Originally Posted by Oceanride007
It'll be nice if we could have 3 ph inverter
if a circuit needed it for high start torque, maybe multispeed (Pole changing), slow for hauling an anchor
in fast for letting out, no brushes
to worry about.
Modern 3-phase squirrel-cage induction motors are a beautiful thing. They have been so well developed that bearing life is now often the limiting factor of the motor's service
These days, 3-phase power is pretty easy to get too. In the old days, you had to use a ronk box or similar rotary phase converter to get 3-phase from a single
phase primary. Those big heavy klunky things are now only used in the more gruesome applications because static inverters have become small, lite, cheap
& readily available. I put one on a 1.5hp Bridgeport milling machine about 10 years ago & it's still running like a champ today. Static phase inverters can be found here: Enco - Guaranteed Lowest Prices on Machinery, Tools and Shop Supplies
If you want to get a little fancier, you can move up to a variable frequency inverter
, that gives you variable voltage & variable frequency 3-phase output so that you can have variable speed & variable torque control of a 3-phase motor
. You can program either speed or torque as the limiting factor.
At speeds below the nameplate RPM
of the motor
, they operate in the constant torque range & can give you full motor torque as the speed decreases. Above nameplate RPM
, they operate in constant horsepower range & available torque drops off as speed increases. At 2 times nameplate RPM, you have roughly half of normal torque available. You can program lower torques if you want to. Speed control is infinite in theory, but in practice, you normally run them from about 10% of nameplate RPM to about 300%. If you run for long periods of time at low speeds with a fan cooled motor, you may need to provide additional cooling
to the motor.
The vast majority of elevators built today use these to control the speed of the car going up & down. The S-curve acceleration & deceleration is why modern elevators are able to give you that smooth comfortable ride with no sudden jerks. They are pretty reliable little units. Depending on the individual model, they can be run off of single
phase, 3-phase or high voltage DC primary power & still give variable 3-phase output. All this stuff is pretty efficient these days too. Losses below 5% are not uncommon.
One of the nice things about a variable frequency drive inverter is that it lets you start a motor gently, so that you don't get the sudden inrush currents that would normally be present if you just throw the switch on a motor the old fashion way. A 2 or 3 second starting curve will often cut your surge current
to about 1/4 of what it was before. A simplified VFD that only has this one function would be called a soft starter. These devices are very good at reducing the beating that your electrical system
takes from surge currents.
I can go into nauseating detail on these things if anybody wants me to, right down to board level construction & various generations of technology from the 1980's to present. I have pretty deep roots in this technology.