Simple circuit detects sequence errors and failure in three-phase power supplies
All-phases presence and correct phase sequencing are both critical conditions for proper three-phase line-powered equipment operation.
Three-phase line-powered equipment is sensitive to phase sequence. For example, if two of the power connections are inadvertently interchanged, a motor will rotate in the reverse direction, which may damage connected equipment such as a pump or the compressor of an air conditioner.
Similarly, if one phase is missing due to, say, a cut wire or a blown fuse while the motor is running, it will overheat and may be damaged in the process. This situation is called single phasing and occurs due to phase failure. Hence monitoring both phase sequence and phase failure are important in three-phase systems. However, inexpensive motor starters such as the Direct on Line (DOL) approach may not offer such protections.
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The circuit shown in Figure 1 monitors for both incorrect phase sequence and phase failure. Relay RL1 is energized only if the phase sequence is correct and there is no phase failure. This relay contact can be connected to the starter’s “stop” button (as described in greater detail further in this Design Idea). Any time a fault occurs, it will then switch off the motor.
Figure 1 Use a common transformer to generate a 5 V DC supply voltage for all elements of this phase sequence-and-failure circuit.
The circuit uses a star-connected three-phase transformer. Individual phase voltages are rectified by D1, D2 and D3, and the resultant waveforms are applied to transistors Q1, Q2 and Q3. The waveforms at transistor collectors are square waves, and are further shaped by U1 Schmidt trigger inverters.
How does the phase sequence detector work? U1C sets flip-flop U5A when the R phase comes first. When the Y phase then arrives, it resets U5A through U1A. Its Q output is connected to the D input of flip-flop U5B. The B phase, which comes last, loads the D input to its Q output. Hence, if the phase sequence is correct (R-Y-B in this particular case), the Q output of U5B will be LOW. Conversely, if the phase sequence is reversed to R-B-Y, for example, U5B’s Q output will go HIGH.
And how does the phase loss detector work? The U1A, U1B and U1C outputs are connected to OR gate U2A. If all phases (R, Y and B) are present, the OR gate output will be HIGH. This output is connected to retriggerable monostable U3A. Since the input is just a DC voltage, the U3A output will remain LOW. However, if any phase is missing, the output of the OR gate will instead be a pulse train. This pattern repeatedly triggers U3A, and its output is therefore held HIGH. With the RC values shown, for example, the monostable pulse width is approximately 12 msec.
If the outputs of both sections of the circuit are LOW, this condition translates to both correct phase sequence and no phase failure. The U2B output becomes LOW, Q1 saturates and Relay RL1 gets energized. Its contact can be connected in series with the OFF button of a motor starter, so that the motor trips when this relay de-energizes, thereby sensing a fault.
This simple and low-cost circuit can be added to a DOL or other feature-deficient starter for motor protection purposes. Power supply and ground pins are not shown in Figure 1. These must be connected to VDD and VSS (ground). The same three-phase transformer can be used to generate the 5 V supply voltage needed for all portions of this circuit using a LM7805 voltage regulator. The three-phase power input is connected to terminal block (TB) J1, with the relay outputs connected to TB J2.
This circuit can be implemented as-is in countries such as India, which uses a 230 V, 50 Hz line power supply. The transformer rating may be changed to 110 V/5 V and 60 Hz to use the circuit in other countries such as the United States, while keeping the other components the same.
Jayapal Ramalingam has over three decades of experience in designing electronics systems for power & process industries and is presently a freelance automation consultant.
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