Friday, March 06, 2015

Variable Frequency Drive and the Affinity Law

Affinity laws are applied in centrifugal loads such as pump, compressor, fan, hydraulic system and the like. Affinity law comes in many form as a manifestation of interactions between power, flow rate, shaft speed, frequency and pressure head in centrifugal loads.
Among the formulas within the Affinity Law is simple equation that states that the electrical power of any centrifugal load is directly proportional the shaft speed of the motor that drives it. 

Affinity Law Formula; P= Power, f= frequecny

In electrical engineering we know from the formula of an induction motor that synchronous speed is also directly proportional  to the frequency of the supply line.

Formula for synchronous speed
Therefore from the given equations we can directly say that an 80% reduction of speed could result into 50% reduction of power consumption. Since speed is directly proportional to frequency (while taking the number of poles of the motor remains constant) we can say that any changes in the frequency has a corresponding linear changes in the synchronous speed of the motor. For example a 4-pole motor is supplied with 440 volts AC, 60 Hz the synchronous speed for that is N= 120f / P is 1800 rpm (revolution per minute).

Variable Frequency Drive is a device that regulates supply frequency of the main source to a desired level before it is connected to the terminals of the motor. If we take the above example that a 4-pole motor has a synchronous speed of 1800 rpm at 60 Hz, we can reduced its synchronous speed into 900 rpm if we reduce the frequency to 30 Hz. And the device that can reduce that frequency is the Variable Frequency Drive. The reduction of the frequency can be attained by operating it manually or automatically.

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 if the full speed of the motor is not needed.

Typical Set-up of VFD Close Loop System

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 case the Resistance Temperature Detector (RTD) (a kind of temperature sensor) is added to the system and its function is to detect 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 is just one of the several examples wherein the VFD is installed to attain energy saving. In many industrial applications such as in compressors, air blast freezer and pumps application of VFD is necessarily important as far as savings is concern.

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.


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