Construction of Saturable Core Frequency Meter
There is a rectangular frame of magnetic material in a saturable core frequency meter. In between the yokes of this rectangular core, there will be two legs. One is magnetic another is nonmagnetic. But the magnetic one saturates at a low value of emf. The cross-section of these two legs is the same.
The supply main feeds a winding wrapped around both legs. Another winding has two parts in a series. The nonmagnetic leg holds one of the parts. The magnetic leg holds the other. This two-part winding then feeds a bridge rectifier.
The entire arrangement behaves as a transformer. The first winding was the primary winding. The second winding behaves as the secondary winding. These both parts of the secondary winding are so wound and connected; the flux of these parts will oppose each other.
Working Principle of Saturable Core Frequency Meter
After saturation, the rate of increase of induced voltage in both parts of the secondary winding is the same. Therefore, they cancel each other. So, the secondary output voltage is no more the function of the primary voltage. Instead, it will be the function of frequency only.
Let us consider, φ is the flux difference between magnetic and nonmagnetic leg. Thus the resultant induced emf in the secondary winding will be
The RMS value of the resultant induced emf in the secondary winding will be
The average secondary voltage will be
Since 1.11 is the form factor of a sine wave.
Because φmax and N are the constant after the magnetic leg of the transformer becomes saturated.
Again, let us consider the secondary resistance of the saturable core frequency meter including resistance of rectifier is R. Therefore,
Then a PMMC instrument attached with the rectifier measures this average current (Iave).
This DC current (the output of the rectifier) is directly proportional to the average voltage which we have determined just. Again this average voltage is directly proportional to the frequency. So, we can say
So, the deflection of PMMC instrument depends on the frequency of the system.
Where, θ is the deflection angle of the pointer of the PMMC instrument.
Advantages of Saturable Core Frequency Meter
- A wide range of frequency measurement is possible.
- Since the deflection angle of the pointer of the PMMC instrument is directly proportional to the frequency, the scale of a saturable core frequency meter is linear.
Application of Satutable Core Frequency Meter
- It is mainly used in tachometer systems.