SINGLE PHASING OF MOTORS BASIC INFORMATION

A 3-phase motor can be subjected to a situation referred to as “single phasing.” Single phasing means one phase is open—somewhere. See Figure 1 and Figure 2.

FIGURE 1

Example of secondary single phasing. The windings in the motor are wye-connected. When a secondary single phasing occurs, the current in one phase drops to zero amperes and the current in the other two phases increases to approximately 1.73 X 10 5 17.3 amperes. Two sets of phase windings are burned out because the motor did not have proper overload protection. 

FIGURE 2

Example of secondary single phasing. The windings in the motor are delta connected. When a secondary single phasing occurs, the current in one phase conductor drops to zero amperes and the current in the other two phase conductors increases to approximately 1.73 X 10 = 17.3 amperes.

Note that the current in one phase winding in the motor increases to 11.6 amperes and the current in the other two phase windings now in series have a current flow of 5.8 amperes. The phase winding with the 11.6 amperes burns out. Proper motor overload protection would have overload devices in all three phases sized at 125% of the motor’s full-load current rating. 

When an “open” occurs somewhere in the branch circuit supplying the motor while the motor is running, properly sized overload devices will sense the 17.3-ampere current flow and cause the relay to open the circuit, taking the motor off line and protecting the motor from burnout.

Proper motor overload protection would have overload devices in all three phases sized at 125% of the motor’s full-load current rating. When an “open” occurs somewhere in the branch circuit supplying the motor while the motor is running, the overload devices will sense the 17.3-ampere current flow and cause the relay to open the circuit, protecting the motor from burnout.

The current values shown in the figures are for a “pure” single phasing condition with one motor only in the circuit. Nothing can prevent single phasing. Single phasing can be caused by such things as a downed power
line, the utility company’s primary transformer fuse or cutout being open, an open connection, a bad termination, an open splice, a broken conductor, burned open contacts in a motor controller, bad contacts in a switch or circuit breaker, an open winding in the supply transformer, or misalignment.

Figure 3 shows a primary single-phase condition in a wye-delta connected transformation. Similar conditions occur in a delta-wye or delta delta transformation. Prior to 1971, the NEC Table 430-37 permitted 3-phase motors to have overload protection in only two phases. At that time, when a primary and or secondary single phasing occurred, there were numerous motor burnouts.

FIGURE 3

Example of primary single phasing on a wye-delta system. An “open” occurs in the primary while the motor is running. All three conductors are still supplying current to the motor. The motor has overload protection in only two phases as permitted prior to the 1972 NEC. One phase has no overload protection.

The current in two phases increases to approximately 1.15 X 10 = 11.5 amperes. The current on one phase increases to approximately 2.3 X 10 = 23 amperes. The winding subjected to the 23 amperes burns out. Depending on which phase of the primary opened, there is a one-out-of-three chance of motor burnout. Had there been motor overload protection in all three phases, the overload devices would have sensed the increase in current and taken the motor off line.

Why so many motor burnouts? Because the motor winding for the phase subjected to the damaging increase in current just might be the phase that did not have overload protection—a one-outof- three chance of motor burnout! The 1971 NEC began to require overload protection in all three phases, NEC Table 430-37.

The current values in these diagrams are theoretical. In the real world, unexpected things can happen. A 3-phase motor will probably not start if one phase supplying the motor is open. However, if a large, lightly loaded 3-phase motor is running at the time of single phasing, and other smaller motors connected to the system are running, the large motor acts as a generator and will actually feed back into the system on the open phase.

This does not create a true symmetrical 3-phase system (120° phase displacement), but it creates a distorted, asymmetrical 3-phase system. This generated phase might be called a “phantom phase.” Smaller motors not fully loaded on the system might start or continue to run, leading to motor burnout unless the problem is corrected, Figure 4.

FIGURE 4

A 3-phase motor running at the time a single phasing occurs. If lightly loaded, this motor could continue to run. As it rotates, it will generate and feed back into the system a distorted third phase. Other motors on the line will “see” this distorted 3-phase supply and might start or continue to run. Motor failure can be expected—not a good situation. Properly sized motor overload protection is the key to keeping motor burnouts to a minimum.