synchronous motor vs induction motor
synchronous motor vs induction motor
(OP)
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.
- 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.





RE: synchronous motor vs induction motor
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
RE: synchronous motor vs induction motor
RE: synchronous motor vs induction motor
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
RE: synchronous motor vs induction motor
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.
RE: synchronous motor vs induction motor
RE: synchronous motor vs induction motor
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%.
RE: synchronous motor vs induction motor
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.
RE: synchronous motor vs induction motor
RE: synchronous motor vs induction motor
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.
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