[Milsurplus] Even Worse

Sat Jul 10 16:12:56 EDT 2010

At a USAF jet-engine test facility there was a motor-starting facility that
directly started large (53,000 HP) synchronous motors.  It started by
applying pulses to the target motor up to 6900V/phase at up to 2000 amperes.
Every time a pulse was applied the motor starter would check for rotation.
The pulsing routine was gradually increased in rate until the motor could be
fed a reconstituted 6-Hz (yes, 6 HERTZ) sinewave.  The frequency of the
sinewave was gradually increased to 63 Hz, with the motor-starting facility
checking the rotational speed of the motor.  When it slowed down to match
line current, the motor-starting facility would lock the synchronous motor
on the line and go on to the next motor.  It could start a sync motor in
about 2 minutes, as I recall.

This was designed and built by Brown Boveri of Germany in the late 1970s.
At the time it was quite an installation.  There were two of them in the
facility, with suitable switching to ensure that either system could start
any motor in the facility.  This was one instance where your tax dollars
really did work...

Dave WB4FUR

-----Original Message-----
From: milsurplus-bounces at mailman.qth.net
[mailto:milsurplus-bounces at mailman.qth.net]On Behalf Of J. Forster
Sent: Saturday, July 10, 2010 2:57 PM
To: Richard Brunner
Cc: milsurplus
Subject: Re: [Milsurplus] Even Worse

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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