How far can you adjust the Power Factor on a Synchronous Motor? 3

[gnortham]

I am trying to decide on the right synchronous motor to install on a paper refiner. The Mechanical Engineers tell me I need 800HP. Because I need all the Power Factor correction I can get, I would like to install a motor and run it to maximize the VAR flow.

Which motor is better: 800HP at .80 PF; 900HP at .80 PF or 1000HP at 1.00 PF?

How is the 1000HP at 1.00 PF different from the 900HP at .80 PF?

Can I get more leading PF from the 900HP motor if I limit it to 800HP (ie run it at .75 PF)? and How do I calculate what I can run it at?

 

[gordonl]

The horsepower and power factor will combine to give a stator current limit equal to the nameplate full load amps or corresponding MVA. With less HP loading the PF could be decreased upto equal FLA, this could be calculated using the 800HP, 746W per HP, and efficiency to get approximate real current, then maximum reactive current would be root(FLA squared minus Real current squared). This could also be done with power instead of currents.

Because the motor needs to be overexcited to produce more vars, to increase var output more excitation would have to be applied, the rotor may be current limited and prevent operating in this mode. The manufacturer would need to be consulted.

A lower power factor can be specified, we have a 14000HP at .39PF which we use for plant voltage control and power factor correction as well as mill rolling. (MG set) I imagine there will be cost considerations as well which I can't comment on.

 

[peterb]

I suggest that you explore the relative cost of supplying some of the VARs by capacitors as against an oversized motor.

 

[iamconfused]

The difference between 1000HP at 1.0 pf anf 900hp at .8 pf is that the 1000 hp motor at 1.0 pf will not draw any vars, whereas the 900hp motor at .8 pf will draw 503 K-Vars at full load.

Another thing is that the nameplate rating states the pf at which the motor can be run continously at full load. that is 1000 hp 1.0pf ( no vars ) in comparison to 900 hp at .8 pf (503 kvars at full load). now what u want is a moor which can provide the required mechanical load and is designed to operate at low PF (for power factor compenstaion). as u also need a little cushion for the mechanical load to be driven ( at least 15% cushion, 800x15/100 = 120hp)
My suggestion would be to go for the 900 at .8 pf motor.
if u operate it at 800hp load continous, then u can control the PF to have a max var flow of 44.6 Kvar.

There are a lot of things i have left out as i was feeling cramped for space. if u have any further questions plz feel free to contact me.

 

[gnortham]

This has helped a lot. The next question that I have is: is there anyway to predict what the field voltage and current will do (based on the nameplate data) when you change the PF?

I'm not sure I can get the information I need from the manufactures (GE and Electric Machinery)to safely run my motor at a different PF. Is there anyway that the limits of a motor field can be determined so that I might run a motor that is rated at 900 HP .8 PF at 800 HP .71 PF? This would increase the field current above the nameplate but perhaps by monitoring an RTD in the field I could determine if I was overheating the field.

 

[coolfish]

V-curves and capability curve will help in analyzing the performance of the machine. If the machine reactances are knowb, you could draw the capabilty curve.

 

[electricpete]

Is there anyway that the limits of a motor field can be determined so that I might run a motor that is rated at 900 HP .8 PF at 800 HP .71 PF? This would increase the field current above the nameplate but perhaps by monitoring an RTD in the field I could determine if I was overheating the field.

I'm assuming all power factors you mention are leading.

It's true that the two loadings would give the same stator current and should therefore give equivalent stator heating.

But as gordon and others have pointed out, it may be the field circuits (not stator windings) that are limiting in this over-excited operating mode (0.71pf leading). The RTD's that you have available are likely on the stator and won't help you judge field circuit loading. So you need some more info from the manufacturer on field circuit capabilities or P-Q capability curves before you load above 0.8pf (unless you feel lucky).