DC motor Control questions

[PILOON]

Still don't know what you need to do.
It just so happens that I have 2 variable speed controllers (new surplus) that can drive a variety of motors, generally they are DC 90 volts.
They are brand name units that can handle (think) up to 3 hp. or so by simply changing a resistor.
As is they have 1/2 or 1/3 motors and can do, like 10 RPM thru to 1750 or so.
I have 'played' with them a bit and am impressed with the quality.
PM me with more details and we can talk.

 

[Redneck in training]

The capacitor generates a phase making the motor spin in certain direction so I thought that I could use one of the phases instead of capacitor.

 

[LD1]

The capacitor generates a phase making the motor spin in certain direction so I thought that I could use one of the phases instead of capacitor.

Still dont understand what you are trying to ask here?

What do you have? and what are you wanting to drive?

 

[quicksandfarmer]

From the original post I understand that what you are looking to build is called a "must pump." ("must" is what winemakers call crushed grapes). Googling, it seems that must pumps are pretty involved and the low-end ones are thousands of dollars. Somehow I don't think that motor speed control is the central problem.

 

[Redneck in training]

Still dont understand what you are trying to ask here?

What do you have? and what are you wanting to drive?

Single phase motors are actually two phase motors. One phase comes from the grid directly and the second is generated by the capacitor that causes phase shift. So if you remove the capacitor can you use a phase generated by the VFD electronics.? I am asking just out of curiosity.

 

[blakester]

Yes a must pump is a complicated pump. Generally speaking, I COULD run the pump from a single speed and not worry about speed control at all. The design of the rest of the pump is underway, and I'm optimistic I have something that will work, but it would be great to have it variable speed. Typically these pumps on the smaller end run a 3000$ price tag for a fraction of what I'm looking for, hence the desire to build one. Even splurging on a nice VFD and a DC motor I would come in way under cost if I can get it right. 4-9 KW seems to be a normal rating, which I believe transfers relatively closely to HP. Everything that I'm hearing here leads me to the conclusion I'm likely better off leaving the electronics to the experts, and focusing on the mechanical. So a nice sturdy DC motor and a VFD is where I think I'm landing on this one. Thanks for all the help so far.

 

[deere 855]

DC motors and VFD do work together. If you are going run a DC motor you need a DC drive. A DC motor needs a field, either permanent magnet or wound copper field. Never weaken a field unless you have feedback control as under light load the motor will run away and can fly apart. A FVD driving into a 3-phase motor works quite well. Bosh makes a nice VFD that operates on single phase and can drive a 5 HP 3-phase motor. I recently installed one on a wheel polisher. I am very familiar with the type pump you plan to use, sometimes referred to as a toothpaste pump in the trades. We ran dozens of them with both DC and AC drives, most were 2 HP or less. We used C face motors direct coupled to a right angle gearbox, ratio set to keep motor speed near 75% of base speed. Keep the speed up for best regulation and longer motor life. Pumping a liquid with solids in it be sure to use overload and over current protection to protect the motor and pump.

 

[arlen4720]

Single phase motors are actually two phase motors. One phase comes from the grid directly and the second is generated by the capacitor that causes phase shift. So if you remove the capacitor can you use a phase generated by the VFD electronics.? I am asking just out of curiosity.

No you can't do that. Two legs off of a three phase system is still single phase. You do not get 2 sine waves. They resolve into 1 sine wave. If you are familiar with vector math, and want to play around with it on paper it will make sense.
You can run a single phase motor off of 2 legs of the three phase system, but it still needs it's capacitors.
You are right that the capacitor creates a phase shift in the 2nd winding to allow the motor to start in a particular direction.
You could design a motor to run off of 2 legs and a neutral of a 3 phase WYE system, but what would be the point? You would still need 3 wires and a special custom motor. Plus it would run like a 3 cylinder engine hitting on 2 cylinders.
Your main "take away"....2 legs off of a three phase system is not "2 phase" system. It is purely a single phase system.

If you use a VFD, there is no need for a single phase motor, The VFD will generate 3 nice, symmetrical phases from a singe phase source.
You can use a DC motor and DC drive too. Just get a AC or DC motor and Drive combination that are designed to work together (obviously).