Email of the day: Brett asks about field power:
Hi George, Recently purchased an ST12 here on east coast set for 240VAC, the rheostat replaced by an AVR. We are seeing it requires upward 5 horse power 1800 RPM just to create the excite field 80 VDC 6.1 amps. That requires over 3730 Watts just to excite the field before any outside load has been applied. The field measures 17.8 ohms and the stator 4.1 ohms, the rectifier checks good on diode check. We also checked there are no high resistance or shorted windings to ground. The excite field is (80VDC@6.1amps) = 488 watts albeit that does not count for expected reluctance of rotor magnetic lines of force pushing electrons in the stator or the moving the weight of the rotor however 488 watts is a far cry from 3730 watts. With the field disconnected from rectifier ST12 required roughly 560 watts to maintain 1830 RPM. The question being is 5 horse power (3730 watts) a typical load while AC is unloaded or might there be some underlying issue at hand? Thanks for assistance!
Brett, Thanks for your question, this topic will be of interest to many in our DIY-AE community.
For the sake of other readers, I’ll mention that field current and voltage gives us a good idea of the power (overhead) we need to provide in order to produce power from our generator stator winding(s). But there are other loads we need consider, The ST12 has a fairly large fan, and bearings, and I have found grease on a few bearings as thick as plastic at 1800 RPMs, I recommend you assume the worst until you inspect the bearings you have. The accurate measure of torque at 1800 rpm with the field powered should give you total overhead for the machine, opening the field should show you only the parasitic loads like the Fan, bearing drag, the rotor beating the air, etc.
I had a problem doing accurate comparative tests between machines, and this lead to the creation of my Variable Frequency Drive powered three phase electric motor mounted in bearings, so I could add a strain gauge if desired. The VFD powered motor allows me to DRIVE the generator at exactly 60hz under test AND apply loads and no load from a test box.
With this tool, I can get a fairly good profile of how the generator performs and I have a few generators in the shop I use to create base lines for compares.
With this mention, I need ask, what is your prime mover in your test, and how are you measuring 5hp? WE know that power in an electrical motor (prime mover) changes with load, and power required is equal to Voltage X Current X Power Factor. And we know we can use this figure and convert to horsepower.
If your prime over is an engine (ICE), we’ll need to know torque times RPM to have a decent idea of Horsepower required to drive your 12kw rotating machine with no load.
With this said (four our audience):
- Your readings for stator windings in series seems reasonable
- Your Field reading seems reasonable
Now the questions:
- How are you calculating the horsepower required to drive the generator at 1800 RPMs?
- What horsepower does it take to drive the generator at 1800 RPMs with the field open?
- Are you able to read the actual current in the field WHEN you are seeing 5hp consumed at no load?
I am always the student, and learn something new every day, but I’d expect lots of smoke from a 12KW ST Gen that was consuming 5HP WITH no load. I’d expect a blue bearing, a smoking field, or a fan beating a rat’s nest till the smoke pours out..
As a rule of thumb, we expect to provide about 2hp for every KW of power (load) we measure across the stator winding(s).
Other readers, jump in with your comments.. just for grins, here’s a link to my test bench