A three-phase sinusoidal power system consists of three AC voltages displaced by 120 electrical degrees from each other. A balanced three-phase supply is essential for the efficient operation of three-phase electrical machines.
If two of the phases get disconnected, than machine will not work. As there no potential difference for the flow of working current. But What if; only one phase get disconnected ? That situation is known as “Single phasing“.
What Is Single Phasing?
Single phasing is a effect of fault in which one of the three supply phases becomes disconnected due to a “blown fuse“, “broken conductor“, “loose connection”, or other fault. As a result, only two phases remain connected to the load.
A three-phase Induction motor requires three balanced phases to produce a constant rotating magnetic field (RMF) in the stator. This rotating magnetic field induces current in the rotor, creating another magnetic field. The interaction between the stator and rotor magnetic fields produces torque, causing the rotor to rotate.
During single phasing, the balanced three-phase supply get lost. The stator no longer produces a perfectly rotating magnetic field. Instead, the magnetic field becomes unbalanced and can be considered as a combination of forward and backward rotating magnetic fields (rather than a pure rotating field). Consequently, the motor develops much lower starting torque and reduced running torque.
- If the motor is already running, it may continue rotating on the remaining two phases, but its torque decreases significantly. To maintain the mechanical load, the motor draws excessive current from the remaining phases, causing rapid overheating and possible winding failure.
- If the motor is stationary, it cannot develop sufficient starting torque and therefore fails to start. It simply draws a very high current, which can quickly damage the stator windings if protection does not operate.
Since the rotor speed decreases during single phasing, the back EMF is reduced, which contributes to the increase in stator current. This excessive current produces excessive heat, leading to insulation breakdown and eventual motor burnout if the fault is not cleared promptly.
An image of terminal box of 4 KW induction motor is shown above. One loose connection can create a single phasing situation.
How Does Single Phasing Occur?
Single phase directly means unavailability of one of the phase out of 3 phases. There are multiple reasons such as fuse blown, Broken conductor etc. Some of the reasons of single phasing are discussed in a table below :
| SNO. | REASONS OF SINGLE PHASING | REMARKS |
| 1 | Fuse blown | blown either due to low rating or due to overload or short circuit. |
| 2 | Broken conductor | Either due to overload, short circuit or physically damaged |
| 3 | Loose terminal | Due to unskilled workforce |
| 4 | Faulty MCCB/MCB | Old MCB, damaged due to overload or short circuit |
| 5 | Contactor problem | Old contactor, low rating contactor, damaged due to overload or short circuit |
| 6 | Faulty energy source | It can be transformer or alternator |
| 7 | Short circuit | Heavy current during short circuit can damage the one of the phase. either its conductor or either a fuse |
| 8 | Wrong connections | one of the terminal is connected to wrong side |
| 9. | Overload | Overload is gradual increase of current more than the rated circuit. Over the time, It damages the circuit. |
Images related to causes of single phasing
What Happens During Single Phasing?
- Torque decreases
- Speed drops
- Motor continues running (if already running)
- Motor runs very slowly or never runs only producing the humming noise (if not running)
- Current increases in remaining phases
- Vibration and noise increases
- Motors temperature increases rapidly
- Motor overheats
- Insulation deteriorates
- Motor eventually burns
- Long exposure to single phasing can lead to permanent damage to the motor
Signs of Single Phasing
- Motor humming
- Low speed
- Overheating
- Burning smell
- Excessive vibration
- Overload relay tripping
- Unequal current
- Voltage imbalance
- no current in one of the phase
How to Detect Single Phasing
- Clamp meter
- Multimeter
- Phase sequence meter
- Voltage measurement
- Current comparison
- Protection relay indication
Protection Against Single Phasing
There are multiple ways to protect the motor from single phasing. All these are discussed in a table below :
| SNO. | PROTECTION AGAINST SINGLE PHASING | REMARKS |
| 1. | Power monitoring relay | Monitors the voltage continuously. In case of phase failure it produces an electrical change. Such as GIC make SM175 relay |
| 2. | Thermal overload relay | Connects in between the circuit. Current passes through it in a series manner. Such as RTO relay by LK |
| 3. | MPCB | Motor protection circuit breaker. A combination of overload relay and MCB. Connects with contactor for motor protection. |
| 4. | Proper distribution | Total distribution should be done Calculatively from source to load. Which includes the source such a transformer, cable for distribution and hierarchy of circuit breakers |
| 5. | Scheduled monitoring | Schedule monitors consists of arranging the data in a record. Such as units consumptions. KW, ampere and voltage readings. This shows the current running trend of the system. |
| 6. | Skilled workforce | Skilled workforce play very important part. They are the one who are working at ground floor. Who are going to tightened the terminals. Who are going to lay the distribution |
| 7. | Variable frequency drives | Since these drive on their own generates the power source. In single phasing scenario, These VFD produces alarm and do not allow motor to get run. |
Images related to the protections from single phasing
Advantages of Single Phasing Protection
- Prevents motor burnouts
- Improves equipment life
- Reduces downtime
- Saves maintenance cost
- Enhances safety
- Improves system reliability
Frequently Asked Questions (FAQs)
What is single phasing?
Single phasing is a fault condition in a three-phase electrical system where one of the three supply phases is lost due to a blown fuse, broken conductor, loose connection, or faulty switchgear. Although the remaining two phases may still supply voltage, a three-phase motor cannot operate normally and may overheat or become damaged if the fault is not cleared quickly.
Can a motor run on single phasing?
Yes, but only under certain conditions. If a three-phase induction motor is already running when single phasing occurs, it may continue rotating on the remaining two phases. However, the motor draws excessive current from the healthy phases, develops lower torque, overheats rapidly, and can suffer severe winding damage. A motor cannot normally start under single-phasing conditions.
Why is single phasing dangerous?
Single phasing is dangerous because it causes unbalanced current, excessive heating, reduced torque, increased vibration, and poor motor performance. If protective devices fail to disconnect the motor promptly, the insulation may deteriorate, resulting in winding burnout and costly downtime.
What causes single phasing?
Common causes of single phasing include :
- Blown fuse in one phase
- Broken or damaged power cable
- Loose or burnt terminal connections
- Faulty contactor contacts
- Failure of circuit breakers or switchgear
- Open conductor in the distribution system
- Utility supply faults
Regular inspection and preventive maintenance help reduce the chances of single phasing.
How can single phasing be detected?
Single phasing can be detected by checking the three-phase supply voltage with a multimeter, measuring phase currents with a clamp meter, or using a phase failure relay. Warning signs include reduced motor speed, unusual humming noise, overheating, excessive current in the remaining phases, and abnormal vibration.
Which relay protects against single phasing?
A Phase Failure Relay (Phase Loss Relay) is the most common protective device used against single phasing. It continuously monitors all three supply phases and disconnects the motor when a phase is lost. Many modern motor protection relays also provide protection against phase loss, phase sequence reversal, phase imbalance, overvoltage, and undervoltage.
Can single phasing damage a motor instantly?
Not always. The extent of damage depends on the motor load, operating time, and the effectiveness of the protection system. A lightly loaded motor may continue running for some time, whereas a heavily loaded motor can overheat within a few minutes and experience insulation failure or winding burnout if it is not disconnected.
What is the difference between phase failure and single phasing?
Phase failure is the loss of one phase in a three-phase electrical supply. Single phasing is the operating condition that results when a three-phase motor continues to receive only the remaining two phases after a phase failure. In practice, these terms are often used interchangeably, but phase failure refers to the electrical fault, while single phasing describes its effect on the equipment.
Does single phasing affect all three-phase motors?
Yes. Any three-phase motor can be affected by single phasing. However, the severity depends on factors such as motor size, load, duty cycle, and the type of protection installed. Motors operating under heavy loads are generally more vulnerable to overheating and winding damage.
How can single phasing be prevented?
Single phasing can be prevented by installing phase failure relays, maintaining healthy fuses and cables, tightening electrical connections, inspecting contactors regularly, and using properly rated overload protection. Periodic preventive maintenance and routine electrical inspections significantly reduce the risk of phase-loss faults.
Conclusion
- A three-phase sinusoidal power system consists of three AC voltages, and a balanced supply is critical for motor operation.
- Single phasing occurs when one phase disconnects due to issues like blown fuses or broken conductors, reducing torque and causing overheating in motors.
- While running, the motor may still operate at reduced efficiency, drawing excessive current, risking insulation failure, and potential burnout.
- Detection methods include multimeters and phase sequence meters, and protective measures involve phase failure relays and regular maintenance.
- Understanding single phasing helps prevent damage, ensures motor reliability, and minimizes downtime. Regular inspections are crucial for mitigation.