Technical Information Needed

[jojhanv]

I am in a bind here guys...I cant seem to explain what a synchronous speed is and its use for electric motor drive selection for our fire pump.

the Design Data point is:

power required by pump: 46.3Hp

Eff of pump: 70%

operating speed: 2900 rpm

The motor size generated by the system was:

Standard: NEMA

Enclosure: TEFC

Motor Size: 75Hp

Speed: 3000 rpm

Synch Speed: 3000 rpm

What is the purpose of the sych speed? what is its importance in the electric motor selection?

We needed 46.3Hp, but why the 75Hp recommendation for the motor drive?

Again thanks in advance...appreciate it if you can explain in laymans terms...been reading a lot of tech books but i cant seem understand them quite a bit...BTW i'm a MECH eng working for a Fire Protection Company and not quite good with my electical background

 

[Sidewinder]

Your flag indicates Kuwait?

I guess this is correct?

We are a UK based forum.

Is your mains 50 or 60 Hz I can't remember.

Synchronous speed is the speed that a motor will run at with no load.

It is the frequency of rotation divided by the number of pole pairs.

@ 50 hz & 1 pole pair this would give 3000 rpm

@ 60 Hz this is 3600 rpm

THe efficiency of the pump is just that.

Given your info I would take your pump output to be 46.3 hp.

Thus the input required would be 46.3/.7=66.12 hp at the input.

The motor will also have losses, say only 90% efficient, this would require a motor of

66.12/.9=73.49 hp at the motor input.

Perhaps the next standard size NEMA (US spec) motor would be 75 hp.

Based on this I would expect that you are on 60Hz supply.

This should be simple stuff even for a Mech Eng surely

Believe me I know!

The synchronous speed will define the rotational speed after slip which is inherrant in an induction motor and it is this slip that generates the motor torque, no slip no torque.

The speed of the motor will define the speed of your pumpe thus the volumetric displacement of the pump on a time base rather than specific displacement.

i.e. cu m per rev into cu m per minute.

HTH

Paul

 

[Evans Electric]

Is this to provide pressure for a sprinkler system ?

 

[jojhanv]

Your flag indicates Kuwait?I guess this is correct?

We are a UK based forum.

Is your mains 50 or 60 Hz I can't remember.

Synchronous speed is the speed that a motor will run at with no load.

It is the frequency of rotation divided by the number of pole pairs.

@ 50 hz & 1 pole pair this would give 3000 rpm

@ 60 Hz this is 3600 rpm

THe efficiency of the pump is just that.

Given your info I would take your pump output to be 46.3 hp.

Thus the input required would be 46.3/.7=66.12 hp at the input.

The motor will also have losses, say only 90% efficient, this would require a motor of

66.12/.9=73.49 hp at the motor input.

Perhaps the next standard size NEMA (US spec) motor would be 75 hp.

Based on this I would expect that you are on 60Hz supply.

This should be simple stuff even for a Mech Eng surely

Believe me I know!

The synchronous speed will define the rotational speed after slip which is inherrant in an induction motor and it is this slip that generates the motor torque, no slip no torque.

The speed of the motor will define the speed of your pumpe thus the volumetric displacement of the pump on a time base rather than specific displacement.

i.e. cu m per rev into cu m per minute.

HTH

Paul

thanks very much for the info. very much helpful and kinda confirms my theory. I'm actually in the process of approving a set of pumps for the local fire department here and I have to be quite prepared for his questioning. Here in Kuwait that would be 50Hz frequency. That means sych speed is 3000rpm. thanks...BTW flag is Philippines

---------- AUTO MERGE Post added at 09:39 ---------- Previous post was at 09:29 ----------

Is this to provide pressure for a sprinkler system ?

nope, I'm just preparing to get approvals for these sets of fire pumps from the local authorities here and I thought to get some information from people who know more than me...as they say...if you want to learn, ask questions... I was working as a site engineer before and I'm quite new to designing...this way I would learn faster...

 

[Sidewinder]

The only thing confuing me is that you have a NEMA motor designed for 50Hz?

If the motor is a 60Hz design it will not normally run @ the sync spd for 50 Hz on a 50 Hz supply.

BTW, the motor will normally run, even fully unloaded a little below sync say perhaps 2890?

This should be in the spec?

 

[Apache]

BTW flag is Philippines

If you hover your mouse over the flag it tells you the country.

 

[jojhanv]

The only thing confuing me is that you have a NEMA motor designed for 50Hz?If the motor is a 60Hz design it will not normally run @ the sync spd for 50 Hz on a 50 Hz supply.

BTW, the motor will normally run, even fully unloaded a little below sync say perhaps 2890?

This should be in the spec?

This IS another problem that I am facing because I have been searching for TEFC motors and they are normally coming in the speed of 3600 rpm which is the speed in a 60Hz frequency.But there is a note:

60/50 Hz rated with no derate on HP; 230/460 volt 60 Hz ratings operate on

190/380 volt 50 Hz, 460V 60 Hz ratings operate on 380V 50 Hz; Full 60 & 50 Hz

data on Nameplate

which means?

 

[Sidewinder]

TEFC = Totally Enclosed Fan Cooled.

This could be NEMA or ISO/BS/DIN.

I think you need an ISO motor as this will be 50 Hz.

Would need full nameplate data/motor specs to make full decision/advice.

Have to keep short, posting via mobile as away from home in hotel, AGAIN!

 

[jojhanv]

The local fire department here is asking for a TEFC, class F insulation UL/FM listed electric motor for the FIRE PUMP SET that we are trying to get approved here.

We dont have the actual pumps yet, we are only relying on the specs given by the suppliers in their brochures. But sadly all I have is a copy of catalogues running on 3600rpm which the engineers in the fire dept are not accepting. That was why I was asking about the synch speed and its use.

 

[jojhanv]

good news today... they have given me the 'go' to start the approval of the fire pump sets...THANKS GUYS FOR THE ENLIGHTENING DISCUSSION...ESPECIALLY SIDEWINDER...now all I have to do is find a catalogue of electric motors in the market for 380V/3Ph/50Hz TEFC, in class F insulation with UL/FM listing.

If any of you guys have a copy...would be a big help.

The supplier is using either WEG or US motors Pump.

Thanks

 

[Sidewinder]

jojhanv,

Sorry I could not post any replies yesterday, I wrote a nice lone explanation & my phone crashed its connection & I lost the whole thing.

Glad you have had some movement.

Why not try WEG direct for the motor data they are a BIG global motor manufacturer, IIRC they have the single largest motor manuf' plant in the world.

HTH

Paul