Variac for speed control?

[nothing to lose]

 A couple projects on hold right now are E-vehicle stuff, large and small. Can't afford controllers comercail made, or projects just not worth the cost of them. I have some good motors maybe, no controllers.

If I take an inverter to go from 12V or 24V  battery bank to 120Vac, then to a 25-30amp variac, then to a motor, or rectify to DC for a PM motor, would that work? Could I adjust the output voltage of the variac as needed to adjust motor/vehicle speed.

If the variac is rated for say 25amps, is that going to be 25 amps for any volts I put out of it, say 36V-48V DC once rectified, or 80-120VDC?

If I were to use this for say a 120AC motor, under 25amps,  as a speed controll should it work for long periods under heavy loads?

What about excess heat or losses in a variac?

I could build up a rectifier system if needed, and the 5k inverter should handle the load if the battery bank will. Also I have 1k UPS's sinewave for smaller loads perhaps.

So could I reasonably use a 25amp variac for a speed control for an E-vehicle?

I found some on E-bay I can afford, about $150 shipped, and I would have other uses at times for one also.

I just can't pay $500 or more for a project controller for just one use that may not even work though.

[commanda]

What wattage of motor are we talking about?

Generally, the variac will turn energy not used by the motor into heat. This is energy that has to come from the batteries. This will kill your available range versus weight of batteries.

Give more details of your project, maybe we can point you towards some reasonable cost pwm controllers and dc motors.

There are other forums (fora ?) where you may get more help. v is for voltage dot net springs to mind.

Amanda

[bob golding]

a 25  amp variac is going to be  very heavy as well as ineffiecient as amanda points out.  if you wait long enough the right bits of expensive silicon will turn up on ebay cheaply. i dont have a circuit in my head but would think some sort of h bridge pwm circuit would do the job. look for high power fet,s there are plenty of controller chips around. this takes care of the complicated part. there are some circuits on the international rectifier site for  power controllers.

bob golding

[drdongle]

NTL what you have in mind is not really practical or recommended, and in fact to complicated to much potentual for failure. You have a 12-24 battery bank to inverter, to variac to rectifier, to motor, two many conversions and links in the process. What you need is a 120 V battery pack to a controller, and to the motor, three steps instead of 5 and smaller ( cheaper) wire over a 12-24 system. By the time you buy and build all that crap you can buy a proper controller, with less likelyhood of failure.

If you must avoid a commercial controller then do surgery on the inverter to allow external control of the frequency and or voltage and drive an AC motor. You could also bypass the step up section and control a 120 V DC voltage with it and do away with the voltage step up.

Also I don't know what size/ power vehicle you have in mind but full size E vehicles can use 100's of amps under certain conditions.

KISS ( keep it simple stupid)

[Bruce S]

NTL;

   Also, if this is for a 550 watt motor , then I can send you a link where you can get the twist grip and controller for ~50USD delivered. These are good for 30Amps max at 24Vdc.

Let me know in private email if you want.

Cheers;

Bruce S

[Ungrounded Lightning Rod]

A permanent magnet field, brush-commutator DC motor can be speed controlled quite well by controlling the voltage across the commutator.  Most other types can't.

But as a result of the way it occurs the current when the motor is OFF speed - which means any time you're changing speed - becomes extremely high.

You're talking a vehicular appliication.  Even a rather small amount of kinetic energy maps to a large amount of electrical power.  (Note that a kilowatt is only about 1 1/3 horsepower.)  Trying to control a motor by voltage controlling the primary power to it using variacs implies you're going to need a VERY large variac and have a very large risk of burning it out.

For loads like that you need to be subtle, because brute-force approaches are opposing a very big brute.

[nothing to lose]

Thanks guy, I'll skip the variac for now then and do something else.

For the smaller project I'll see what BruceS mentioned, it sounds good.

For a 3hp or so app I could maybe use golfcart motor and controller. I can get another junk cart cheap I think.

For the large stuff like a real car or small truck van, I'm still looking for a source for low cost motor and controller. I have a couple good vehicles with bad motors that would be nice E-vehicles if I found the parts.

[scottsAI]

Hello NTL,

Just how cheap can you get a junk golf cart?

Exactly what type of motor are you trying to control?

AC, DC Specifications?

Have fun,

Scott.

[nothing to lose]

Golf carts depend on how junky or just old. 2 for $50 once, complete but no batteries.

 I'll look for another. A friend hauls scrap sometimes, he cut one up and hauled to scrap yard awhile back

You know I get alot of motors, so it was kinda a general question about doing that with the variac. Also would probably been a temporary solution, if it worked ok and everything else was right. I'd buy the correct controlers later of course if projects worked and worth the costs.

[scottsAI]

Thanks NTL,

I looked into a used one around here, stopped when I saw the price!

I don't have a place for one just yet, looking to move and have room to play.

Scott.