N mosfets

[TKS]

Hi,

i have paralleld 6 N mosfets

the motor i wanted to control has only one wire (positive) the case is ground..

am i able to hookit up after the mosfet??

whats the downside on this?

Tks

 

[chemelec]

Hi,

i have paralleld 6 N mosfets

the motor i wanted to control has only one wire (positive) the case is ground..

am i able to hookit up after the mosfet??

whats the downside on this?

Tks

First Question is, Why did you parallel 6 Mosfets?

The Motor has 2 wires. Since the case is Ground, it is a Wire also.

 

[kchriste]

What you need is called a "high-side driver". The MOSFETs drain can then go the postive supply rail and the source terminal to the motor's + terminal.
What it does is supply a voltage capable of going above the motor supply voltage so the MOSFET remains on when it's source terminal approaches it's drain voltage.

 

[Russlk]

You could insulate the motor from the vehicle frame and drive the negative side, or perhaps the grounding wire can be brought out so the motor frame is not grounded. Driving the high side involves using a capacitor to obtain a voltage higher than the battery voltage.

 

[zevon8]

I would try and do it low side, as suggested. Parts are far cheaper, circuit is simpler. International Rectifier has some good info on this, check this link, and those that branch from it.

http://www.irf.com/product-info/auto/autogdic.html

 

[TKS]

Ok its clear need to bring the wire outside, since its a brush type motor i think its easy..

just drill the case and disconnect the brush and connect it to the wire.

Wy i parallel 6 wires its because i need huge amps..
the motor is a starter motor.

Its not used very often and not for very long times...

Thanxs for the info

Tks

 

[chemelec]

Ok its clear need to bring the wire outside, since its a brush type motor i think its easy..

just drill the case and disconnect the brush and connect it to the wire.

Wy i parallel 6 wires its because i need huge amps..
the motor is a starter motor.

Its not used very often and not for very long times...

Thanxs for the info

Tks

As well as the 6 Mosfets, They need to be quite high current ones and You better have a Very BIG Heat Sink for them.

Also, How do you want to Contol it?????

Just On or Off?
Or Variable Speed?

 

[Nigel Goodwin]

As I mentioned before, starter motors are SERIOUSLY NOT SUITABLE for this type of use. Check the robot wars type of sites, who all seriously advise NOT to attempt it. The only moderately successful Robot Wars machine that ever used starter motors didn't do any speed control, just switched them ON and OFF with big contacters - it wasn't pretty, it wasn't very controllable, but it was very sturdily built!.

 

[audioguru]

As I mentioned before, starter motors are SERIOUSLY NOT SUITABLE for this type of use.

I frequently propelled my 1967 car with its starter motor whenever it ran out of gas. It would drive pretty far with its starter motor. Sometimes it got going fast enough to gear down to 2nd. After many years, the car died with its original starter motor.

 

[jpanhalt]

I have been running a Ford 6V long-shaft starter motor at 12V using a 1/2H configuration of N-channel Mosfets (IRF3205; 5 in parallel on each leg) for several years. I have no idea what the current draw is, but it's high and probably approaches or exceeds 200A at its peak. It is used as a variable speed/power winch for launching RC sailplanes. There is no need to pull a wire from the starter case.

My system uses an LT1158 as the Mosfet driver, and I was able to get by with a single 2400ufd/50V capacitor.

Good luck.
John

 

[TKS]

mhhh

i have paralleld 6 N channel IRF1104 (2400 peak amps output) 100amp continous per fet.

its all cooled by an amd cooler

no driver yet wanted to use just a bc547c witch put the power on the gate, the gate is also connected to negative with a resistance... (for bulbs this works)

what means halve bridge?

Tks

read somewhere that the starters are series wired?? Dont can regenbrake?

do i need a mosfet to connect both ends to positive rail on the low side of the pulse?

i have a driver wichof i guess is suitable for me..

will post reference

 

[jpanhalt]

Hi TKS,

A British manufacturer of large DC motor controllers, such as those used in golf carts and mobility assistance for the disabled, has a web site: https://www.4qd.co.uk/

There is a lot of valuable information there that I relied upon for design of my controller. 4QD has a section on theory and defines the concept of quadrants for a controller. I believe you are correct that common starter motors (such as the Ford starter that I used) cannot regen brake and also cannot be easily reversed. That is, you can reverse the leads to the motor, and it will run just fine, but in the same direction as before. To reverse the motor, you need to add leads so you can reverse the armature with respect to the field coils. I do not know if the same applies to more modern, high-speed, geared starters.

Best regards, John

 

[TKS]

Hi John

many thanxs for that link but obviously it was already given to me by aother forum.

Also i want to make clear to nigel that it isn't for a robot but for a 500Kg Locost style car.

Its true that you need to add leads because the windings are in series (what ever) it means.

I'm gonna try to make my engine run with the N fets connected to positive

Will post the scheme to see some comments.

also you can make it regen brake but then you need to power up one coil i tought it was.. to create the field

Tks

 

[TKS]

Driver as driver i use the IR2112..

wanted to hook the 6 gates the the HO output pin and 1 mosfet to the LO output pin.

Remember that positive of the motor s then connected to the 6 Sources and to the drain of the last mosfet.

Negative is on the engine permanently and also on the last mosfet.

Would this work?

Link: to data sheet: http://www.datasheetcatalog.com/datasheets_pdf/I/R/2/1/IR2112.shtml

As source for the pwm i use a PIC16F877a if it is possible i would like to use a mosfet (buz11) and a pullup resistance??

I now its overkill, but else i will need to change the circuit?
Will look for a way of doing it different

Tks

 

[zevon8]

I think the point trying to be made about cranking motors are that they are brute force technology, they are designed to do a job, not effieciently, not cleanly and DEFINITELY not for very long. Think hammer, big honking stone hammer.

Even the example of a sail plane winch, I bet this setup does not run for very long. Way less time than than one would expect to drive around in a car.

I have a simlar motor at work that operates a mechanical ram on a gear rack, the thing can draw nearly 300 Amps at 12 volts. Pretty much a starter motor in design, it only operates for about 15 seconds per cycle, and it gets about as warm as you care to touch after each 15 second cycle.

Point being, you can design all the power electronics you like for this operation, and it is quite do-able, bigger power is switched every day on any subway train, however connecting it to a starter motor is not going to work, the motor will not last very long.

 

[Oznog]

First off, what do you want this motor to do?
Then we can recommend a cheap or free alternative that is more appropriate to your task than a starting motor.
Unless you actually want to start a car. In that case, use a starting motor but forget about MOSFETs and just use the stock relay.

 

[Nigel Goodwin]

Also i want to make clear to nigel that it isn't for a robot but for a 500Kg Locost style car.

It makes no difference, it's still basically the same thing - I was suggesting robot wars sites so you can see what people use, and it's NOT starter motors!.

 

[TKS]

people use starter motors for this purpose because they have the power needed what is the problem in their desing? the series windings?

i think every electric motor wich operates at 12volts and has 1BHP will get hot not?

wouldn't it get less hot if speed controlled then when just a relais is used?

Tks

 

[Nigel Goodwin]

people use starter motors for this purpose because they have the power needed what is the problem in their desing? the series windings?

The problem is it's designed for a VERY specific purpose, and NOT as a general purpose motor - they are also only designed to run a few seconds at a time, and only in one direction.

i think every electric motor wich operates at 12volts and has 1BHP will get hot not?

Yes, but other motors are designed to run for longer periods, so have better forced cooling than a starter motor.

wouldn't it get less hot if speed controlled then when just a relais is used?

Running a motor slower generally makes them hotter, as the cooling is reduced as the slower speed - this is a problem when applying speed reductions to motors not designed for it (not just starter motors).

Starter motors have two advantages:

1) They are cheap.
2) They are freely available.

And that's all! - everything else about them is a negative!.

 

[zevon8]

Starter motors are perfect examples of how to make a powerful motor cheaply..taken to extreme. They use as big a winding wire guage as possible, and draw an insane amount of current.

Many automotive use motors, except maybe fan blowers, are really poor designs, since they don't have to be efficient, don't need to run long, have low duty cycle, and most importantly, need to be CHEAP.

If you are Toyota and you make something like 9,000,000 vehicles a year world wide,( and you put a starter in each one,) and likely 6 or more small motors, believe me every penny saved adds up to alot of money.