p.f. of a locked-rotor ind. motor

Why would one want to use a locked-rotor induction motor as a convection heater?

Once the overload operates, the power factor falls to zero.

Ineffiecient, big copper losses small power ind. motor may stay long under the condition in question without burning out (the protection is often oversized). And often they (1ph ones) will run fine after you have taken apart and oiled its bushes.

Invitation declined.

Homework?

Homeporc!

Dear all,

Power factor of locked rotor motor is around 0.2 to 0.4.

Regards

Thank you. I was always convinced it is about 0.91 -0.99. Am now going to make an actual check with a small one - by adding cap-s.

I was always convinced it is about 0.91 -0.99.

How? Locked rotor means -

a) no mechanical power transfer

b) Secondary is like a transformer short circuit.

Actual power = only the copper losses of the twin - pri and sec

magnetising current remains same

hence Pf will be as the previous post said.

BTW: please put current limiter unless you want to destry even the small motor.

Have just carried the test:

An ind. motor, locked rotor. 2 cap-s at my hands : 1 and 2.5 microfarads (no others at the moment)

App. current being drawn byt eh mot. 0.20 A;

Add 2.5 mf cap: current 0.23 (becoming capacitive);

Add 1 mF cap: current again 0.20 A:

Add the both cap-s (0.71 mF): current remains 0.20 A;

The clamp meter is not of a best precision, so, evidently, are the last 2 measurements.

Clear one: p.f. is rather 0.9 than 0.2 - otherwise the capacitive load added would have compensated the inductive.

Dear Friend,

The Power Factor will NOT be 0.91 to 0.99. Pl. refer Post No.4 of Mr. AkihitoShigeno, and my endorsement for his views/statement, as "

"Dear Mr. AkihitoShigeno,

You are right.

The P.F will be as you said 0.2 to 0.4, for the SIMPLE REASON, the LOCUS of the CURRENT of the Induction Motor WILL FOLLOW THE CIRCLE DIAGRAM, and at the extreme end of the Loci Circle the current and the phase angle between the voltage and the current will be around 70 Degree, and POWER FACTOR is Cos70 and it will be 0.342."

Pl. refer Standard Elec.Engg. Text Book - CIRCLE DIAGRAM of the Induction Motor.

DHAYANANDHAN.S

Dear Mr. AkihitoShigeno,

You are right.

The P.F will be as you said 0.2 to 0.4, for the SIMPLE REASON, the LOCUS of the CURRENT of the Induction Motor WILL FOLLOW THE CIRCLE DIAGRAM, and at the extreme end of the Loci Circle the current and the phase angle between the voltage and the current will be around 70 Degree, and POWER FACTOR is Cos70 and it will be 0.342.

DHAYANANDHAN.S

What may be the PF of the transformer whose second winding was short curcuited ? (untill it is burned)