Power Factor and Saturation

[KatyDave]

Hello:

I have an application where there is a 40 HP submersible pump that is properly installed for cooling flow. The pump is used to pump water from a lake up to a reservoir. The induction motor is a 2-pole Franklin Electric, 575 Volt, 3-phase. The motor is rated at 42.8 FLA with a service factor of 1.15. It is driven by a FVNR starter with class 10 overloads.

The service voltage at the site varies between 615 to 627 volts. In Canada we have a 600 volt service which the utility must assure falls within +5/-10 %. When commissioned I noted that that the running current was about 47 amps. For this reason I connected a AEMC power pad and have the following:

PF: 66%
kW: 33 kW (44.2 HP)
kVAR: 37.5 kVAR
kVA: 50 kVA

I was told by Franklin that the motor is being driven into saturation by the high voltage, resulting in a poor power factor. Can someone explain this? I have never seen such a low power factor for a motor operating in the service factor. What was also interesting is that in the trend the kVAR followed the service voltage while the kW remained relatively constant.

I do not like the operating conditions on this pump and I am recommending that the client use VFDs. Opinions, suggestions?

Thanks.

 

[Skogsgurra]

The motor needn't be driven into saturation to have a bad power factor. As the voltage goes up, so does excitation current. And, as you have noticed, the kvar. Since power (HP or kW) is determined by the load and stays constant, the PF (kW/kvar) gets lower at higher voltages.

The utility seems to keep their voltage within limits so the problem is the 575 rated volts. A VFD is a very good solution. But if you don't need the other benefits, it seems to be an unecessarily expensive way of doing it.

If the motor is rated 42.8 HP (I assume), then you are a bit into the SF. So the bad PF cannot come from light load. It is the voltage that is causing it. A simple buck transformer (auto transformer) seems to be a simpler and cheaper way out.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[aolalde]

A magnetic circuit (core laminations and air gap) will demand a magnetizing current to handle the flux density B that the voltage, frequency and winding turns are producing.
Im=HxL
H=B/mu
B=phi/A
phi = 100xE/(4.44fKpKdN)
Im = magnetizing current in amperes
H= magnetic field intensity in Ampere/m
mu = permeability of the magnetic circuit
phi = flux per pole in megalines
A= magnetic circuit effective cross sectional area.
E= RMS volts per phase
f= Line frequency in HZ
Kd= winding distribution factor
Kp = winding pitch factor
N = number of turns in series per phase

From the expressions above, when the voltage increases phi
increases, since the area is constant B increases, the permeability mu is only constant before saturation, when saturation starts it reduces tending to zero at 100% saturation. With that H increases a lot and finally the magnetizing current since L ( average length of the magnetic circuit) is constant.

The magnetizing current is 100% reactive with 90° offset from the voltage. that is the reason why the kVAR increase and the PF is reduced.

Motors designed under NEMA Standards shall work allowing +/-10% Voltage deviation from the nominal or nameplate voltage but the efficiency and PF could deviate from those at nominal voltage and frequency.

 

[dpc]

Submersible motors are a different breed. I've seen submersible motors with horrible power factors - even at normal voltage. What does Franklin say the power factor should be?

627 V is only 109% of 575 V so I don't think you're doing any harm to the motor, and probably not even in saturation. But submersible motors don't have to meet normal NEMA motor standards, so who knows.

A lower voltage at the motor might improve power factor, but it might be easier to just add some capacitors if power factor is the concern.

I would have Franklin give you a voltage range that the motor can live with and then decide if you need to reduce the voltage. I agree that a VFD is probably overkill unless there are other reasons why it would be needed. But I suspect the motor power factor may never increase to the level that you are expecting, regardless of voltage.