Static Frequency Converter, SFC Vs MG set

[easyser]

31 May 2006

 

Hi All !

 

We have about 55 nos. 60 Hz motors, with KW totalling to about 2000 KW, which have to be driven through an SFC / MG set, supplied from a 50 Hz network. The motors may be started in groups or individually. Some motors may possibly be started like 'DOL', if required, after the SFC / MG set has reached the full voltage.

 

For the above application, which is a better option ; SFC or MG set ? Any other comments / solutions ?

 

Thanking in advance for the replies,

 

easyser

 

 

[jraef]

First off, both solutions pose the same major hazard, that being a single point of failure for the entire system. In that light, the MG set would likely be more reliable than the SFC (as long as it is properly maintained, see below). As far as starting ability, that is totally dependant upon the size of either system you select, which of course relates to the price you pay. In general, on a single system of that size, the MG set with enough capacity for DOL starting (assuming staggered) is going to cost less than a SFC of equal capacity. Maintenance is another issue however. The MG set will be another piece of rotating equipment, needing routine maintenance to stay reliable. The SFC would have no moving parts, hence the term Static, therefore requiring far less maintenance. In either case you will need to deal with the voltage difference as well.

 

I would also investigate another option: Check out each motor as an individual system and determine if it is necessary to actually run at full speed. You didn't state the voltage, but often the voltages match up well to the frequency difference, just the speed will be lower. Motors work based on a ratio of voltage and frequency. If for instance your motors are rated 460V 60Hz, the V/Hz ratio is 7.6. If your supply is 380V 50Hz, that V/Hz ratio is also 7.6, so the motors will work fine, just slower. Some of your applications may be fine with the slower speed. Once you determine that, then buy individual VFDs for all the critical speed motors and a transformer to feed only those circuits. This will likely cost the same as a single SFC, but if one VFD goes down it will not cripple your entire system, plus it is far easier to keep spares ready.

[kens]

Hi

It is worth while to look at the relative efficiencys of both systems also. M/G sets can have a fairly low efficiency. You are looking at a 2000 kW system so if your M/G set is 80% efficient you will require a input power of around 2500 kW. Depending on the loading and running hours these losses can cost a lot of money. I dont know where you are located but if you add up all the network charges for the additional capacity and also the cost of the energy losses you may find that the static converter ends up being cheaper in the long run. If the motors are old there could also be an argument for replacing them with modern high efficiency 50 Hz motors.

The point that jraef makes is also very valid. If the process can handle the motors running at a slower speed then you may not need to do any thing, if the speed is critical on certain motors then you could go down the route of VSDs and transformers. One other thing to think about is that you may be able to simply overspeed some of the motors with a VSD without using a transformer if the torque requirement is not too high. It could be that the motors are oversised for the application and at the higher frequency even without the higher voltage still provide enough torque to do the job. One word of caution is that if you go down this track you will need to oversize your drives to handle the additional current required at the higher speed.

 

Good luck

Ken

[easyser]

First off, both solutions pose the same major hazard, that being a single point of failure for the entire system. In that light, the MG set would likely be more reliable than the SFC (as long as it is properly maintained, see below). As far as starting ability, that is totally dependant upon the size of either system you select, which of course relates to the price you pay. In general, on a single system of that size, the MG set with enough capacity for DOL starting (assuming staggered) is going to cost less than a SFC of equal capacity. Maintenance is another issue however. The MG set will be another piece of rotating equipment, needing routine maintenance to stay reliable. The SFC would have no moving parts, hence the term Static, therefore requiring far less maintenance. In either case you will need to deal with the voltage difference as well.

 

I would also investigate another option: Check out each motor as an individual system and determine if it is necessary to actually run at full speed. You didn't state the voltage, but often the voltages match up well to the frequency difference, just the speed will be lower. Motors work based on a ratio of voltage and frequency. If for instance your motors are rated 460V 60Hz, the V/Hz ratio is 7.6. If your supply is 380V 50Hz, that V/Hz ratio is also 7.6, so the motors will work fine, just slower. Some of your applications may be fine with the slower speed. Once you determine that, then buy individual VFDs for all the critical speed motors and a transformer to feed only those circuits. This will likely cost the same as a single SFC, but if one VFD goes down it will not cripple your entire system, plus it is far easier to keep spares ready.

 

 

Thanks for the replies.

 

The process needs that the motors be run at speed corresponding to 60 Hz.

 

The motors are rated 440 V 60 Hz. The network supply is 415 V 50 Hz.

 

We are considering going in for system with 100 % rated redundant inverter. The largest motor in the group is rated 90 KW. The inverter will be sized to handle the simultaneous DOL starting current of at least two of these largest motors in the system.

 

easyser

 

 

[marke]

If you use a MG set, you should be able to run at 440V 60Hz, whereas an inverter will be limited to the line voltage, i.e. 415 volt at 60Hz. This could be enough to limit the torque / power output and create issues with motors overloading.

 

Best regards,