Synchronous Generator speed

[bartb102]

We have a hydro powerhouse with 750KW 40Hz generators with 40 poles. We cannot increase the rotor speed to produce 60Hz. power. We have been told that the stator can be rewound to produce 60Hz. It seems like this would need 1.5 stator windings per rotor pole. There is only one winding per slot, so the slots are fairly shallow. It would be much easier to restack the stator core than to modify the rotor.

Has anyone tried this? Any thoughts? A vendor has said it might be possible, but has offered no details.

 

[davidbeach]

Frequency is dependent on how often a rotor pole passes a given location on the stator; you can raise the frequency by rotating faster or by having more poles. Nothing you can do with the stator to change the frequency.

 

[waross]

Why can't you increase the speed? A speed increase may be an easier problem to solve.
Changing the number and/or spacing of the stator poles without changing the number and/or spacing of the rotor poles may change the phase angle of some of the windings in a phase group and may not change the frequency.
Changing the number of rotor poles will change the frequency but with many coils generating at the wrong phase angle the terminal voltage at a given excitation current will be down.
Why can't you change the speed, The set is not that big and not running that fast.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bartb102]

The Francis wheel turbine chokes off water flow and will not rotate any faster than 140RPM, even at full gate no load, so speed cannot be increased. Rotor is cast iron, also cannot handle anything faster than that. No room for a gearbox (or its expense). We're familiar with re-poling the rotor, but due to its construction, we don't believe it will work on this unit.

 

[oldfieldguy]

Might be a case for a rectifier-inverter arrangement, basically a frequency converter, all solid-state and low maintenance. Depending on the voltage, you could almost re-purpose an off the shelf VFD for hardware.



old field guy

 

[waross]

Thank you for the explanation bartb102.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bartb102]

The inverter is a good idea, but not economical for 750KW, 12KV, 40Hz input. Could put in a stepdown transformer, but not cheap for 40Hz.

 

[davidbeach]

There's always the rotary converter option, a four pole motor driving a six pole generator.

 

[petronila]

Hello bartb102

Maybe you could take a look wheel and mechnical design in order to study if you can redesign something that can help you increasing the mechanical speed like an alternative you could replace the actual generator by one with 54 poles and regulate.

I.M.O a comparisson between mechanical modification and generator replacement most done in order to have a technical- economic solution.

Regards

Carlos

 

[rasevskii]

That powerhouse was the subject of a thread in Eng-Tips some couple years ago.
It is an historic intallation, and has a web page. I will not reveal more, only the OP can do that.

The 6 40HZ generators are coupled to the 60Hz grid by two synchronous/synchronous frequency changer sets, or that was the arrangement. Perhaps the idea is to eliminate the frequency changers, or there is a fault in one of them??

A static frequency changer is possible of course and have been built for many different frequency combinations by amongst others, ABB, and Siemens. The 60/25 HZ
SFCs feeding Amtrak and SEPTA rail operations have been in service by now many years.
The Swedish Railways are fed by many SFCs working at 50/16.66 HZ.

It is an economic problem, likely, the cost of a HV SFC would far exceed the available funds for this relatively small plant.

rasevskii

 

[squeeky]

Sorry. Protection required by the utility normally includes some sort of direction current limitation and either ROCOF (rate of change of frequency) or vector control. This prevents the alternator from adding to a utility fault or the utility supplying energy back into the alternator. The system is further complicated if the utility supplying has auto-reclosers on them. If the utility disconnect from the grid due to fault and then recloses automatically, the frequencies may have slipped out of step and this may result in pole slipping and destroy your machine. This is not good!

 

[bartb102]

rasevskii, you are correct. The two freq. changers are still in operation. We have restored six units to full output, and would like to restore the seventh. Problem is, the changers can handle only 3 units each. Thus our search for a better way to make the conversion. As always, economics will determine what happens.

 

[electricpete]

I assume your rotor operates at 2 hz and you have 40 poles to produce 40 hz.
If you wanted to produce 60hz, you would need to rewind both your stator and rotor for 60 poles

Regarding the stator:

There is only one winding per slot

Do you mean one coil per slot? i.e. single layer winding?
How many stator slots does this machine have?
If it is a multiple of 60, that makes the job easier. If it is not an exact multiple of 60, it would be a fractional slot winding. We can look at options for fractional slot winding when we know the number of slots.
Someone would have to look at physical installation /coil design so the end portions of the coil have adequate clearance


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(2B)+(2B)' ?

 

[electricpete]

Seems like that rewinding the rotor from 40 to 60 poles might be even more of a challenge than the stator

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(2B)+(2B)' ?

 

[bartb102]

There are 120 slots in the stator, 20 coils per phase. Single layer winding (makes it relatively easy to change coils when one fries.)

 

[electricpete]

do you mean 40 coils per phase?

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[electricpete]

Sorry, I got it. Single layer winding, twice as many slots as coils.

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(2B)+(2B)' ?

 

[bartb102]

If we needed to double the frequency, I could envision trying to install two layers of stator windings (so each pole was over two coils, not just one). But I don't see a way to do it for a 50% increase.

Remove half the rotor poles & install three layers of stator winding? Now that would be fun to design and build.

By the way, at 60Hz. we want only 4160V, not 12KV.

 

[waross]

You don't need a special build 40 Hz transformer. A 60 Hz transformer rated for 150% voltage will maintain the V:Hz ratio. Reading and talking to old timers when I was young indicates that the during the great change-over from 50 Hz to 60 Hz in Ontario Canada many transformers and motors were re-rated and continued in service. Changing from delta to wye may do the job on delta delta transformers.
Due to the increasedd voltage rating at the same current, the KVA rating must increase by 50% to 150%.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

There are 120 slots in the stator

Let’s say we want to try to put a 80 pole stator into a 120 slot core.
2/3 slots per pole per phase.
If we attempt to develop a fractional slot winding, some pole phase groups have 1 slot allocated and some have zero allocated. Already we suspect that will result in a lot of harmonics, but we need to figure out the coil layout to balance the fundamental among phases first:
A reference here:
thread237-261447

Here we are talking about the three-phase windings with fractional number of slots per phase and per pole , such as: q = 1⅓ or q = 2⅜ etc .
This fractional number we can write in this form: q = A + b/c
If q is an integer then the winding is always symmetrical. But if q is fractional number then symmetrical three-phase winding MUST meet two additional requirements:
1/ - Number of poles must be divisible with "c" and
2/ - Number "c" should not be divisible by 3 [electricpete’s note: or else the fundamental will be unbalanced among phases] .

You have q = 120/(3*60) = 2/3
A =0, b = 2, c = 3
With c=3, you cannot perfectly balance the fundamental in this configuration. You would have to leave some slots empty to do that. I’m sure there are quite a few more challenges even in designing the stator and balancing fundamental of stator among phases may be among the smaller ones. But in my view it is first step to find the winding configuration to balance the fundamental and second step to evaluate harmonic performance of that winding configuration. I am doubting this existing stator core can be rewound for 60 poles without substantial derating and lots of harmonics and vibrations. If your shop thinks they can do it, then I would ask them to tell me how they are going to lay out the coils (with that info we can evaluate the harmonics).


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(2B)+(2B)' ?