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Controlling a large DC motor 12volts 350Amps

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TKS

New Member
How could i control this motor?

i thought of PWM and a solid state relais.

is it true that if the relais / mosfet rates

30amps at 50volts that it can calulate the watts?
and say that at 12volts that mosfet can have 125amps ((50/12) * 30)

or is it an or or situation (30amps is limit and 50volts is limited)

anyone knows of siemens solid state relais?

Ideaas??

Tks
 

dknguyen

Well-Known Member
Most Helpful Member
Maybe you should use a higher voltage so you can use lower amperage?
 

Papabravo

Well-Known Member
Ratings are ratings for a reason. If you want to fudge around with them by all means do it and win a Darwin.
 

Oznog

Active Member
If I read your question correctly, using a MOSFET or solid state relay at less than the max rated voltage does not in any way increase the max current.

I doubt you have a motor rated for 12v 350amps, unless maybe you're talking a starter. It's not very practical to wind a motor for that much current at a low voltage.

I don't understand your setup but this sounds just silly. For one, it would take an enormous number of MOSFETs to even avoid losing too much voltage due to voltage drops. The cables would have to be enormous, and this would require a phenomental battery bank to sustain it for more than a few moments. Nobody winds a motor like this. So a lot about your problem doesn't make much sense.
 

oldtimer

New Member
It sounds like a motor from a golf cart which is usually 48 V and around 3-4 H.P. or 2.25 Kw - 3Kw which would give a current of 62 max. and they're controlled by limiting the current in the stator.
 

dknguyen

Well-Known Member
Most Helpful Member
If that is indeed the case, it's a silly idea to run a 48V motor at 12V (I think we can all agree with that). THe winding resistance would be too high at for the 12V supply to be able to supply enough current and insufficient current will flow through the motor. The motor will be VERY inefficient (if it can even build up enough power to overcome mechanical losses to start spinning), greatly underpowered, and it's stall current at 12V would be vastly lower than the stall current at 48V.

If you figured out a current draw of 350A by taking the power output of the motor at 48V, and assumed the it would require [email protected] to supply the same amount of power, that is false because of the resistance in the motor windings. The stall current at 12V would be much much less than the stall current at 48V (and the stall current at 48V is much much less than 350A).

If anything, the switch for 12V operation doesn't even need to handle anything near the 48V stall currents.

For a given motor, more voltage increases current and produces more power and heat. Less voltage decreases current and produces less power and heat. A motor with a small voltage supply cannot draw higher currents than the same motor with a larger voltage because of winding resistance. Therefore, you cannot get the same amount of power from the motor when using a low voltage as you could when using a high voltage supply. If you attach a current source to try and drive a higher current through the motor to try and achieve this, you fry the windings due to IR^2 heat dissipation (or something like that anyways).
 
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TKS

New Member
Its a small starter motor from a bike (CBR 600)
i had calculated also a small margen.
the winding resistance is about 0,6ohms.. but i think its the startoff curren draw wich is that high..

the voltage is indeed 13,8volts (while car is running / charging)

have seen mosfets reted for 150amps 30volts...useable??

Tks
 
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dknguyen

Well-Known Member
Most Helpful Member
If it's just the startup current then get a switch that can handle the spikes, but can handle the continuous current rating. Sizing switches for 350A spikes is a lot different than sizing for 350A continuous, which is what you made it sound like.

What is the stall current or rated current that the motor is probably going to be running at?

I don't know how much current it draws normally...but here are some giant MOSFETS:
http://ec.irf.com/v6/en/US/adirect/ir?cmd=catNavigateFrame&punchInID=355

These are what I use for large motor drivers. You could probably find a suitable relay if you look for one based on continuous current loads of the motor rather than 350A continous. After you find a suitable relay during for continous operation, then check the surge rating to make sure it's suitable with the 350A spikes.

I would assume a 150A CONTINOUS rated MOSFET would have a surge current rating that could handle the 350A, but I can't be sure. Use the continous current rating to find your switch, then check to see if it can handle current spikes of 150A. My guess right now is it would work, it's just a guess.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
What are you trying to do with the motor anyway?, starter motors are really completely useless for anything except starting an engine - they are designed for that one specific purpose, and are not really a 'normal' electric motor.
 

Oznog

Active Member
TKS said:
Its a small starter motor from a bike (CBR 600)
i had calculated also a small margen.
the winding resistance is about 0,6ohms.. but i think its the startoff curren draw wich is that high..

the voltage is indeed 13,8volts (while car is running / charging)

Tks
How did you calculate this current?
What do you intend to use this motor for? A starter motor is just not made for continuous use. It will overheat in a very short period of time.
What would you intend to power it with? What sort of battery?
Or do you just want to replace the starter relay with MOSFET? If that's the case the answer is that the relay's by far more suitable.

That is not the winding resistance. 13.8v is only 23 amps short circuited across 0.6 ohms. You may have included a lot of wire lead resistance. It takes a special 4-wire connection and an accurate meter to read in the low-ohms range.
 

TKS

New Member
Well its said in the hayness manual.

I want to use the motor for a reverse in a locost car.
Because of the weight of the vehicle i need a torqui motor.

I want to control its speed with the mosfet...
Normal builders just use a relay with a press button, but wanted to do it a bit
nicer...

Its a bike starter, sow it hass allot less power then a car starter.
I will try to measure the amps with a clamp (altough i think mine canonly measure AC amps??)

THe power will come from the car battery while the engine is running..

its only used as often as you use reverse on your car..

also it will be very very low geared sow the speed backwards will be about 5Km/h

any ideas? could though of a more suitable motor for this.
because you need lets say 1Hp 12volts engine..for this..

sow the wiper one won't work, and my feeling said that the scooter ones will allso be to small.

Tks
 

TKS

New Member
Nope,

i have a ring type meter and a multimeter when i try to measure up suchs amps i wont put my meter in series just clamp the cable trough the meter...

it then measures the amps..


Tks
 

TKS

New Member
except if you have a more expensive model then you can also measure DC current..

but offcours like always you think you don't need it..

Tks
 

oldtimer

New Member
If you're thinking of building a hybrid vehicle using this motor forget it, you need a higher voltage motor to reduce the current to levels which can be handled without losing the power in external wiring.
 

Hero999

Banned
Oznog said:
I doubt you have a motor rated for 12v 350amps
Its possible, I work on military vehicle electrical systems and a rocket boom mechanism uses 300A continious at 24V with a 600A surge at turn on.
 
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TKS

New Member
oldtimer shouting loud doesn´t make sense please read thread and also look at the section.

Wow!! well atleast we have a mission now! :D:D
 
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