Merry-go-round

[dreamproject]

Hi all,
Can anyone suggest a source of a small dc motor with enough torque to turn a power-assisted steering of a car. I had a starter motor in mind but Mr.Nigel quickly dispelled all my ideas. I am also thinking of a Wiper motor .

1. Will the torque be sufficient ?

2. will it be reversible .

3. can low-cost MOSFET driven H-Bridges be used for this ? .

4. What will be the approx. current draw and torque (kgm) ?


thanks

 

[jrz126]

It should be reversable, but i dont know about the torque though. The worst case for it would be trying to turn the wheels when the car isnt moving. it takes quite a bit of force to do turn them on my 95' Chevy monte carlo.
what kind of car is it going on?


And what is up with your topic titles? this is about a car, not a merry-go-round. I think you're alittle too creative with them

 

[zevon8]

One thing to keep in mind when using automotive electric motors. Many of them are pretty crudely constructed, with little thought towards efficiency. The reasons for this are that many are used for short duty cycles, as in window motors, so long term heating or efficiency is not a concern. Being able to draw very large currents from a storage battery allows for sloppy designs also. Many of them have over sized windings to help generate greater torque with less turns, resulting in massive stall currents, and often very high in-rush currents on start-up.

For example, a common Bosch window motor runs no-load around 5 Amps, but stalls at well over 30 Amps.

Wiper motors are not as bad, they are designed for more continuous use, but they do draw pretty hefty currents when loaded. One problem you may have with a wiper motor is the case being grounded to the motor, and the other connections being for speed and park return.

A wheel chair motor with the gearbox attached would be the perfect thing, not sure where you would get one though.


PS, if your questions are all about one project.... maybe they should stay in one topic? It makes it easier to follow.

 

[Nigel Goodwin]

Hi all,
Can anyone suggest a source of a small dc motor with enough torque to turn a power-assisted steering of a car. I had a starter motor in mind but Mr.Nigel quickly dispelled all my ideas. I am also thinking of a Wiper motor .

1. Will the torque be sufficient ?

2. will it be reversible .

3. can low-cost MOSFET driven H-Bridges be used for this ? .

4. What will be the approx. current draw and torque (kgm) ?

I would suggest you get a wiper motor from a scrap yard and have a play, as someone has already suggested one connection is probably connected to ground - it's probably a good idea to take it to pieces and bring that connection out seperately.

They are often used for various models and projects (due to their cheap cost and easy availability). Rex Garrod (a famous Robot Wars competitor) set up a robot format for the magazine RealRobots, using wiper motors for drive motors - he's also used many of them (often overrun at 24V volts) for various exhibits he's constructed.

 

[ante]

A couple of cautions here, some wiper motors are tuned to run in one direction only for maximum durability and torque. When opening one of them up you sometimes can see 3 brushes of which two are at 180 degrees and the third close to one of the “main” brushes. Another way they can be arranged is a turned brush bridge, which means that the brushes are not in alignment with the field making it a very poor motor in one direction. It is possible in some motors to align the brushes to the field poles or magnets (if a PM) but it can be a lot of work to get it right. And when doing this remember it’s not always the gear has been made to take the forces in both directions and also the motor will not give 100% of the original torque in it’s normal direction since it’s no longer “tuned”.

Ante :roll:

 

[magno_grail]

I am using Denso window motors for driving a small mill table. Worm gear drives are non-reversable due to the right angle alignment of the gears. Straight gears or parallel shafts will reverse drive unless you add locking clutches to the shafts. The wires are separate from the case. It does use a plastic output gear (though the shaft is steel) so there may be a safety issue. I support the output shaft with an external bearing to reduce wear. Not sure what the MOSFET H bridges can handle but HEXFETs will work. You would have to over drive them to get enough speed.
Wiper motors generally run one direction and do not have the bearings to support reverse thrust.
Forget about american starter motors. They are as ancient as the engines they drive. Bosch and Mitsua make plantary gear motors. Lots of torque and speed but a lot of current as well. It could be mounted directly on the end of the steering shaft. I doubt you could stall one unless you hit the steering stop. I believe they can draw 100 - 150 amps in stall.
Might be easier to use a hydraulic motor to drive the rack so a much smaller motor can control the hydraulic valve. You already have the hydraulic source from the steering pump.

 

[ante]

Magno,

It must have been a long time since you had a good look at american engine technology and starters. They too have gears now and there are not any big differences today. The current in a starter motor is way beyond 100 – 150A when stalled. If you make a calculation you will find that 150A is at normal cranking with a small to medium starter. Just think about it, 2hp and about 10Volts when cranking.

Ante :roll:

 

[magno_grail]

Hi Ante,
Not to get off the subject but I made the mistake of buying a 2002 dodge dakota V6. It is a Piece of Crap. 20 year old V8 design when they cut two cylinders off to make a V6 30 years ago. Almost all the modern american alloy block OHC motors come from the foreign manufacturer connections.
Starters that drive through a single ring/pinion gear drive draw a lot more current than multiple gear reduction drives as the motor has little speed and efficiency. Motorcycles in particular use a high gear reduction to get HP from speed instead of torque.
Regardless, a starter motor is not, in my opinion, the way to go in this application. The hydraulic power already exists and does not take high cost MOSFETs to control.

 

[ante]

Hi Magno,

There are crappy cars on all continents but the average car is good. I think that US Fords for example is better than the European Fords (they couldn’t possibly be worse) :lol: . Well the starters will still need there current no matter which gear ratio they got, they still have to put out 2hp and at 12Volts….. The difference is a little less current from standstill to normal crank rpm because the (geared) motor runs to a higher rpm much quicker. OK back to the subject, a powerwindow motor with a belt drive will work fine and with no extreme currents.

Ante :roll:

 

[panic mode]

motor starter for turning steering wheel is huge overkill
(and very dangerous for the car). i made post earlier with
some specs of the similar equipment we made for one
of the major carmakers. i had to limit max torque to ca
20Nm to prevent damage to the car - and this is in a process
before power steering is functional!
starters draw running current in excess of 150Amp on most cars.
cheap little motors like used in power seats, windows etc. have
running current of ca 5A and stall current ca. 25-30Amp.

quick look into digikey catalog shows that IRF2804-ND is good
for continuous current of 280Amp and max current of over 1000Amp.
Price less than $5 in small quantities, $3 if you buy 10 or more.
I guess transistor doesn't have to be overly expencive after all
(well it depends on budget...) although it does require some research.

 

[panic mode]

forgot to mention, prices i posted are Canadian dolars so it's
like $3 and $1.8US...

i guess they'll get lot's of new orders ;-)

 

[holabr]

www.herbach.com has all kinds of DC gear head motors.