Dear All,
During no load trial of motors, it is advised to switch off the capacitors. my doubt is the PF during no load is much lower than on load, then why to switch off the capacitors?

Thanks for any inputs.....

 

Charz... can you provide motor rating (V, A, Hp, Rpm) and its associated capacitor size, Mfd?

If there is a group of motors, then submit the information in Table form!

Regards, Phil

 

> Charz... can you provide motor rating (V, A, Hp, Rpm) and its associated capacitor size, Mfd? <

1)voltage= 6.6 KV
2)Amps= 56
3)KW= 520
4)Rpm= 1488
5)capacitor size= 150 Kvar
6)Mfd= 0.541 (i don't remember the exact value..but it starts with 0.5)

 

Charz,

The power factor correction capacitors are used to lower the full load running current of induction motors. The removal of capacitors during a test is so the current readings will be more accurate.

The typical loaded 40 hp induction motor running on 3-phase 480 volts may draw 50 amps. With a properly size capacitor it may run 10% lower current when the current is measured upstream of the capacitor connection.

The same 40 hp motor will typically draw 20% (or 10 amps) of its full load current when running under no-load. With the capacitor this current will reduce to 2 amps or less.

The motor current readings will still be correct when measured at the motor leads if the capacitor is connected at the motor starter.

I hope this helps,

William Hinton

 

Charz... The capacitive reactance, 150kVAr(c), could lead to severe over voltage if it remains connected to the motor you cited during the run-down trial.

PF Correction or Compensation (PFC) should not be greater than the motor's no-load magnetization reactance which, for this motor, is estimated to be about 8-15% of full load (say 75 kVar(c))

Please consult with the motor manufacturer before proceeding with the test!

Phil Corso

 

Dear Phil,

could you plz explain how the motor extracts high voltage or the capacitor supplying excess voltage to the motor at no load?

 

Charz... following are two reasons PFC for large motors should be limited to less than 80% of the no-load magnetization requirement:

1) Overvoltage.
When a motor and its PFC are switched-off together, the motor becomes "self-excited" because the capacitor supplies the motor's magnetization current. Consequently, a terminal voltage is developed whose magnitude could exceed rated-voltage. The degree of overvoltage is determined by capacitor's kVAr rating, while its duration depends on the motor and driven-machine inertia. As an example, if the PFC offsets the equivalent no-load kVAr, generated-voltage is about equal to rated-voltage. If PFC approaches the motor's full-load kVAr, voltage will exceed rated-voltage! This phenomenon is more prevalent in larger machines than smaller ones because, for the latter, speed decreases much more rapidly.

2) Transient Torques.
If the PFC is large, and the motor is inadvertently switched-on immediately following a switch-off action (e.g., reacceration following a power interruption), the developed torque could bew several times the motor/load shaft rated-torque. Admittedly, such an occurrence is rare, but it could explain some of the observations by Control.Com members!

Regards, Phil Corso

PS: What is the driven-machine.

 

Dear phil,

The driven machine is a centrifugal pump..

 

Charz... Thanks! Phil Corso

 

Motors are compensated for Inductive reactance by capacitive reactance. However if there is no load one cancels the other and likely to go to resonance and with high current as the Impedance Z is very low. That is why the capacitors to be disconnected.

Regards
Seshadri
hvseshadri2010 [at] gmail [.] com

 

Dear seshadri,

can u explain me in detail about the resonance? If so, in that condition the power factor is unity. kindly clarify me regarding the unity PF Vs resonance in that conditions..

 

I remember an example of this from my apprenticeship days. I wired pf correction capacitor directly to the motor of a large grinder. When I turned off the grinder the lights on it went to about 2 x normal brightness. I thought that was pretty cool but the journeyman made me connect it up properly.

Roy

 

Roy... that was not Resonance!

Regards, Phil

 

>Roy... that was not Resonance!

>Regards, Phil

True, it was acting like an alternator

 

Roy... I would like your help. I am developing a mathematical model for the phenomenon covered in this thread. And recollection of your personal experience with the grinder would help:

1) Single-Phase or 3-Phase.

2) Cap size in uFD or kVAc or any info relative to motor size.

3) If 3-phase were caps connected as wye or delta

4) If 3-phase was center point grounded?

Thanks, Phil