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'[ee] Best way to control a large dc motor'
2006\10\31@223950 by peteHVAC

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I'm adapting a forklift motor to power my trusty 72 beetle.
So far the mechanical part has been easy. The motor is a 36V 90lb
monster and I'm guessing it can draw upwards of 500A. Controlling
this much power has me stumped. I have the SCR control board from
the forklift but no schematics. I have found a lot of sites selling
controllers but not too much info on doing it yourself and looking up
the components for the original controls hasn't turned up anything. If
anyone can steer me in the right direction I'd be very grateful.

Thanks, Pete


'[ee] Best way to control a large dc motor'
2006\11\01@020107 by Jake Anderson
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peteHVAC wrote:
> I'm adapting a forklift motor to power my trusty 72 beetle.
> So far the mechanical part has been easy. The motor is a 36V 90lb
> monster and I'm guessing it can draw upwards of 500A. Controlling
> this much power has me stumped. I have the SCR control board from
> the forklift but no schematics. I have found a lot of sites selling
> controllers but not too much info on doing it yourself and looking up
> the components for the original controls hasn't turned up anything. If
> anyone can steer me in the right direction I'd be very grateful.
>
> Thanks, Pete
>  
take a look at the OSMC for a place to start.
some big power mosfets IRF1405's and some decent fet drivers and your
done ;->
easy lol

2006\11\01@173635 by peteHVAC

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> peteHVAC wrote:
> > I'm adapting a forklift motor to power my trusty 72 beetle.
> > So far the mechanical part has been easy. The motor is a 36V 90lb
> > monster and I'm guessing it can draw upwards of 500A. Controlling
> > this much power has me stumped. I have the SCR control board from
> > the forklift but no schematics. I have found a lot of sites selling
> > controllers but not too much info on doing it yourself and looking up
> > the components for the original controls hasn't turned up anything. If
> > anyone can steer me in the right direction I'd be very grateful.
> >
> > Thanks, Pete
> >
> take a look at the OSMC for a place to start.
> some big power mosfets IRF1405's and some decent fet drivers and your
> done ;->
> easy lol
> --

Thanks for the link. Very nice I'm now a new member.
The fets look nice too. I know what the datasheet says but can they really
be run
at that current? The leads are so small. It doesn't seem right 6 gauge wire
hooked
to a few little fets. I guess there is only one way to find out.

I heard tell that measuring resistance of the motor and multiplying by
voltage would
give a close value of the lock rotor amps. any truth in this?

Any good books about dc motor control?

2006\11\01@215236 by John Chung

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with that many amps you can use relay in parallel.

John


--- peteHVAC <spam_OUTpetehvacTakeThisOuTspamoptonline.net> wrote:

{Quote hidden}

> --

2006\11\03@000858 by Jake Anderson

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peteHVAC wrote:
{Quote hidden}

the FETs are only good for ~50A each, 75 is the package limitation.
with a fan though 50A per fet isn't unusual. At least for a robot.

direct resistance measurement is probably pretty ok for stall current.
don't forget to take into account the resistance of the leads in your
test gear, and rotate the motor (SLOWLY) to see if it has less
resistance in some places than others (as brushes make contact etc)

2006\11\13@073925 by Martin K

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Yes that is true.
How big is your big motor?
--
Martin K

peteHVAC wrote:
> I heard tell that measuring resistance of the motor and multiplying by
> voltage would
> give a close value of the lock rotor amps. any truth in this?
>
> Any good books about dc motor control?
>
>  

2006\11\13@074734 by Martin K

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Sorry, I replied before reading your original question -
The answer is that you should probably get one of the large Curtis
controllers or one of Cafe Electric's Zilla controllers (with the wait
time over 6 months probably)
The Curtis controllers are designed for forklifts. They have many medium
power MOSFETs in parallel. The Zilla controllers use large and powerful
IGBT modules. The Curtis is the least expensive way to get going unless
you want to use what they call a contactor controller in which your
batteries are combined between series and parallel sets with which to
vary the voltage that reaches the motor. Sometimes a high power resistor
is also used so that you can have a smooth transition between changes.
It's not the "cool" solution but if you're just running your beetle on
60 volts or some such, it should be ok. You aren't going to run your
beetle on 36v are you? You won't be very happy.
Check out evdl.org too. They would be able to provide much more
specialized help than the piclist.
The general consensus with everyone who asks about building their own
500A motor controller because it might be cheaper is that it won't be,
not even close.
If you want to build one and you have lots of time and money and you
enjoy difficult challenges, it would be fun.
--
Martin K

peteHVAC wrote:
{Quote hidden}

2006\11\13@081807 by olin piclist
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> peteHVAC wrote:
>> I heard tell that measuring resistance of the motor and multiplying by
>> voltage would give a close value of the lock rotor amps. any
>> truth in this?

Martin K wrote:
> Yes that is true.

Huh!!?  The units don't even match.  Ohms times volts comes out to Kg m**2 /
S C**2.  I can't think of how such a quantity is meaningful, but it
certainly isn't C / S.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2006\11\13@184905 by peteHVAC

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Hello, Thanks for the info. I would really like to build the controller
myself even if it's with alot of help :) The OSMC Yahoo group has been
a great source of info and I'm basing most of my circuit on it. The nice
thing
is I dont need to reverse the motor since it's mounted to my trans. I'll
need alot
less fets to get the job done. Your right about the voltage, 36v will move
it but
I want to pull the front wheels. When I let the magic smoke out I'll be
investing
in higher voltage fets.
I'm not building this to have an electric car but to learn how to make one.
So far the
money hasn't been to bad. I've spent around 250 bucks and its been alot of
fun to think
about so even if it doesn't work it was still worth it. Besids Im sure It'll
sell on eBay :)

My next stop is evdl.org thanks for the help.

> {Original Message removed}

2006\11\13@190642 by peteHVAC

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Yup, that was quite the brain fart sorry about that. Since nobody knows
me here I'll risk further imbarrasment. It would be V/R wouldn't it?
I calculated 120 amps from a measurement of .3ohms and 36 volts. When
I locked the rotor and applied voltage, from a good distance away, I
measured 643 amps. The armature and feild are connected in series.
It was very exciting to actually measure the current but I don't think
I'll ever do that again.
How can I make a quick estimate of lock rotor amps for a series motor?

> {Original Message removed}

2006\11\13@191738 by peteHVAC

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Sorry to reply to my own post but "motor current when voltage changes"
thread
has cleared things up a bit. Thanks

> {Original Message removed}

2006\11\13@191814 by Martin K

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I thought he said I=V/R
Honest mistake. Read what I mean, not what I write.
--
Martin K

Olin Lathrop wrote:
{Quote hidden}

2006\11\13@193845 by Richard Prosser

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I think your method is OK - but possibly there was an error in the
initial measurement of 0.3 ohms. Could be multimeter lead or
connection errors?

You may need to modify the method a bit. If you have a power supply
you can set to constant current mode it is a good way to measure low
resistances. Setup for a known currnt (1A?) and measure the voltage
drop at the motor terminals.

RP

On 14/11/06, Martin K <.....martin-distlistsKILLspamspam@spam@nnytech.net> wrote:
{Quote hidden}

> -

2006\11\13@201232 by Bob Blick

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> Yup, that was quite the brain fart sorry about that. Since nobody knows
> me here I'll risk further imbarrasment. It would be V/R wouldn't it?
> I calculated 120 amps from a measurement of .3ohms and 36 volts. When
> I locked the rotor and applied voltage, from a good distance away, I
> measured 643 amps. The armature and feild are connected in series.
> It was very exciting to actually measure the current but I don't think
> I'll ever do that again.
> How can I make a quick estimate of lock rotor amps for a series motor?

Hi Pete,

It's hard to use a multimeter to measure such low resistance. You will do
better to apply some small voltage and measure the current. Even that is
not going to be accurate because the armature/commutator is not
predictable at zero speed. But having designed quite a few motor
controllers, some over 1000 Amps, I will impart a piece of design
information that you should consider.

Cycle by cycle current limiting.

All commercial controllers pretty much have it. It's a good thing. It
doesn't need to be particularly accurate, but it should be part of your
design. Even if you don't care about protecting your motor's armature, you
need to protect your mosfets every microsecond (or at least every 10 or 20
microseconds).

One reason is simply that it's impossible to predict how much current your
motor may draw under some unusual circumstances. Let's say you roll
backwards on a hill and then give it full throttle. You could draw 1400
amps. Or if your potbox gets a loose connection and the throttle toggles
on and off 10 times in a second (and you didn't filter the potbox inputs
properly).

Another thing you need to look at is gate drive. Where does the supply for
that come from? Do you have a good undervoltage lockout circuit that
doesn't turn into a dirty oscillator? That can toast your mosfets in a
hurry.

But let's say you're serious about hotrodding, you think you want as much
current as possible. Fine, but there is a limit. You've measured 643 amps
already. Maybe you should design around limiting to 450 amps. Now how do
you make it into a hotrod? More voltage. Your 36 volt motor will not draw
anywhere near 450 amps at cruising speed if you only give it 36 volts.
Think more like 60 volts (but watch your RPM - don't goose it in neutral).
You can get 30 to 40 horsepower that way, which is a respectable amount.

60 Volts * 450 Amps = 27000 Watts. This is no beginner project.

Do you have a  radiator fan motor or something like that you can mate up
to a bicycle or scooter and then design a controller for that first?
You'll learn a lot, especially if you try to scale it as you would your
27KW controller (in other words, in the 27KW model you won't be able to do
massive overkill, it needs to be smart and protect itself. Use those same
principles to guide you in a 1/4 horsepower version).

I haven't been following this thread so I don't know where it's gone
before, I hope I haven't treaded old ground or anyone's toes.

Cheerful regards,

Bob




2006\11\13@202355 by Harold Hallikainen

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> Yup, that was quite the brain fart sorry about that. Since nobody knows
> me here I'll risk further imbarrasment. It would be V/R wouldn't it?
> I calculated 120 amps from a measurement of .3ohms and 36 volts. When
> I locked the rotor and applied voltage, from a good distance away, I
> measured 643 amps. The armature and feild are connected in series.
> It was very exciting to actually measure the current but I don't think
> I'll ever do that again.
> How can I make a quick estimate of lock rotor amps for a series motor?


I would think the DC resistance you measured would be a good way of
determing the locked rotor current. I can't see how the current could be
any higher than V/R using the measured DC resistance. Did you check the
zero on your meter before measuring? 0.3 ohms might be your test lead
resistance.

Harold

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