Electrical motor

[Sandro Baptista]

A label plate of a motor 1~230V 50 Hz with electrical condenser (to "create" a second phase). The label plate says:

230V
0,55A
120 W (that should be the nominal shaft power)

230V x 0,55A = 127 W

How is possible to achieve 120 W? It would mean almost 100% of electric efficiency and 100% cos phi. An for this size of motor it should be much less (maybe about 60%)

Can the reason be on the "second" phase and so the calculation needs be multiplied by a coefficient?

[taz24]

A label plate of a motor 1~230V 50 Hz with electrical condenser (to "create" a second phase). The label plate says:

230V
0,55A
120 W (that should be the nominal shaft power)

230V x 0,55A = 127 W

How is possible to achieve 120 W? It would mean almost 100% of electric efficiency and 100% cos phi. An for this size of motor it should be much less (maybe about 60%)

Can the reason be on the "second" phase and so the calculation needs be multiplied by a coefficient?



I don't know if this explains what you need but I copied it from an old document I have.


Single-phase induction motors
The single coil of a single phase induction motor does not produce a rotating magnetic field, but a pulsating field reaching maximum intensity at 0o and 180o electrical.

Single phase stator produces a non-rotating, pulsating magnetic field.
Another view is that the single coil excited by a single phase current produces two counter rotating magnetic field phases, coinciding twice per revolution at 0o and 180o . When the phases rotate to 90o and -90o they cancel. At 45o and -45o they are partially additive along the +x axis and cancel along the y axis. An analogous situation exists. The sum of these two phases is a phase stationary in space, but alternating polarity in time. Thus, no starting torque is developed.

However, if the rotor is rotated forward at a bit less than the synchronous speed, It will develop maximum torque at 10% slip with respect to the forward rotating phase. Less torque will be developed above or below 10% slip. The rotor will see 200% - 10% slip with respect to the counter rotating magnetic field phase. Little torque (see torque vs slip curve) other than a double frequency ripple is developed from the counter rotating phase. Thus, the single phase coil will develop torque, once the rotor is started. If the rotor is started in the reverse direction, it will develop a similar large torque as it nears the speed of the backward rotating phase.
Single phase induction motors have a copper or aluminium squirrel cage embedded in a cylinder of steel laminations, typical of poly-phase induction motors.



taz

[Sandro Baptista]

Thanks for your info. However still persist the doubt.

I will take this opportunity to study some books I could get about this subject.

[Peter_1]

I think they should better state 120 VA instead of 120W.
Or, adding a cap improves cosphi a lot.

But the capacitor isn't there to create a second phase but to create a starting torque in the right direction.

[Sandro Baptista]

I think they should better state 120 VA instead of 120W.

that would be logic. And if it is that so there is an error on the plate data from the manufacturer. But I can advance you that on the data sheets of the evaporators this also appears from a lot of years ago.

[chilliwilly]

120w/127va=0.95 cos pf. 95% efficient. You won't get it any better than that without introducing other losses.

[Sandro Baptista]

On a motor plate 120W it should be the nominal mechanical power. So you also should count with the electric efficiency...that's why this label plate don't make any sense. Even for a great motor you would could achieve this results of efficiency and cos phi