Oscilloscope AC voltage measurements

One importent function of the oscilloscope is to make voltage measurements. The scope os capable of measuring both DC and AC voltages.

Voltage is determinated by using vertical grids on the display.each square on the grid is called a divison.

Just as a voltmeter has a range switch , so does an oscilloscope . this switch selects volts per divison for example , at the 2 VOLTS/DIV position , one vertical divison repersents tow volts.

To make an AC  voltage measurement, the coupling switch can be placed either the AC or DC position.

To make an ac measurement.

Step 1: click on the coupiling switch to move it to the GND position.

Step 2: move the horizental center line on the grid by adjusting the vertical position knob.

Step 3: connect the oscilloscope probe to a circuit that has 40 peak to peak AC,

·         The ground is connected to a reference point of the circuit.

·         The probe is connected to a location that has a diffrent potential than the reference point.

·         Set the  probe 10*/1* switch to the 1* position.

Step 4: click repeatedly on the VOLTS/DIV knob to set it the 10 VOLTS/DIV position.

·         Click on the coupiling switch to move it from the GND to the AC position

The voltage measures 40 volts peak to peak because the vertical trace moves 4 divisitions with the range switch at 10 VOLTA/DIV. The sine wave appears with the positive alternation above the center line and the negitive alternation below the center line.

·         Click on the coupiling switch twice to move it from the AC to the DC position.

Get it a fullscale reading. Deascreses the VOLTS/DIV setting to the 5 volts position.

Click on the VOLTS/DIV knob below to change the setting

The scope still measures 40 volts peak to peak , but at this VOLTS/DIV setting. The trace moves 8 seconds.

·         The most accurate measurement is a achived when the VOLTS/DIV knob is reduced to lower positions untill as much of the screen is used as possible.

It is usually recommended to have the coupiling switch in the DC position because it can measure when the AC signal is riding on a DC voltage . for example , if the 40 volts peak to peak

Sawtooth voltage is combined with a positive 20 volts signal with the VOLTS/DIV switch in the 10 volt position, it would apper on the screen ,

When the coupiling switch is in the AC position ,it is only capable of measuring the AC sawtooth signal . ti is not capable of measuring the dc voltage . observe the signal when the coupiling switch is moved from DC to AC .

The largest voltage the oscilloscope in this learning object can measure is 400 volts.

·         To make a higher reading, a times 10probe . labled 109, s available.

·         It alternates(reduces) the incoming voltage by a factor of 10.

·         By using the 10* probe on this sample, you can measure 400 volts. Some scopes have a VOLTS/DIV bracket at which the numbers on the knob are alinged when using this probe.

·        

This display shows an 8 volt peak to peak sine wave when the probe switch is possible in the *1 position . by moving the switch to the *10 position ,the waveform is reduced by 1/10 its size.

Power factor

A large portion of electrical power generated by utility plants is supplied to factories . much of the equipment in an industrial plantconsists of motors and transformars.

A large portion of electrical power generated by utility plants is supplied to factories . much of the equipment in an industrial plant consists of motors and transformers.

 Some of the current from the feeder line provides true power , which is consumed by electrical energy being converted to heat energy and mechanical energy. True power performs work , and its consumption is from;

A large  portion of electrical power generated by utiluty plants is supplied to factories . much of the equipment in an industrial plant consist of motors and transformers.

Some of the current from the feederline provides true power , which is consumed by eectrical energy being converted to heat energy and mechanical energy. True power performs work ,and its consumption is form:

·         The mechanical energy of the motor required to physically turn the load.

·         The mechanical energy required to overcome the bearing friction and windage (actual air resistance to the rotor turning.)

·         Heat dissipation from the resistance in the wire, eddy currents and hysterisis loses in the  iron cores

The remainder of the current from the feeder line is used to produce magnatic fields that explained around the coiles. When these fields collaps during each AC a;ternation, they induce a voltage back into the coils , which causes current to be returned back through the feeder line to the generator , this type of power , called reactive power , called does not perform any work

The combination of true power and reactive power sent to the factory is called appearnt power . because moter coils are inductive . the reactive power and the true power that pass through the feeder line are not on phase.

Therefore, apperent power is the phasor sum true and reactive power.

The ratio of true power to apperent power is reffered to as the power factor , abbrevaiated (pf) . it is expressed by the following formula

The power factor is an indication of the efficency of a circuit , which refers to how much energy supplied by the source performs work, an efficent circuit is one that uses most of the energy from the feeder line to perform work and consumes a high amount of true power . when all of the energy form the source is used , the power factor is equal to 1.

An efficent circuit is one that uses very little energy to performs work an example is when the motors are driving light loads that don’t require much physicalpower to turn . in this situation , more of the line current is used to produce the reactive power , which creates the magnatic field around the coils , then the true power that performs work . if allof the current formthe sorce is used to create maganatic fields and none of it performs work , the power factor is equal to 0 .

A common power factor for a factory is in the range of 0.6