what is single phasing in motors and what is its remedy?

Answer 1

In the operation of a three phase motor, the term "single phasing" refers to operating the motor with one of the three phase connections missing. The cause is usually one of the three phase fuses blown in the local supply or loss of a phase from the utility. It could also be a loose connection or some damage in the motor starter or other equipment supplying power to the motor.

The remedy would be to find and repair whatever caused the fuse to blow or the connection to open and replace the fuse.

Since two power connections are required for single phase power and three for three phase power, the loss of just one connection is called single phasing.

Answer 2

Most larger motors are 3-phase. The rotating field depends on all three phases. The most common electrical fault is line-to-ground. When this happens, only two wires are left, leaving only a single phase. If the fault is "downstream" of the over-current protection device, it may trip the motor off until someone comes along to find out what is wrong and fix it. If the fault is in the mains (upstream) then a "relay" or
electronic device may be needed to detect that there is no current and trip the motor.

Answer 3

3 phase 4 wire system. Phase to phase = line voltage. Phase to neutral = line voltage/ root 3. When one phase is broken rotating field is gone. Motor stops. This is called single phasing. Remedy = (1) Attentive operator, who notices the change in motor sound and stops. (2) C.B.T. This makes the tripping by acting on unbalanced load by inducing emf.

Answer 4

*Loads using three-phase power sources are subject to loss of one of the three phases from the power distribution system. This condition is known as "single-phasing." The loss of a single phase on a three-phase line may be due to a downed line or a blown pole top fuse on the utility system. Loss of a single phase may also result from a single-phase overload condition causing one fuse to blow, or an equipment failure within the end-user's facility.

The loss of one phase, or "leg," of a three-phase line causes serious problems for induction motors. The motor windings overheat due primarily to the flow of negative-sequence current, a condition that exists anytime there is a phase voltage imbalance. The loss of a phase also inhibits the motor's ability to operate at its rated horsepower.

If single-phasing occurs when a motor is rotating, the torque produced by the remaining two positively rotating fields continues to rotate the motor and develop the torque demanded by the load. The negatively rotating field, the field associated with the lost phase, produces currents in inductive loads resulting in voltages in the faulted leg of the three-phase supply. These voltages may be nearly equal to the phase voltage that was lost. Therefore, detecting a single-phasing condition by measuring the voltages at the motor terminals is usually unproductive.

Three-phase motors may continue to run, but they are not capable of starting on a single phase. If after the overload devices on the energized phases isolate the motor, the motor is not then isolated from the lost phase, later attempting a restart on that single-phase supply will cause the motor to draw locked rotor current.

http://www.wisconsinpublicservice.com/business/phase.asp

*http://www.bussmann.com/library/docs/Psp.pdf

Interesting:
[Effect of Single-Phasing on Three-Phase Motors. The
effects of single-phasing on three-phase motors varies with
service conditions and motor thermal capacities. When single
phased, the motor temperature rise may not vary directly with
the motor current. When single-phased, the motor temperature rise may increase at a rate greater than the increase in current. In some cases, protective devices which sense only current may not provide complete single-phasing protection.
However, PRACTICAL experience has demonstrated that
motor running overload devices properly sized and maintained can greatly reduce the problems of single-phasing for the majority of motor installations. In some instances, additional protective means may be necessary when a higher degree of single-phasing protection is required. Generally,
smaller horsepower rated motors have more thermal capacity
than larger horsepower rated motors and are more likely to
be protected by conventional motor running overload devices.]

Answer 5

A motor that connects to a single phase system with a capacitor(remedy)