motor can act as an inductor?

- A
- Alan Horowitz
  
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posted
20 years ago

Sun, Feb 1, 2004 10:03 PM

the scenario is a large vessel where service electric power is usually supplied by either a straight diesel-driven alternator, OR a mechanical power-take-off (from shaft main propulsion) driving a (different, but identical) alternator.

the manual states that when PTO is selected (this is preferred for fuel-economy, a "synchronous compensator", which is just a large motor of its own, is in circuit to "add inductive reactance".

How can a motor do that, and why does the PTO's alternator need the additional inductance thrown into the circuit?

- J
- John Larkin
  
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posted
20 years ago

Mon, Feb 2, 2004 12:13 AM

An unloaded synchronous motor can act as a capacitor or an inductor depending on whether its rotating field is over or under-excited (can't remember which is which!) Utilities sometimes use them as virtual capacitors for power factor compensation. I can't guess why additional inductance would be desired... capacitance is what makes most power systems happy.

Does the shaft speed have to be constant for the PTO alternator to make 50/60 Hz? Is the main propulsion steam or diesel?

John

- B
- Bill Shymanski
  
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posted
20 years ago

Mon, Feb 2, 2004 4:17 AM

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

As the field current goes up, the reactive power of a synchronous machine goes from lagging (inductive) pf, through unity pf and minimum line current, up to leading (capacitive) power factor. I had the chance to try this out one idle day at the steel plant, turning the big knob that regulated field current to a 30 MVAR synchronous condensor used in the arc furnace circuit to compensate for lagging power factor. You can plot line current vs. field current and come up with a "V-curve" so-called because of the shape - the minimum of the V corresponds to unity power factor. For loaded machines the "V" isn't very sharp, and I found that on the machine I had access to, the ammeters weren't terribly well calibrated for the low-current end of the range so it was hard to accurately plot a V-curve.

Bill