SCR control???

[Trippple P]

Hello you all, so far i'm glad i found that site.
I have a big project...
I would like to use a DC motor from a Fork lift to replaced a 8 hp gas engine(
I beleive it is a Brigg &... on a "special" post hole digger drill it need to reverse. I'm thinking about a Dc motor because of it hi torque and "ease" to control the rotation.
My first question is what size of motor that would be?
I thought utiluse a 24-volt motor because my ac welder can produce a constant source of about 90A @ 25 volts. Saying that if any one got a SCR bridge schematic on hand that could be nice.
I know i have to do more home work, that i a new idea who came in a couple of days ago.
I thanks' everyone in advance.

P.S. I'm a french canadian so please forgive the translation...

 

[Mithrandir2008]

No.

Just No. Ok. DONOT use the 25v,90A source to drive anything but welders, period. That's just madness.

I don't get what you mean exactly. Why exactly do you need high torque? What is it driving? Besides, if you have a DC motor from the fork lift, design the schematic based on the motor rating.

Maybe it's just the translation screwing things up. Please be clearer.

 

[tcmtech]

The power conversion rate for gas horse power to electric horsepower is about 3 electric = 5 gas. The high duty cycle fork lift motor will likely have more than enough power to replace a 8 hp gas engine.

I have yet to ever find a reverse rotation B&S engine though. They only ever built them with one direction of rotation as far as I have ever seen.

I would have no concerns with powering it off of a constant voltage welder power supply (wire feeder type). They are basically just a large adjustable voltage DC power source. I use mine that way all the time!
Just dont use a constant current welder (stick welder type). Those dont work so well as electrical power sources for motors.

 

[Trippple P]

Hello tcmtech! Thanks for the information, i would like to clarify about the gas engine. i know or don't think so either than there is a reverse rotation on small combustion engine.

Now about the stick welder, (**** that is what i have) i was thinking to it as a fix drop down transformer and build an SCR bridge or power converter to control the motor?
And maybe if i can find a old (most important cheap) 24 volts battery charger.

Any way thank again for your time.

 

[Mithrandir2008]

Hmmm

This may just be me, but I don't think this will work.

Basically because you are using a constant current source, dont connect a t/f. You WILL burn the windings at 90A

You need a different way of transforming power.

 

[Sceadwian]

The power conversion rate for gas horse power to electric horsepower is about 3 electric = 5 gas

How is this possible? What horse power rating are you talking about? BHP doesn't care about the type of motor 1 BHP = 1 BHP no matter what type of motor is involved. I've seen many horse power ratings on electric/gas motors always been a little curious about how they're measured because BHP is not a common listing on motors as it requires direct testing at the motor shaft, or at the output of the transmission. There's a whole lot more going on with RPM vs torque as far as break horsepower goes, but I've always wondered what the rational behind horse power rating on electric motors vs combustion engines.

 

[mneary]

Gasoline engines lose torque rapidly when operated outside their sweet spot. Hence, an electric 3hp motor probably has as much or more low speed torque as a 5hp gas engine.

At the sweet spot on the power curve, the gas motor is putting out 5hp while the electric is stuck at 3hp.

 

[tcmtech]

Unfortunately typical most gasoline engines are peak power rated while electric motors are continuous rated.
Your 3600 RPM 5 HP garden tiller engine could easily be replaced with a 3hp 3600 RPM electric motor and you would never see any performance difference. Likely you would in some instances see an overall improvement in peak power being a typical induction motor with adequate supply wiring can hold a 2 - 3 X peak loads for short periods without any problems.

 

[Sceadwian]

Ahh so it's peak horse power? So the gas motors come out artificially high because of their peak power band where electric motors are much flatter with their power band? This is one of those things I hate about motor specifications, horse power is almost a useless number without a torque/rpm and power curve.

With things like motors I wonder why they put such stupidly limited specs on the nameplates in the first place when suitability for an application actually depends on real life curves being suitable for a specific application or not.

 

[tcmtech]

Exactly! Even the ratings between engine types vary as well. Your typical vehicle engines are rated at peak power while any common diesel is usually rated for its continuous output power.

A diesel to electric horse power comparison in an industrial application is usually close to 1:1.