[Milsurplus] Even Worse

[J. Forster]

It was almost certainly analyzed in the late 50s / early 60s by Kingsley
(of Fitzgerald & Kingsley fame). It was his Electrical Machinery course
that took the Magnet Lab tour. The analysis almost certainly is on file
somewhere. I will enquire a bit.

-John

==============


> Yes, starting large motors has always been a problem.  AC motors pull
> locked rotor current from zero to about 85% of speed, and it tapers off
> above that, locked rotor current being 4 to 20 times full load current,
> depending on motor design.  Common practice was to use a Pony Motor to
> get it up to acceptable speed before energizing the large motor.
> Starting motors is hard on them, and is best done quickly to minimize
> heating, so maybe your large VFD is not a good idea, also a large step
> in load would not be welcome to Cambridge Electric.  This would be fun
> to analyze.
>
> Richard, AA1P
>
>
> On Sat, 2010-07-10 at 12:43 -0400, Peter Gottlieb wrote:
>> Darkening Cambridge wouldn't be the best PR move.
>>
>> What a crazy setup.  These days they use a single large VFD to do this
>> job and just set current or power limits.  It wouldn't be hard to even
>> incorporate a data feed from Cambridge electric which specified every 4
>> seconds how much power they could pull.  That would even help smooth out
>> grid fluctuations.
>>
>> This same problem exists in Chile with the big mines.  You don't just
>> pull in a contactor to start a 20,000 horsepower crusher motor.
>>
>> Peter
>>
>>
>>
>> J. Forster wrote:
>> >> All the dynamics of all the control systems across the grid can make
>> for
>> >> an "interesting" day if something goes wrong.
>> >>
>> >
>> > The general philosophy seems to be "don't rock the boat". In
>> Fitzgerald's
>> > grad school electrical machinery course, we went on a tour of the MIT
>> > Bitter National Magnet Lab, which has giant AC motors hooked to DC
>> > generators for powering the experiments. These things were easily 15
>> feet
>> > in diameter with buss bars like I beams.
>> >
>> > Anyway, when they put the machines on-line, they would gradually bring
>> > them up to operating speed with smaller motors and look at the phase
>> > difference with a thing like a differential synchro hooked to a dial.
>> The
>> > pointer would turn CW or CCW, depending on whether the machine was
>> above
>> > or below sync speed. When the pointer was stationary, the machine was
>> > operating at the right speed. Then they would goose it a bit up or
>> down to
>> > bring the phase difference to zero, and then adjust the field to match
>> > voltages.
>> >
>> > The final step was to connect the thing to the grid with a giant
>> circuit
>> > breaker.
>> >
>> > The objective was to be as seamless as possible. A screwup and the
>> city
>> > would go dark.
>> >
>> > Best,
>> >
>> > -John
>> >
>> > ================
>> >
>> >
>> >
>> >
>> >> Generally, most plants are looking to have a 1.0 PF at their point of
>> >> connection to the grid even if it takes significant reactive to get
>> there.
>> >>
>> >> (Yes, I know, gross oversimplification).
>> >>
>> >>
>> >>
>> >> J. Forster wrote:
>> >>
>> >>> By raising or lowering the field current, you can make the reactive
>> >>> power
>> >>> look capacitive or inductive respectively, but not instantaneously.
>> The
>> >>> L/R time constant of the field circuit is nowhere near zero. The L
>> is
>> >>> huge.
>> >>>
>> >>> -John
>> >>>
>> >>> ===============
>> >>>
>> >
>> >
>> >
>> >
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