DETECTION OF PHASE REVERSAL AND SINGLE PHASING OF AC POWER SUPPLY

WITH D FLIP FLOPS

Problem: Design a circuit that will trip the power supply in case of phase reversal or single phasing.

Key words: Three phase AC power supply, phase reversal, single phasing, D Flip Flop

Solution:

Step1: The three phase AC supply is converted into square wave suitable for ICS (say TTL logic).

Step2: Three inputs are connected to D flip flops (7474), Fig. 1a.

Step 3: The S (PR) terminals of FFs are connected to Vcc.

Step 4: Output of Q1 is connected R (CLR) of D2 FF, Q2 to D3 and Q2 to D1.

a) The input sequence is RYB (Fig. 1a).

If the sequence is RYB, at the positive edge of the clock, D inputs of all FFs are HIGH, i.e., the

output of all FFs will be HIGH. Output will be HIGH.

R

Y

Push Button

Vcc

S

S

S

Y

B

B

R

R

R

R

Q1

Q2

Q3 D3

Fig. 1a

Three Phase Square Wave Input

R

R

B

B

Y

Y

D1

D2

Output Output

b) The input sequence is changed to RBY from RYB, Fig. 1b. (It will cause three-phase motors to run

in reverse direction.)

Now, at the positive edge of the clock, D inputs of all FFs are LOW, i.e., the output of all FFs will

become LOW. LOW at Output can be used to trip the circuit/sound an alarm.

System can be reset by pressing the Push Button.

R

Y

Push Button

Vcc

S

S

S

Y

B

B

R

R

R

R

Q1

Q2

Q3 D3

D22

Fig. 1b

Phase Reversal (Y – B interchanged)

Three Phase Square Wave Input

R

R

Y

B

B

Y

D1

Output

c) Single phasing (say phase - B is disconnected), Fig. 1c. This condition is also dangerous for three-

phase systems.

Now, at the positive edge of the clock, D3 input of FF #3 is LOW, i.e., Q3 will go LOW. It will reset

Q1 to LOW, and LOW at Q1 will reset Q2 as well. Output of all FFs will go LOW. LOW at Output can

be used to trip the circuit/sound an alarm.

System can be reset by pressing the Push Button.

Fig. 1c

Single Phasing

(B – Phase disconnected)

R

Y

Push Button

Vcc

S

S

S

Y

B

B

R

R

R

R

Q1

Q2

Q3 D3

D2

R

R

B is disconnected

B is disconnected

Y

Y

D1

Output

Note: The inputs to D FFs should follow RYB sequence of AC supply, and should become LOW if

R/Y/B is disconnected so that inputs to Ds of FFs don’t remain unconnected in case of fault in the AC

supply.

Annexure: Function Table of 7474 is given in Table I

.

Table I