Affinity Laws

The more poles a motor has the slower it turns but the more torque it has, roughly speaking...

power = RPM * torque

http://www.engineeringtoolbox.com/electrical-motors-torques-d_651.html

By going to a 2 pole motor you would double the rpm, so you could add a 2 to 1 pulley to make it work....

Sorry, I meant to say it was for a fan.

Is it direct drive or are there pulleys involved?

Direct Drive,

Well obviously you can't double the speed of the fan....unless by chance it's an inverter duty motor and you have a properly sized VFD laying around....I don't see this working...

The power needed for a fan is proportional to around the cube of the tip speed. So doubling the speed by fitting the wrong 2-pole motor instead of the right 4-pole one requires an increase in power by a factor of around 8, with attendant issues regarding the incoming supply, cables, starters and overloads; the concept is nonsensical.

Dear Mr.eltech,

You have mentioned that you want to use 2 poles motor instead of 4 poles and it is direct drive.

The Affinity Law states that

Capacity is Proportional to the Speed.

Pressure or Head is Proportional to the Square of the Speed.

Power is Proportional to the Cube of the Speed.

Accordingly the fan capacity will be double, the pressure (or draught) will be 4 times, and the Power Demand will be 8 times.

This type of subject was discussed earlier few times

DHAYANANDHAN.S

If you exclude the differences in efficiency between the old fan (turning at the original rpm) and the new fan (turning at twice the original rpm) and look only at moving the same mass flow rate of air under the same conditions against the same pressure; the new power requirement will be the same as the old.

....but you understand there are other factors involved

~44kW; worry also about centrifugal force and possible supersonic blade tip speed.

The way the affinity law for speed of a centrifugal machines works is the the power REQUIRED by the machine (pump or fan) changes at the cube of the speed change. So if your fan was designed to work with a 55kW 4 pole motor and you use a 2 pole motor, the speed will double, so the fan will demand 800% of the power it used at the original design speed. So you would need a 440kW motor.

if you use a VFD to return to the original speed, then you will have insufficient torque with that motor. With the VFD, you are reducing the power rating of the motor. A 2 pole motor already has 1/2 of the torque of a 4 pole motor of the same power rating, then when you reduce the speed you are reducing the power rating at the same rate. So at 1/2 speed it has 1/2 of the power capability; 27.5 kW at what was the original motor speed using a 4 pole, which means the 2 pole motor still doesn't work. You would need to use a 110kW 2 pole motor and run it at half speed

Bottom line, you can't do this.

Beat me to it, GA!

Minor typo: his motor is 5.5kW rather than 55.

Yeah, when I use my iPhone, I can't see the previous post so I was going from memory.

But hey, whats a decimal point among friends?

Why change to a 2-pole motor?

As others have said you will double the speed. So if direct drive you need a smaller diameter fan or change the pitch of the blades to give the original air flow.

If you just change the motor and nothing else it is likely to run on overload and get very hot if it doesn't trip out on current to start with when it gets up to speed, but on the plus side the extra cooling of the high speed fan will reduce the motor temperature to compensate - and introduces some interesting factors into the equation. Regardless of the effect of fan speed on the evaporator process.