Variable speed motor theory help

[Contactor]

Hi :D

Would anyone care to educate me about wiring a variable speed motor?

Take a single phase 6 pole extract fan.

My question is:

Presumably speed control is on the frequency, therefore why the need for 6 poles?

Thanks:D

[frank]

2 pole, 4 pole and 6 pole motors will run at different speeds due to the formula ( fx60/pole pairs )-f, where f is the friction factor of the motor (usually around 4%)

With a fixed frequency (in the UK) and fixed poles the speed is determined as follows:

2 pole motors

50 x 60/1 - f = 2880 RPM

4 pole motors

50 x 60/2 -f = 1440 RPM

6 pole motors

50 x 60/3 -f = 960 RPM

Etc.

To control the speed of a motor where the voltage is fixed and the poles are fixed, the other option is to vary the frequency (frequency inverters) - normally expensive due to the electronics involved but precise control is available.

One other method used to control the speed of a motor is to use phase cut electronics (normally on the neutral). This is a cheaper way and is used on head pressure controllers quite a lot.

To answer your question, first define 'variable speed' Do you mean multiple windings?

Edit: Not all motors are suitable for speed control.

[Contactor]

I'm confusing poles in motors with poles in switches.

I'm not sure how it's wound, I understand it's variable speed from the supplier.

[Electrocoolman]

If you contact POLESTAR motors...their latest catalogue has a lot of info regarding what you are asking about.

As I understand the motors you are asking about are normally used on fan decks and have several speed settings selected by a multipole switch.

[goshen]

Hi :D

Would anyone care to educate me about wiring a variable speed motor?

Take a single phase 6 pole extract fan.

My question is:

Presumably speed control is on the frequency, therefore why the need for 6 poles?

Thanks:D
Hi:
if u are dealing with a 3 phase motor they make them with 6 leads so u can start it in many ways ,star delta and so on.
some motors come with only 3 leads and are internaly bridged .
single phase is more complicated but for resons of speed noise and so on will come with 3 ,4,5,6 speeds
and many wires.
if u want theory on vsd then try this:
http://www.joliettech.com/variable-frequency-drives_and_cooling-towers.htm
http://www.automationdirect.com.au/Welcome/drives/Danfoss.html
:D

[Contactor]

Thanks, all very helpful.

Goshen I like your comments about learning from mistakes.

Learning from other peoples mistakes is also a good idea, if you know who you can listen to. ;)

[hvaclover]

[quote=goshen;186356]Hi:
if u are dealing with a 3 phase motor they make them with 6 leads so u can start it in many ways ,star delta and so on.
some motors come with only 3 leads and are internaly bridged .
single phase is more complicated but for resons of speed noise and so on will come with 3 ,4,5,6 speeds
and many wires.
if u want theory on vsd then try this:

I am a residential heating tech by occupation as well as installer of the HVAC equipment I sell.

Variable speed or VS as applied to equipment I service consist of an "electronically commutated" motor.

AC power is converted to a half wave DC and software built into the motor head slows up down and speeds the motor blower wheel to meet the the needs of the conditioned space.i.e. when the thermostat calls for less humidity only, the VS motor will run at a slower speed to extract more moisture from the room air while the OD condenser run at constant speed.

[goshen]

hi i would be happy if u could elaborate on this motor control i
am not familiar with this type ,
i am wondering if u are talking about an integrated vsd with a motor like danfoss and abb sell .
control of a the motor is not done by dc ! it is done by converting ac to dc and via a igbt back to ac with a changing frequency?

[hvaclover]

hi i would be happy if u could elaborate on this motor control i
am not familiar with this type ,
i am wondering if u are talking about an integrated vsd with a motor like danfoss and abb sell .
control of a the motor is not done by dc ! it is done by converting ac to dc and via a igbt back to ac with a changing frequency?


I think yes on vsd (variable speed drive?).

The motor has the drive and a head that has software programed to the specs of the equipment MFG.

In most cases there is also an external board where the desired CFM can be programmed to VSD.

In my example the gas heating appliance is two heat and two stage cooling.

So there would be a different cfm requirement for low stage heat and and second stage heat.

Same for first stage cooling and second stage cooling or dehumidifacation.

[goshen]

I think yes on vsd (variable speed drive?).

The motor has the drive and a head that has software programed to the specs of the equipment MFG.

In most cases there is also an external board where the desired CFM can be programmed to VSD.

In my example the gas heating appliance is two heat and two stage cooling.

So there would be a different cfm requirement for low stage heat and and second stage heat.

Same for first stage cooling and second stage cooling or dehumidifacation.

Yep this sounds like an integrated vsd with preset speeds!!:)

[Pole Star]

If you contact POLESTAR motors...their latest catalogue has a lot of info regarding what you are asking about.

As I understand the motors you are asking about are normally used on fan decks and have several speed settings selected by a multipole switch.

Hi Electrocoolman, i am glad you found our latest catalogue useful.

Should you need any further assistance "contactor" please don't hesitate to ask, however looking at the responses given it would seem these guys know their stuff too!

Regards
Pole Star UK

[cadillackid]

the ECM motors that hvaclover speaks of are typically GE by regal beloit.. they are not fixed speeds.. the speed is controlled by an input signal.. the VSD is integral to the motor and the power supply in the USA will be a standard 115 volt or 220 volt AC supply found in the furnace...

the ultimate control is handled by the furnace board which varies the speed based on the temp / humidity requirements as hvaclover stated...

standard PSC motors can also be converted to variable speed.. single phase motors can be driven by Dual-TRIAC drives.. most PSC blower motors have an AUX winding on them.

a DUAL TRIAC drive will use both the AUX and the main windings set off-phase to drive the motor quietly and without the typical heat build up associated with under driving a PSC motor....

the reason for variable speed indoor motors in central residential systems is as hvaclover stated the need for service varies greatly across different weather and indoor conditions that just a 1 or 2 speed fan cannot accomodate correctly...

in the past most forced air systems had a 4 speed motor installed... multi-taps from the windings... you would typically wire in 1 speed for heat and 1 speed for cool and cap off the rest of the wires...

multi stage units made use of more speeds but still the spped was never varied based on any conditions other than heat / cool, stage 1 or 2...

I designed and built a retrofit variable speed control for my PSC blower 5 years ago and have loved every minute of it compared to the old days of fixed speeds
-Christopher

[goshen]

Hi :
you are 100% correct life just isnt the same once you have installed a vfd ,
not to mention the electrical savings.