are dual fan with excellent flat spaced diodes for cooling. If more is needed, 300 amp version are sold using a dual stator wire arrangement. Full fielded, any alternator produces about 85-90% of its rating, but when operating, they supply the amperage needed to run the devices, not the batteries. A fully charged battery is 12.8 volts and conductance test will show 100% state of health if good. The batteries do add a small load on the alternator, using some horse power. Since 2004, GM uses active battery monitoring resulting in the amperage to charge a good battery is under 1 amp, and duty cycle to control output. 1 wire regulators are under $25-30. Therefore the alternator(S) would supply the current required to run an electrically powered vehicle. Using 36-42 volt system would reduce weight, cost of cabling, and electric motor size. This was used in GM’s “Parallel Hybrid” full sized trucks, but a battery pack of Lithium Ion cells, as well as regenerative braking. The holy grail is 3 phase A/C pancake wheel motors or torrodal motors. To provide 0-100 MPH, GM used PWM controlled shielded LEDs on the low voltage side, triggering photo transistors on the high voltage side, producing a 3 phase and speed control. High speed relays used magnets next to contacts to prevent high voltage arcing. This becomes complicated quickly. Inverters / converters are required for some high voltage items but required if running standard 12-14 volt lights, fans, wiper or other devices. It depends on how far one want to go but test and calculate amperage needs on paper before getting the credit card out.
I prefer the PEM hydrogen fuel cell that produces 180 volts DC. GM had 4 facilities working on these in 2008. The PEM is one of five types of hydrogen fuel cell types and the PEM works great at low temperatures. The inverter & converter where cooled with de-ionized water & “Dex-Cool” coolant thru a radiator and had a standard heater core. Hydrogen can be liquid or gas and produce 7% more tailpipe water. Our vehicle used liquid high pressure in twin carbon fiber tanks with a 240 mile range in a 5 passenger minivan configuration. This is information only but the AD244 is considered one of the best alternators on the market. They can be found at any salvage yard (2002 & up GM trucks/SUVs) and rebuilt easy with a few tools and updated newer parts. Alternator-man.com has parts, complete units and videos. I bought my slip-rings & parts kit on eBay. I hope some information will help your project. I grew during the age of rebuilding stuff, not boxed parts.
ASE Master Tech since 78 - Retired (ACDelco instructor)9AD66E7D-E3CB-4B3C-AA52-76E1D5CB8674.jpegF6F78267-04A3-46D1-BE2E-1B1F2A80EE9A.jpeg9687A887-7A40-4E7E-9CA3-BAD48937EC67.jpeg7BC548CD-5D32-4915-AB16-F5EF7FD2B2CD.jpeg