Hi, I am new to Arduino. I have a 300HP DC Motor. I need to control its speed ( as 75Hp, 100Hp, 200Hp, 300HP,etc ). How can I dot it with Arduino UNO ? 
What type of current controller is the arduino to be controlling?
Hi,
300HP, Horsepower, or are you talking about RPM shaft speed?
Tom....
10k+ motor and wants to control with a 10 dollar board.
You need an industrial motor controller with all the safety features if you have a 1/4 MW DC motor.
as much as it sounds like you're a beginner, you have ambition! I like that. First place I'd start out with is a good transistor.
http://www.ebay.com/itm/281491687939?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
I jumped on that deal as soon as i could, been tracking down a deal like that since 2011. a CM1000HA-24H Igbt. 1000 amps, 1200 volts max, suitable for most applications switching at less than 20kHz (so you will probably have motor wind. then again if you're driving 300HP your equipment is going to be a lot louder than the noise.
The basics:
1.) a gate driver. you're going to need a good gate driver to keep the igbt out of linear mode, this means you need to get the gate voltage up to around 15 volts in as little time as possible, once there the current draw is negligible. second, you need to turn it off by discharging the gate, and for good measure bias it below ground with a negative voltage to MAKE SURE it turns off.
2.) power connectors. KEEP THE POWER CONNECTIONS SHORT. You want to minimize inductance in the power connections to keep huge voltage spikes off your expensive igbt
3.) build in some hardware protection. IGBT's fail short circuit, so if it fails your motor will draw full current and won't be able to turn off. put some big contactors and take A LOT OF TIME CAREFULLY ENSURING THAT YOU CAN CUT THE POWER. DON'T SKIMP ON FUSES AND CONTACTORS.
my reccomendation is to start small. get a hold of a small tiny motor and make your circuit on a table, get your gate driver working well, iron out the bugs in your code, make your safety interlocks, and get an idea of where you want to go with this project. move up to some bigger dc motors, maybe a car starter would be a good place to continue. observer how inductive loads cause spikes and test your heat-sinking. a car battery is a good enough power supply for these tests. then maybe get an old forklift motor and continue testing your design. There are lots of tutorials and app-notes online that have plenty of information. keep in mind at 300HP you will have massive amounts of power that can cause EXPLOSIONS/FIRES/TONS OF TORQUE THAT CAN SEND METAL FLYING. this is a very dangerous project that really shouldn't be attempted big right off the bat. that's why it's important to start very small.
this isn't going to be a weekend project. A good controller will probably take months to design, and for a beginner maybe even longer. READ READ READ. get your electrical theory solid, make your mistakes, learn, and ask for help keep those big dreams in your head, because if your dreams are small and easily attainable, they aren't really dreams are they 