What is affinity law?
Affinity laws are applied in centrifugal loads. Centrifugal loads such as pump, compressor, fan, hydraulic system and the like. This is a well settled scientific law that was discovered long time ago but not widely used until advent of
advance motor control devices. Affinity law comes in many form as a manifestation of interactions between power, flow rate, shaft speed, frequency and pressure head in centrifugal loads.
Affinity laws are applied in centrifugal loads. Centrifugal loads such as pump, compressor, fan, hydraulic system and the like. This is a well settled scientific law that was discovered long time ago but not widely used until advent of
advance motor control devices. Affinity law comes in many form as a manifestation of interactions between power, flow rate, shaft speed, frequency and pressure head in centrifugal loads.
There are several formula that belongs to this law that shows the interaction of the foregoing parameters. But one simple form states that the electrical power of any centrifugal load is directly proportional the shaft speed of the motor that drives it.
We know from the formula of an induction motor that synchronous speed is also directly proportional to the frequency of the supply line.
From the figures given we can directly say that an 80% reduction of speed could result into 50% reduction of power consumption.
Application in the industry
Without so much dissecting it's theoretical background lets get straight on how can we be benefited on the application of the same. This can benefit us by using the VFD or the Variable Frequency Drive. VFD is a motor control device mainly used for speed control eliminating the use of gear box, belt, damper, and throttle should the full speed of the motor is not needed. However because of this affinity law the logical consequence of energy savings transformed itself to be the main factor that affects why engineers used VFD.
 | ||
| example of VFD system schematic diagram for a grain drying plant |
 |
| grain dyring bin |
The process are as follows:
1. A blower is inducing hot air to the bin in order to dry the grain products. This motor is designed for the maximum capacity of the drying bin.
2. But sometimes the production volume cannot reach the maximum capacity or cannot even the reach the half of its capacity. In this case careful operator attention shall be done so as to prevent the product from being over dried or burning.
3. To overcome this problem they installed valve at the side of the bin that would evacuate the excess hot air whenever necessary. This excess hot air manifests a waste of energy.
4. The solution is the installation of VFD. (see above schematic diagram)
5. In this situation the Resistance Temperature Detector (RTD) which is a kind of temperature sensor detects temperature level and send the detected value to the temperature controller as an electrical signal.
6. The temperature controller now evaluates the received information and transmit analogue signal to the VFD (0-10 volts or 4-20 mA). The amount of current or voltage to be sent by the controller depends on the set value, set by the engineers or the operator.
7. The analogue signal will then be received by the VFD and serves as the determining factor on how much speed will the motor run.
In this method energy is saved!
The scenario that is mentioned above is just one of the several examples wherein the VFD is installed to have energy savings in industrial applications such as in compressors, air blast freezer and pumps.
Guides in VFD selection
In selecting VFD's you have to properly consult the manufacturer or its authorized representative on what type of VFD is suited for your requirements. From different manufacturers there are different types and models of VFD. We will not choose VFD by considering the required power and voltage only but we have to consider also the environment that the VFD is to be installed whether it is exposed to dust, damp areas, humidity and other factors that if overlooked could affect the performance of the unit.
-end-
