power factor of mains operated extractor fan

Thread Starter

denison

Joined Oct 13, 2018
250
I have a 100 watt extractor fan with metal blades. The fan has built in capacitors. The measured power factor is 1. There must be some advantage of using the capacitors.
I have another fan without capacitors and the power factor is 0.72.
The only possible advantage I can see is that the motor can run faster when the voltage and current are in phase. So if I put in capacitors in the other fan would this speed up the fan? I would like it to run faster.
The reason it is called a power factor must be that the fan motor has more power to move more air and therefore run faster with capacitors.
My question then is if I put in capacitors in my 0.72 power factor fan will the motor run faster? Raising it to a power factor of say 1. If the answer is yes what size capacitor should I use?
I know that the watt reading is the same with or without capacitors. I also know that increasing the watts of the fan motor also increases its power.
 

MaxHeadRoom

Joined Jul 18, 2013
23,802
If induction motors they run at a RPM dependent on supply frequency.
Some AC fans are constructed using a shading ring, i.e. do not use capacitors and are not very efficient.
 

crutschow

Joined Mar 14, 2008
27,949
The reason it is called a power factor must be that the fan motor has more power to move more air and therefore run faster with capacitors.
No, that's not the reason it is called the power factor.
The power factor is how much of the volt-amperes the motor is consuming is generating power.
if I put in capacitors in my 0.72 power factor fan will the motor run faster?
No.
As MHR noted, the motor speed is primarily determined by the line frequency.
All that increasing the power factor does is reduce the current to the motor.
It has no effect on the motor speed or power.

The power companies like a high power factor since it reduces the reactive current, and thus their line resistive losses.
It has no particular advantage at your end, since you are only charged for real power, not the total volt-amps.
 

Thread Starter

denison

Joined Oct 13, 2018
250
No, that's not the reason it is called the power factor.
The power factor is how much of the volt-amperes the motor is consuming is generating power.
No.
As MHR noted, the motor speed is primarily determined by the line frequency.
All that increasing the power factor does is reduce the current to the motor.
It has no effect on the motor speed or power.

The power companies like a high power factor since it reduces the reactive current, and thus their line resistive losses.
It has no particular advantage at your end, since you are only charged for real power, not the total volt-amps.
Mr Crutschow, What you are saying here is contradictory. Without capacitors only 0.72 of the volt-amperes is generating power. With a power factor of 1, 100% of the volt-amperes is generating power. Therefore you are getting more power with the capacitors. But further on you say it has no effect on motor power.
I don't believe the makers of the motor with capacitors just want to keep the power company happy.
This is what I think 'power' means in this context. The motor with fan attached is experiencing drag because of the air resistance to the motion of the fan. If you have more power you have more ability to overcome this drag and therefore more motor speed. The motor without a fan attached would run much faster than a motor with a fan. I am sure these rpm figures would be different and are probably quoted by the manufacturer.
 

AlbertHall

Joined Jun 4, 2014
11,534
The motor without a fan attached would run much faster than a motor with a fan.
This is not true if it is an induction motor. It might run a very little faster as the slip will be lower. Have a look at the table of speeds in the link below and compare the synchronous and full load speed. Synchronous speed is the absolute maximum speed if there is no load on the motor, no friction, etc.
https://www.engineeringtoolbox.com/synchronous-full-load-speed-induction-motors-d_1448.html
 

MisterBill2

Joined Jan 23, 2018
9,526
Power factor capacitors adjust the relationship between voltage and current. Most motors that use capacitors use them to adjust the phase relationship between two windings so as to produce torque. The motor without capacitors is not designed to use capacitors, and adding them in any place will not result in more power or speed.
You might be able to improve the power factor of the motor without the caps by adding some capacitors in parallel with the motor, BUT it will not alter the motor performance. The motor speed will not change.
Some fans are more efficient than others and sometimes the difference is hard to see.
 

MaxHeadRoom

Joined Jul 18, 2013
23,802
. Most motors that use capacitors use them to adjust the phase relationship between two windings so as to produce torque.
Not so much to use torque, but to implement a phase shift WRT the main winding in order to start revolutions happening!
Otherwise the rotor remains stationary.
e.g. PSC motors as commonly used in many fans.
 

MisterBill2

Joined Jan 23, 2018
9,526
Not so much to use torque, but to implement a phase shift WRT the main winding in order to start revolutions happening!
Otherwise the rotor remains stationary.
e.g. PSC motors as commonly used in many fans.
It is the phase shift that produces the torque that produces the rotation. with no torque there will be no rotation. Thgat is how it works. Torque produces the rotation..
 

crutschow

Joined Mar 14, 2008
27,949
It is the phase shift that produces the torque that produces the rotation.
In a single-phase motor, the phase-shift generates a rotating field for the rotor (which a 3-phase motor generates naturally), which drags the rotor with it, generating a rotating torque.
Otherwise you have an oscillating field with no net rotating torque and no rotation.
 

MaxHeadRoom

Joined Jul 18, 2013
23,802
It is the phase shift that produces the torque that produces the rotation. with no torque there will be no rotation. Thgat is how it works. Torque produces the rotation..
Without the phase shift (start) winding the field oscillates across 180°, result, no rotation.
The capacitor provides the phase shift in the start/run winding to achieve this, in some motor the start winding is simply removed once the rotation begins.
The start winding is often operated at a very much lower current.
Motor technology 101!
 

MisterBill2

Joined Jan 23, 2018
9,526
Without the phase shift (start) winding the field oscillates across 180°, result, no rotation.
The capacitor provides the phase shift in the start/run winding to achieve this, in some motor the start winding is simply removed once the rotation begins.
The start winding is often operated at a very much lower current.
Motor technology 101!
the start winding, with a different resistance than the run winding, also has a phase shift. That is how, for many years, induction motors started, and how many of them still start. Certainly having a mechanical stsrting switch is less reliable, but it worked well enough for many years. Adding a capacitor is probably more efficient now, and possibly now that is more important than minimum cost.
 

MaxHeadRoom

Joined Jul 18, 2013
23,802
The phase shift using just the winding is not sufficient to provide the 90° aimed for.
In a PSC (1/4hp max) motor, the windings are identical, therefore the method of reversing can be achieved with just uses a SPDT switch.
1ph induction motors have used a capacitor plus start winding long before my first time working on, and re-wireing motors, quite some many decades now.
 

Thread Starter

denison

Joined Oct 13, 2018
250
Mr Crutschow, What you are saying here is contradictory. Without capacitors only 0.72 of the volt-amperes is generating power. With a power factor of 1, 100% of the volt-amperes is generating power. Therefore you are getting more power with the capacitors. But further on you say it has no effect on motor power.
I don't believe the makers of the motor with capacitors just want to keep the power company happy.
This is what I think 'power' means in this context. The motor with fan attached is experiencing drag because of the air resistance to the motion of the fan. If you have more power you have more ability to overcome this drag and therefore more motor speed. The motor without a fan attached would run much faster than a motor with a fan. I am sure these rpm figures would be different and are probably quoted by the manufacturer.
In a single-phase motor, the phase-shift generates a rotating field for the rotor (which a 3-phase motor generates naturally), which drags the rotor with it, generating a rotating torque.
Otherwise you have an oscillating field with no net rotating torque and no rotation.
The motor is an Australian made Vevor. How is this motor different from others I have which don't need a capacitor? You say this is a single phase motor but aren't they all? Can I get names for the fan motors that need capacitors for torque and rotation and those that don't? Would all the other types be shaded poll motors? The fan motor I have and measured a 0.72 power factor presumably wouldn't have a capacitor. What type of motor is this? It is a centrifugal fan and the maker is EBMPAPST. It is 20 watts at 50hz and 19w at 60hz at 230v.
I really didn't want to get into this subject so deeply. But I guess the answer to motors that use capacitors is that they need them to have rotation. While others that don't have capacitors don't need them for rotation like a shaded pole motor. It is interesting that I have yet to come across a 240v motor that doesn't respond to my speed control circuit. Even the Vevor responds well and I can get it down to very slow speeds. I know it will start stuttering if I take it too low.
 

MaxHeadRoom

Joined Jul 18, 2013
23,802
While others that don't have capacitors don't need them for rotation like a shaded pole motor.
A shaded pole motor also needs a phase shift, otherwise it suffers the same problem as its larger counterparts that use capacitors.
The shading pole or shorted turn causes a phase shift in that part of the magnetic field of the stator, effectively producing a 'split phase',
They are very poor, efficiency wise, due to the wasted energy in heat produced by the shorted turn..
 
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