synchronous motor vs induction motor 1

[unexpert]

We are going to buy a motor for compressor drive. The motor is 3 MW, 333 rpm, 6600 V, 50 Hz. We were advised that synchronous motor is more suitable but it seems to have more initial investment and we have no experience with it.
- We were told that synchronous motor may cause harmonics problem, is it true?
- How to reduce starting current? Can we use auto-transformer (reduced voltage) for synchronous motor?
- The reliability of sychronous vs induction motor for the above application. Which one is better?
Hope somebody can advise or provide link to the good information. Thank you.

 

[spargher]

Synchronous motor are more expensive but have their advantages, otherwise shy should we buy it?
Let's see your questions
1: Harmonics problem depends ok the kind of motor and it's control
2: Starting current: if you use a VFD soft start or a pony motor for start up you should have no starting current, you just need to sinchronixe to the network and close the breaker, without any starting current
3: depends on motor type

Let me me just say a think. One of the advantage of this kind of motor is the possibility of generating reactive power, so you can adjust the power factor of you factory by acting on the excitation current... more words about synchronous machine can be writte here... just do a google search

 

[PWR]

Your application sounds like the decision of whether to go with induction vs synchronous will easily be made for the latter. You will learn there are many factors to consider. One advantage of synchronous that does not get the attention it deserves is reduced inrush. Because the starting winding of a synchronous motor is separate and distinct from the running situation, the starting winding can be altered to lower the inrush significantly without penalizing the run efficiency. Thus you can often avoid the capital expense and maintenance costs of a reduced voltage starter by using a synchronous machine.

 

[aolalde]

Yes, a Motor 3 MW (4000 HP), 18 Poles will have better performance with a Synchronous construction as compared to Induction.

The power factor of a Synchronous motor can be adjusted even to work at unity (PF=1.0). This means your stator current will get the minimum value to handle the electric incoming power. Resistive losses on the stator windings, line cable and transformer windings follow the squared ratio of current (R*i^2). A High Torque-High resistance cage winding can be designed to reduce starting inrush.

All this mean lower energy operating cost. (Min. kW-HR) for the life of the equipment.

If the compressor is a reciprocating type direct coupled, an inertia wheel could be required to avoid current pulsations.

See this link for more data and help.

http://www.electricmachinery.com/products/synmotor.htm

 

[unexpert]

Thanks for all advice.
spargher : Could you give more explanation about harmonics please? What kind of motors and controls that should be awared of?
PWR : I understand that starting synchronous motor is more complicated than induction (and more protective relays?). I'm not quite sure that maintainance cost of its starter is less than induction. Can you give more comment?
FYI, we will not use VFD for this motor.
aolalde : I'm reading your recommended link. Thanks.

 

[PWR]

The synchronous starter is more complicated because it requires a means to supply and control the field of the machine. The added level of complexity is a relatively small factor in a machine the size you are considering however. The protective relaying costs are slightly higher. The unique requirements of the protection related to the field can be provided with the excitation control package.

 

[motorspert]

I do not understand the comment about synchronous motors and harmonics - I have designed a good many and am struggling to understand that one.
I do agree that the synchronous motor is more efficent and lends itself to the recip compressor quite well. Inertia can be cheaply added, starting currents are quite low.
As to reliability, the important thing is to specify the motor correctly and ensure the motor manufacturer is aware of the duty. Theoretically the induction motor has one winding and one cage winidng. The synchrous has four windigs including the exciter and a field controller and sop has to be less reliable haowver we are talking small differences here - difference maybe between 99.9999% and 99.99999%.

 

[coconutalley]

Unexpert,
Have you considered brushless dc motor with sinusoidal drive?
The motor is more efficient, has more power to weight ratio.
One drawback: for the voltage and MW sizes, you are probably looking at custom motor/drive. There are companies that specialized in making these. But, should do the tradeoffs so that you cover all there is out there.

 

[coconutalley]

I was referring to permanent magnet brushless motor.

 

[jraef]

3MW PMDC motor? I suppose if cost is not a consideration!

Forget harmonics in the motor decision, that is related only to the control method. Any slight increase in harmonics involved in the field excitation is more than made up for by virtue of the PF correction inherent in the synch motor. In other words, the benefits far outweigh the detractors.

In that size of application, any motor is going to be an engineered system (or should be), so long term reliability should be a design factor in either case. State what you want and expect to your vendors. IMHO, any motor, synchronous or induction, can be built to be reliable. If you base decisions on cost alone however, you will likely get what you pay for.

Synch motors need a little more in terms of control, but good high-end protection relays are capable of protecting either type anyway so that is not a major consideration.

Overall, I agree with Aolalde, slow speed applications are generally best done with synchronous motors even though the initial cost is higher.

Eng-Tips: Help for your job, not for your homework Read faq731-376