[Milsurplus] Even Worse

[Peter Gottlieb]

That's the beauty of a modern VFD.  You can independently set torque to 
limit heating.  Of course, you need enough torque to actually accelerate 
the motor and load.  Most motors used in variable speed operation have 
independent cooling blowers.  Large motors are characterized for 
thermals (e.g., thermal inertias of different parts) so you can control 
startups so as to not get near insulation degradation temperatures in 
the cores of the windings.

Even a small (by grid standards) 10 MW battery system will have a max 
specified ramp rate of no more than a few MW per minute.  You don't want 
people's lights changing brightness.

A VFD can start nice and gently, it's all a matter of how you program it. 

Peter


Richard Brunner wrote:
> 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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