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Large motor to run a water pump - induction or synchronous motor? 4

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Bronzeado

Bronzeado

Electrical
Jan 6, 2008
271
Hi Folks,

In the first stage of a big water project, it was specified 6,9 kV synchronuos motors (2 to 13 MW) to run the water pumps. These pumps are supplied by 230/6,9 kV transformers fed by a dedicated 230 kV radial power system (150 km) with several susbtations.

Now, there is a doubt about if that choice was correct. So, what kind of motor should be better to run the new water pumps in the second stage, induction or synchronous motor?
Also, which motor nominal voltage should be more adequated?

I wonder if the experts in this forum could give us some tips about that choice, showing the advantages and disadvantages in each case. Thanks!

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil
 
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General rule of thumb is that if the motor horse power (in HP) exceeds the motor speed in RPM the motor should be synchronous and if the HP is lower than the speed it should be induction.

So, that 2MW could go either way depending on speed. But if there's a large project with a bunch of motors and a small number are close to that HP/RPM break point I'd consider go with the predominant type.

Sounds like you'd be looking at synchronous pump motors and probably lots of induction motors for various support functions.

I’ll see your silver lining and raise you two black clouds. - Protection Operations
 
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One advantage of synchronous motors is that they can be used to control power factor. Being on a 150 km radial line, this ability might be useful. However, using various power electronic controlled capacitors (i.e. STATCOM, etc.) might also be effective at controlling voltage.

Keep in mind that many things have changed in decades since the rule of thumb was developed last century.

The motors have different efficiencies, so it may also be important how much of year the pumps operate.
 
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Did you get the issue with your existing 13MW motors sorted out?
 
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You usually want the same motor in a pumping station, for spares, maintenance knowledge, swap-ability purposes, etc.
 
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There can also be the political reason of if you switched and anything went wrong including nothing to do with the motors someone would blame it on the "different motors". Go with what's proven, perhaps a more modern take on the proven.

Keith Cress
kcress -
 
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Dear Mr. Bronzeado (Electrical)(OP)
There had been numerous valuable advice posted. I would like to add the following for your consideration.
1. All MV motor manufacturers have asyn and sym motors say 6.6kV-15kV, 1-30MW in their production line. This would indicates that each has their pro and con. However, both co-existing.
2. Comparison:
a)Asyn //.... b) b]Syn[/b]
1. Motor: cost (low) // (high).
construction (simple + very robust) // (complicated , less robust).
1.1. Starter cost (low + simple control) // (much higher cost + complicated control).
2. Speed/load (drooping). May be acceptable. // (constant). May be not needed.
3. Starting current (high). Consider VD 150km // (low).
4. Starting torque (low) // (high).
5. Efficiency (good) // (better).
6. Running Pf (approx 0.8 lag ) // (adjustable. Can be adjusted to lead).
Important when low pf is been penalized. // May be used to improve other low pf loads.
7. VFD control, if required (simple) // (complicated). Not designed for it.
3. In general:
Sym motor with (higher cost but with higher efficiency) should be [kept continuously running] in order to recover the higher initial+ maintenance cost, within sat 25-30 years.
Che Kuan Yau (Singapore)


 
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Dear che12345 (Electrical),

Great post. Short and Straightforward. I realy appreciate it. Thank you!

Comments:
...
3. Starting current (high). Consider VD 150km // (low).
4. Starting torque (low) // (high).
- Remember that synchronous motor starts as a induction motor
...
6. Running Pf (approx 0.8 lag ) // (adjustable. Can be adjusted to lead).
Important when low pf is been penalized. // May be used to improve other low pf loads.
- In our case, pf is not a problem as we need to compensate the line capacitive power.
7. VFD control, if required (simple) // (complicated). Not designed for it.
- We have soft starters, but they were not effective. The motors are starting straight.

In general, the motor will be kept continuously running.

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil
 
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Herivelto; I'm very sorry to even have to bring that up.
1exd88.gif


Back in the day we had the IBM PC. The rule was: "Always buy IBM because no one could fault you". It was extremely galling because equivalent non-IBM hardware was 30% less expensive or for the same price as IBM you could get something at least twice as fast.


Keith Cress
kcress -
 
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Bronzenado A study was done to compare initial cost and operating energy cost over the longer term for synchronous and squirrel cage induction machines in the petrochemical industry. Several thousand machines were investigated and monitored over a span of a couple of years, running everything from simple centrifugal loads (pumps and fans) to more complex processes (compressors, extruders, and pelletizers). The results, in a nutshell, are as follows.

If V > 4000 AND RPM < 900 Choose Synchronous. (Initial cost for synchronous is roughly +15%, but efficiency payback at energy cost of $US 0.10/kWh covers the difference in about 30 months at unity power factor operation. Faster if also providing reactive power to balance remainder of plant load.)
If P > 3750 KW AND V > 6000 Consider synchronous - it will be a case-by-case thing, dependent on application and number of machines involved.
If P > 7500 KW ... ALWAYS Choose synchronous (Synchronous has lower initial cost AND better operational costs, even at unity PF.)

Bottom line: Induction loses a lot of its "low initial capital expense" when the rotor exceeds the maximum diameter that can be manufactured from a single sheet of electrical steel. This happens to be at roughly 1.15 meters. As the machines get physically larger, the manufacturing expense swings progressively more in favor of a synchronous design. As an added consideration - induction machines operating at speeds below about 750 rpm have atrocious power factor - which usually translates to MUCH higher operating costs (since it is based on having the KVA to provide the kWh).

Converting energy to motion for more than half a century
 
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I'm not BIG motor guy like others here are. Their experience transcends mine. But. From the sage advice garnered in this thread I'd say, "yes", you'll be happy with syncs even if for a while you wonder about what it might've been like down induction road.

Keith Cress
kcress -
 
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I would vote for Induction motor if the motor rating is around 13MW. The reason being simplicity in construction leading to ruggedness, less maintenance and, better availability. This more than compensates a little loss of efficiency.
I have seen Synchronous motors tripping often on out-of-step protection due to extraneous reasons such a fault elsewhere. Since, you already have Synchronous motors running in the station, you may want to share your experience. Of course, one advantage in your case is the load is not high inertia type (unlike the compressor drives I worked with).
 
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Hi RRaghunath,

Thank you for your imputs.

Sorry! I have no experience on synchronous motors running as I was out of the Project when it started to operate. I have just returned to it.
The motors that are running just now are 6,9 kV synchronuos motors from 2.2 to 13 MW. I will see how they have been operated during the time I was out.

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil
 
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