Control DC High Voltage

Look into using a FET...with resistor control.

How much current?

Even a small pot could do. Depends on the load resitance you wish to throttle. They made some pots three to five hundred watts that were used as speed controls on some the old golf carts. Also on some the grocery market riding carts used one. They may not be a high enough wattage for the ungiven load.

I've never seen a 500 volt golf cart, I'm thinking the line needs to be chopped, I'm not sure

5mW, ,5W, 500W, 5kW, 5MW?

The voltage is not the problem, you can find a pot or a rheostat rated for 500VDC very easily. But if your load is 5MW, not so much...

It's impossible to give a good answer not knowing much about your power supply (output impedance, e.g.) or the impedance of the load it is supplying.

What powers the supply? Possibly you can more easily control the input to your supply (if the output is not regulated). If it is AC powered, possibly with a variac (variable auto transformer). If DC powered, a variable DC power supply.

If the impedance of the load is very high, a high resistance pot on the output will work. If the resistance of the pot is R, the power dissipation (watts) of the pot = 250000 / R. So a 1 MegOhm pot would dissipate 1/4 watt. To use a pot, the impedance of you load needs to be many times the pot's resistance.

I hope this is helpful.

if it were AC I'd just go VFD, its pure DC

You still haven't said how much current/power you are dealing with! No one can give you a useful answer without some idea of the current involved.

If you only have the household voltage of 120VAC, step it up with a 110//440 transformer and then "throttle" it with a variac, rectify the output and you're done.

I had never heard of a variac before.interesting little toy. it wont work but the concept did lead me to autotransformer's with a handwheel to adjust DC output

http://www.danaherspecialtyproducts.com/uploadedFiles/Siteroot/Superior_Electric/Products_and_Solutions/POWERSTAT%C2%AE_Variable_Transformers/1296D-c.pdf

thanks

Transformers don't work on DC.

"Powerstat" and "Variac' are just two brand names of variable autotransformers. Since "Variac" is shorter and easier to say, it has, I think, become a generic term not necessarily referring to the brand. I'm amazed that someone like you was not aware of them. I keep several on hand at my home, and have quite a few at work.

They are hardly a toy! We have a multi-million dollar machine whose main operating power is controlled by a bank of six of them all linked to a servomotor.

As TNC indicated, they do not work with DC, but they can control the voltage/power entering a DC supply. The one mentioned above can produce up to 60kV.

Better to have the variac on the line side of the step-up transformer.

Us knowing about the power source and load would be much more helpful here.

What kind of variability do you want in the control? Would on, off, and three intermediate settings be enough? 30 intermediate settings? Or must there be very fine control?

What is the budget?

Will it be stationary?

One thing comes to mind, but it is not for the accident prone:

A watertight fairly nonconductive container filled with aqueous electrolyte....baking soda and water or lye and water for example (avoid table salt) , with two metal plates (avoid stainless), one stationary and one movable in a controlled way so that the plates are close to p

arallel, not touching and the distance between the plates can be adjusted well using insulating material.

The plates are mostly immersed in the fluid except at the top where the electrical connections are made. The surface needs to be well ventilated to insure gas produced is of low concentration.

.

This is a cheap, and potentially deadly (depends on you) way to 'throttle' high voltage DC effectively. To 'throttle', just make the adjustment to move the plates further away. Just make sure the tank is big enough, structurally sound and well insulated.

This will take it down to 12 to 24 volts....is this what you had in mind?

http://www.vanner.com/manuals/HIGH-VOLTAGE-DC-DC-CONVERTER.pdf

We need more valid information, without that, no one can help you!

Maybe you can find a cheap used liquid resistance starter on e-Bay?

You want cheap?

I'll show you cheap.

A real rednek method!

Link below:

https://www.youtube.com/watch?v=P9UjxG8sN1c

cool stuff but I'm still looking for the same control of DC, I've got the AC down

OK.

Here's how it works:

The A/C is varied using the Variac.

The output of the variac is rectified,using diodes.

This results in a variable dc output.

Take a close look at post #8.Yahlasit nailed it.

He shows how to do it.

The same method can be applied to the rednek method output voltage to get DC output.

Filtering of the DC is probably going to be required,depending on application.

I agree with you and Yahlasit, except that I'd have a 120V variac feeding into a 120-400V fixed transformer, rather than the other way around. This, of course, depends on what kinds of transformers and variacs are most easily available to the device builder.

Fredski STILL hasn't told us how much current he is trying to control, so we may be wasting our time...

less than 100A continuous. I'm not sure if peak amp draw is an issue 40-50 would probably be very close. I'll reread Yah's comments

100 Amps at 500 Volts!!!! That's 50kW! This is no DIY project. It's major industrial power. you don't need help from CR4, you need a major industrial supplier.

IS this for a winch,or similar motor?

Are the fields brought out separately,or can they be separated internally?

If so, you could use field weaking to control the speed by putting the fields in series

with the armature,and varing the field resistance,via rheostat, to control speed and

torque.

Rheostats are available in 1000 watt and more for controlling speed in industrial

applications,such as a "dancer" roll to keep a certain tension on a web of paper or

cloth.

They are very rugged,and seldom fail.

I do not know your particular application.

More info would help us to help you.

Sounds like he needs a "Ward-Leonard system"!! As they used on cranes for many years....

See here:-

Ward_Leonard_control

Similar,but not quite the same.

The ones I referenced are a little different:

The rheostat is directly in series with the motor field winding.

These were used mainly for "Lead motors" of a section of motors,usually around 10 or so 15 HP motors.

The other motors followed the lead motor speed.

The roller with the pot was in an accumulator rack,that was geared to the pot to increase or decrease speed automatically as required.

When properly adjusted,the roller wound "dance" a small amount as conditons

changed.

Hence the name"Dancer Pot or Dancer Roller ".

There could me smaller groups of motors,depending on the need.

Back in the '70's the DC was provided by a 1000HP motor-generator set,and a

mercury vapor rectifier.

This sytem was superceeded by solid state controls,and later,by PLC's for

programmability and flexibility.

Reliance Electric was one company that provided such controls,and so was EMCO among others.

The potentiometers were about 10 inch diameter,heavy duty,weighed about 25 pounds,and resistance was only around 10 to 100 ohms.(A fine-tune pot).

Wattage was in the thousands.

Depends on the autotransformer (variac).

Higher current lower voltage of primary,or higher voltage lower current of secondary.

Results are basically the same.

The set up in that video will also work for DC.

Take a look at magnetic amplifiers. You can make one from simple transformers and control the high impedence side of the magamp with a pot and low DC voltage and current. It is the old-school way of controlling high-voltage, high-power equipment.

It is still used for control rod control in a nuclear power plant, and it is used in fighter aircraft for control surface actuator control because of it's reliability and consistent no-moving-parts contruction, and it robustness.

Agreed.They are very reliable,and immune to most things that destroy solid state components.

I have seen them used for speed control on battery powered fork trucks as well as the applications you mentioned.

They can also be used as amplifiers,and properly designed,can give gains exceeding 1000.

They are not a "Glorious" as modern amplifiers,but they will get the job done and are better and more durable for certain applications.

Glad to see someone that still remembers.

We will be much valued after a doomsday event caused by a solar flare or nuclear

blast wipes out most of the solid state components and civilian computers.

MagAmps are indeed robust and very dependable, but those I'm familiar with require special alloys for the core laminations that rapidly saturate above a certain magnetic field intensity.

I suspect that if you tried to make one by putting DC into one winding of a standard multi-winding transformer, the transformer is going to get hot quickly. Have you done it, or do you have a link to someone has tried it?

Here is a link using standard transformers:

http://sparkbangbuzz.com/mag-amp/mag-amp.htm

Thanks! That's a very interesting link. I'm really surprised that the current from the 9V battery through a 1kΩ pot is able to saturate the transformers. I can't believe that the battery would last long...

I agree,but I think this was used primarily as a teaching aid.

Of course,a very high resistance secondary would increase battery life.

Mag amp was really interesting. Ok for using DC to control/modulate AC load current but not DC load current/voltage which is what Fredski asked.

Took you a while to hand over the load current info Fredski....totally out of character.

What are you trying to apply a satisfactory solution to? 50kW doing what?

Please.

Since the details of your purpose are sketchy,here is a link where you can specify voltage,amps,HP and other criteia for just about any drive combination that you desire.

I don't think a home grown simple potentiometer will accomplish what you need to do.

A DC drive seems to be the real answer,if you have a real application for the power range you described.

Good luck.

http://www.driveswarehouse.com/s-83-dc-drives.aspx?select1=&select2=s-77-380-480-10&select3=s-81-1000-9999-amps&select4=s-83-dc-motors&select5=&select6=

http://www.electroschematics.com/8640/pcb-drill-speed-controller/

Here is a link with schematic drawings for DC control that should help.

Not much good for 100A @500+V. unless you can suggest an FET with those ratings.