# car starter motor mod

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#### dstich

##### New Member
i am lookin at using a carstarter moter for my bot because i have a low budgit and can get one at a scrap yard for a couple of bucks i was wondering if they can go in reverse if the curent is reversed i know that i have to cut the field winding coil wires and conect them directly to a battery and take out the selanoid but if i reverse the curent after that will the motor reverse or is there some kind of mecanical catch to stop it from reversing

#### sam963258

##### New Member
starter motor not really suitable.

A car starter motor is a very special device. It is designed for extremely high current, high horsepower, and low duty cycle.

The current will depend on the type of car it came off of and the load that is put on it.

The current can run anywhere from 150 amps to 300 amps or more.
the horsepower for these motors can be anywhere from 25 to 100.
This may seem like a lot of horsepower and it is but the duty cycle for these motors is extremely low.
running for 10 seconds may require as much as 10 to 15 minutes to cooldown. although most people don't realize that.

These motors are made with bushings no bearings, some of them don't even have that.
A lot of them is just steel on steel.

Any prolonged use of these motors such as in a BOT would be very disappointing.

Also you have to contend with a starter Bendix.
a specialty gear that only runs in one direction
it would have to be removed and replaced with something else. normally the shaft that the Bendix rides on has a special spiral spline. You would have to have a lathe to remove this.

I recommend you keep looking.

#### IRQ57

##### New Member
Starter

Hello,
I agree with Sam in the first part, but I think that there must be an error in the HP calculation : (300A x 12V) / 745W/HP x .75eff= 3.6 HP.

There are two kind of starters with respect to the field: With a permanent magnet and with a coil. The first type will reverse with a change in polarity, the latter no.

Anyway, this kind of motor, as said by Sam, is not appropiate for a long period of use, instead, I suggest you to use a generator or dynamo, used in old cars, before the use of alternators. If you get a six volts one (very difficult), this is your motor, reversible and continous duty, and if you vary the field current, you can vary the speed. At 12 volts a 6 volt generator will spin a (aprox) 300 RPM. Try it and good luck.

#### ante

##### New Member
Well Sam, I’m sorry but you are way of. A starter for a normal car is between 0.8 and 1.2 HP. If you go for a big truck (12 litre diesel engine) the starter put out around 25 HP. I don’t know where you got your figures from, but they are not correct.

Ante :roll:

#### sam963258

##### New Member
starter motors

First of all you your right my figures were a little high but considering there's no real rating on a starter motor and never has been to my knowledge it would be a little difficult to quote an actual horsepower rating. I was simply trying to make a point that they would not be suitable for any other use.
Also you can't use the same math with starter motors that you use with standard motors, it doesn't work because of the way the motor is made internally and its intended use. A normal motor is designed to be used for extended period of time at a certain load and RPM. These motors or designed to basically self-destruct.

As far as reversing starter motors yes, it's easier to reverse motors with permanent magnets than motors with a coil. In order to reverse a motor with a coil you must either reverse the polarity of the Armature or stater but not both.
Because you cannot change the polarity of a permanent magnet, all you would have to do is change the polarity to the motor itself.

A dynamo or generator can be used as a motor but because of the brush alignment they are normally very inefficient. The size, weight and current draw of these motors versus the power they would produce would probably make them unsuitable for your use.

Even on alternator can be used as a motor if you want to do some Elaborate electronics to support it. An alternator woodwork much like a stepping motor. Of course you would have to strip the diodes and regulator out first.

Why don't you try going to some of the surplus stores on the Internet such as (www.surpluscenter.com)or even (eBay) you might find what you need their for the right price. and I'm sure there is plenty of other places as well.

my point is it is morally better to spend an extra $5 or$10 on the right part then trying to make the wrong part work and maybe going through several different parts before you get one to work.

sam963258

#### IRQ57

##### New Member
Starter motor

Hello,
Sam, math is just one, I used your numbers to calculate the highest power output with the higher current that you stated. Normally it is half than this, in an average automotive motor, after the initial static friction and inertia conditions have been overcomed. Anyway you may see some HP specifications on automotive starters at: **broken link removed**

Dstich, some 25 years ago, I build a electric gocart using a Ford 6 volt generator, a 6 to 1 reduction, 20cm wheels and a 12V 70Ah battery enough to 4 hours of fun for my 8 years old kids with power enough to spin the wheels when fed without a 0.06 ohm 50W limiting resistor and about 20 KPH.

I agree with Sam that efficience is not very high, but may be enough to have a pleasent ride, without using sofisticated motors and electronics to drive them.

#### ante

##### New Member
These Castle Auto Hi Torque starters are non-standard and 20-50% higher powered. And remember the power get out from your starter is not even close to the power it consumes. A modified old dynamo is a much better alternative; it has bearings and is easier to run in both directions. There was a kind of combination way back called dynostarter, which served as starter and generator. They were hooked up to the fan belt and used on small boat motors and some farming equipment. Don’t try an alternator for this! Good Luck!

Ante :roll:

#### andyphelps

##### New Member
starter will not work

I agree with Sam, the car starter really is not meant for long times of usage, and will over heat in 20-30 seconds. I am just starting to builld an electric go kart, and I have decided to use a 6-12 volt generator, and one or two 12 volt motercycle batterys. But anyways, a 6 volt generator will work fine for a BOT, and shouldn't be too hard to find on the web or at a store in town.

#### tcmtech

##### Banned
Welcome to the forums! but you are about 6 years too late for this thread!

#### andyphelps

##### New Member
haha

oops, I did it again for like the fith time. Hey, do you know how to make an electric switch that switches the polarity of a battery so a moter spins backwards? I already made one that works, but I didn't go very far with it yet.

#### Zac67

##### New Member
6 volt, 12 volt Electric starter motor generator

Hi is it possible to turn a generator into a starter motor, I am rebuilding a 1942 motor bike for my daughter and it is very hard for her to kick it over to start it has an old chain drive generator/magneto setup on it already can such a thing be made

#### Diver300

##### Well-Known Member
It's normally quite easy to turn a dynamo into a motor. However, I very much doubt that the dynamo would have enough torque to start the engine. Car starter motors are geared down to increase the torque. Most will only get the engine up to about 200 rpm because of that gearing. If your bike has a decompressor there is more chance.

A dynamo with brushes that generates DC will run as a motor if the field and armature windings are connected to a battery. However alternators, including the ones that directly power the lights without a battery, would be just about impossible to get to run as a motor.

Any photos or more details would help.

#### Zac67

##### New Member
Thanks for the reply, yes the engine has a decompression lever I have thought along these lines it would be the only way to rotate the engine because of the high compression in a big 500cc single, will take some pictures. Of what would be best just out side as it fits to the engine or the inturnals.

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