Single-Phase Operation of Electric Motors: Causes, Effects, and Protection Methods

Single-Phase Operation of Electric Motors: Causes, Effects, and Protection Methods

Any three-phase induction motor must be connected to a three-phase AC power supply with rated voltage and load for regular operation. Once started, the motor will continue to run even if one of the three-phase power lines is interrupted. The current loss on one of the power lines is called single-phase operation.

Ships are equipped with hundreds of electric motors to drive various pumps, machines, and systems. Critical equipment, such as steering systems, prime movers, generators, and boilers, is equipped with three-phase motors driving main or auxiliary systems.

440-volt three-phase motors are typically standard squirrel-cage induction motors designed for 440-volt, 60-Hz three-phase AC. Small-power motors with a power rating of 0.4 kW or less are mainly used for lighting and other low-power systems and are typically 220V, 60Hz single-phase motors.

Causes of Single-Phase Operation

Single-phase operation is an electrical fault related to the induction motor power supply. When one of the three phases of a three-phase motor's circuit breaks, single-phase operation occurs, causing the remaining circuits to carry excessive current. This typically happens under the following conditions:

• One or more of the three spare fuses blow (or the fuse itself if it's a line fuse).
• An open circuit exists in the motor's power supply circuit.
• Improper adjustment or error of any motor protection device can lead to single-phase operation.
• Scaling or oxidation can occur on the contacts if they are not regularly maintained, resulting in single-phase operation.
• Damaged or broken motor relay contacts.
• Damaged motor circuit wiring.
• Power system failure.
• Short circuit in one phase of the motor in a star or delta connection.
• Blown power supply or transformer fuse.

Impact of Single-Phase Operation

As mentioned earlier, a three-phase motor is an AC motor designed for a three-phase power supply. Both types of motors have similar structures, containing a stator and a rotor. Single-phase motors do not have a rotating magnetic field; instead, they use a 180-degree reverse magnetic field. Single-phase motors typically cannot self-start. Therefore, additional equipment is required, such as replacing the starting coil or capacitor. Single-phase operation of a three-phase induction motor can cause the following effects:

• If the motor stops running, it cannot start because single-phase motors cannot self-start (as mentioned above), and the three-phase motor's built-in overheat protection system will also prevent it from starting.
• If a single-phase operation fault occurs while the motor is running, the motor will continue to run due to the torque generated by the remaining two phases (depending on load demand) (unless equipped with an additional safety disconnect system).
• Because the remaining two phases must carry the extra load of the faulty phase, they will overheat, potentially causing serious winding damage.
• Single-phase operation can cause the current to reach 2.4 times the average current of the remaining two phases.

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Motor junction box.

Single-phase operation will reduce motor speed, and the motor speed will fluctuate.

• The motor will emit abnormal noises and vibrate. This is due to the uneven torque generated by the remaining two phases. – Almost all shipboard motor systems are equipped with a backup. If a motor is set to backup mode and a problem occurs during single-phase operation, the motor will fail, causing a system malfunction.
• If the problem is not resolved and the motor continues to run, the windings will melt due to overheating, potentially leading to a short circuit or ground fault.
• In this situation, if crew members touch the motor, they will suffer potentially fatal electric shocks. The flow of negative series current primarily causes the winding overheating.
• This can cause the generator (i.e., the auxiliary motor and its alternator) to overload.

How to protect a motor from damage caused by single-phase operation?

In this case, the motor must be equipped with a protective device to disconnect it from the system before permanent damage occurs.

All motors with a rated power exceeding 500 kW must be equipped with protective devices or equipment to prevent damage from single-phase operation.

The above rule does not apply to motors in a ship's steering system. An alarm will only be sounded when single-phase operation is detected; however, the motor will not stop running, as its continuous operation is crucial to the ship's safety and propulsion, especially in congested waters or during maneuvering.

The most common single-phase operation protection devices include:

1) Electromagnetic overload protection devices

In this device, each of the three motor phases is equipped with an overload protection relay. If the current exceeds a specified value, the relay will automatically activate, and the motor will stop.

The device works by utilizing the electromagnetic effect generated by the current.

As the current increases, the magnetic force of the electromagnet in the coil also increases, thereby attracting the relay and activating the trip relay, causing the motor to stop.

This system provides a delay function to prevent the motor from drawing excessive current during startup, which could otherwise damage it.

2) Thermistor

A thermistor is a small thermal element used in conjunction with an electromagnetic overload relay. They are installed in the three windings of the motor. Any increase in current causes the windings to heat up, which the thermistor detects and sends a signal to an amplifier.

Related Reading: Amplifier Circuits or Operational Amplifiers Used on Ships

The amplifier is connected to the electromagnetic relay. When the thermistor sends a temperature-rise signal, the amplifier increases the current to the electromagnetic relay coil, triggering a circuit breaker to stop or shut down the motor.

3) Bimetallic Strip

In this method, the bimetallic strip is positioned to detect a temperature rise in the circuit. Once a temperature rise is detected, the bimetallic strip attempts to expand due to the use of two different metals with different coefficients of thermal expansion. The strip bends towards the metal with the higher coefficient of thermal expansion, ultimately closing the circuit breaker and shutting down the motor.

4) Standard Overload Protection for Motor Starters

This protection device is used for three-phase motors to handle single-phase operation. All phases are equipped with overload heaters to detect any overload conditions. If the load exceeds the motor's rating, the heaters will shut off the starter before the motor windings are damaged.

How to Detect Single-Phase Faults?

For seafarers, knowing whether a motor has been converted to single-phase operation is crucial. Three-phase induction motors are typically equipped with single-phase overload detectors. However, motors can fail at any time, and as an experienced marine engineer , you should be familiar with the motor's operating status, including sound, feel, and running conditions.

When checking for single-phase operation problems in a marine engine, always keep all sensors on high alert:

• Unusual humming sound from the engine
• The engine vibration frequency is higher than normal
• Smell of burnt copper (insulator) (Learn how to use a potentiometer for insulation testing to help prevent accidents)
• Visible smoke emanating from the engine casing

To troubleshoot a single-phase-to-three-phase motor fault and restart, immediately stop the engine and switch to standby mode. Check the motor parameters indicated on the casing nameplate and identify the motor fault.

Perform a thorough visual inspection of the motor windings, checking their continuity and grounding resistance. If the motor itself cannot diagnose the fault, a power supply test is required to determine the problem.

After identifying and resolving the problem, prepare the motor. Before connecting the motor to the load, turn on its controls and test all relevant parameters (e.g., voltage, current, speed, temperature), comparing these values to those printed on the nameplate.

Ensure all measurements conform to the specifications on the nameplate. After completing the no-load test, run the motor and monitor its performance to confirm that the problem has been resolved and the motor is operating normally.