Condenser Fans

[bernard]

Hi

What would be the advantages of fitting a fan speed controller instead of pressure switch controll.

Regards Bernard

[US Iceman]

Hi Bernard,

The use of variable speed regulation on condensers provides the following:
- Smooth & better control of the discharge pressure
- Reduced energy use by the condenser fan motors
- Reduced start-up strain on the fan motors
- Less flash gas generated in the liquid lines

Pressure switches used for condenser fan cycling can be difficult to set especially on condenser with many fans. The fans can turn and off too quickly if the differential pressure is not set high enough.

When a fan motor starts at full speed it has the highest amperage input and can increase the overall kW electrical demand. The VFD's can be started slowly and adjust to the operating conditions required quite well.

When a fan motor is operated with a VFD you gain some noticeable reductions in energy use. For example: If you have a 10kW fan motor; at full rated speed it consumes the highest energy.

If the fan speed is reduced by one-half, the required power input decreases according to this simple formula:

(((RPM2 / RPM1 ) ^3) X kW) = kW2

Using a 60 Hertz power supply this translates into:

30 Hz / 60 Hz = 0.5

0.5^3 = 0.125 (Note: the 0.5 is being raised by an exponent of 3, or the cube, which is a fancy way of saying you are multiplying 0.5 X 0.5 X 0.5 = 0.125)

0.125 X 10 kW = 1.25 kW

Therefore at half speed (30 Hz), the input power is now reduced 87.5% to only 1.25 kW. A considerable decrease. Most systems will operate between 30-50 Hz (on a 60 Hz power supply) as an average range.

Multiple fan motors starting with full-voltage start (across-the-line) require higher in-rush current and this can contribute to early motor failure if the start/stops are too frequent.

When the condenser fans start and stop you can see flash gas (vapor bubbles in the liquid line sight glass). This is a notorious event where fan cycling control is used.

Does that help you?

[NoNickName]

These days, frequency controllers are less and less common than chopper fan speed controllers.
Choppers do really "chop" waves reducing the average voltage supplied to the motor. They are much cheaper than frequency inverters, but put more strain on the motors.

[US Iceman]

Bernard,

I forgot to add some other comments about the VFD's.

The motors should be provided for what is called "inverter duty". An inverter duty motor is built specifically for use with VFD's. Special winding insulation and other to protect the motor from high voltage spikes called corona discharge. These will fail a motor quickly if it happens.

You may also need what are called "line chokes, line reactors, or filters". VFD's can generate harmonics which are a pain in the butt to fix. All sort of strange things, noise in the wiring, interference on solid state devices, etc.

If you use VFD's please advise the VFD supplier of all installation requirements before you use VFD's. He should be able to help you immensely.

You will also have some minimum speed/Hertz that the motors can run at. The motors have internal fans that provide cooling for the windings. If the motor RPM is too low, the motors can overheat.

If the motors are placed inside of a 0C freezer, the problem is no longer a factor since the cold air will provide good cooling.

[bernard]

Hi

Thanks for the valueable information.

I have worked on some little NRC packs, 2 x scroll

compressor and two condenser fans.The fans are

controlled via a small FSC on top of the reciever.I

had two go faulty,I changed one like for like and the

other I fitted an hp switch.


Regards Bernard:)

[NoNickName]

The motors should be provided for what is called "inverter duty".

Also called "class F motors".