Measurement of RMS, Average and true RMS values for different firing angles using HCB.

AIM:

            To take measurements of V_RMS,V_DC ,V_MEAN  on single phase half controlled bride rectifier for resistive load.

APPARATUS:  1)  Circuit board.

                           2)   True RMS meter(GWINSTEK GDM396).

   3)   DMM (MASTECH M3900).

                           4)   Powerscope (SCIENTIFIC 30MHz SM901).

THEORY:

The ‘Root Mean Square’ magnitude of an alternating current is the value of equivalent direct current that would produce the same amount of heat in a fixed resistive load.  The amount of heat produced in a resistor by an alternating current is proportional to the square of the current averaged over a full cycle of the waveform.  In other words, the heat produced is proportional to the mean of the square, so the current value is proportional to the root of the mean of the square or RMS.  (The polarity is irrelevant since the square is always positive.)

Block diagram description:

RMS value of the sinusoidal waveform is meas­ured by the average reading voltmeter of which scale is calibrated in terms of rms value. This method is quite simple and less expensive. But sometimes rms value of the non-sinusoidal waveform is required to be measured. For such a measurement a true rms reading voltmeter is required. True rms reading voltmeter gives a me­ter indication by sens­ing heating power of waveform which is proportional to the square of the rms value of the voltage.

Thermo-couple is used to measure the heating power of the input waveform of which heater is sup­plied by the amplified version of the input waveform. Output voltage of the thermo­couple is proportional to the square of the rms value of the input waveform. One more thermo-couple, called the balancing thermo-couple, is used in the same thermal environment in order to overcome the difficulty arising out of non-linear behaviour of the thermo-couple. Non-linearity of the input circuit thermo-couple is cancelled by the similar non-linear effects of the balancing thermo-couple. These thermo-couples form part of a bridge in the input circuit of a dc amplifier, as shown in block diagram.

AC waveform to be measured is applied to the heating element of the main thermo­couple through an ac amplifier. Under absence of any input waveform, output of both thermo-couples are equal so error signal, which is input to dc amplifier, is zero and therefore indicating meter connected to the output of dc amplifier reads zero. But on the application of input waveform, output of main thermo-couple upsets the balance and an error signal is produced, which gets amplified by the dc amplifier and fedback to the heating element of the balancing thermo-couple. This feedback current reduces the value of error signal and ultimately makes it zero to obtain the balanced bridge condition. In this balanced condition, feedback current supplied by the dc amplifier to the heating element of the balance thermo-couple is equal to the ac current flowing in the heating element of main thermo-couple. Hence this direct current is directly proportional to the rms value of the input ac voltage and is indicated by the meter connected in the output of the dc amplifier. The PMMC   meter may be calibrated to read the rms voltage directly.

By this method, rms value of any voltage waveform can be measured provided that the peak excursions of the waveform do not exceed the dynamic range of the ac amplifier.

 

True RMS Reading Voltmeter

PROCEDURE:

1. Make connections for half wave controlled rectifier as follows:

a.       Connect load

b.      Connect Powerscope and DMM or True rms meter across load.

c.       Connect variac.

d.      Complete the gate connection.

2. Observe the waveforms on powerscope for different firing angles.
3. Note down Vdc and Vac on DMM.
4. Note down Vtrms on true rms  meter.

OBSERVATION TABLE:

Sr.. No	Firing Angle	DMM reading	True RMS Meter Reading
	α	Practical	Practical
1.
2.
3.

Conclusion: