02-05-2011, 10:13 AM
PRESENTED BY:
Jayabratha Saha
Sohel Ali Mandal
Sourav Mondal
Rajesh Kumar
Pushraj
Avik Mahapatra
[attachment=13190]
INTRODUCTION
In India there are so many industries in different fields. For example steel sector, Oil sector, Irrigation etc. All industries have many drives and equipment’s like conveyor belts, pumps, Mills etc.
All the drives of industries use electrical motors. Most of the electrical motors are designed for three phase, 50Hz (in India) supply. The starting of three phase motors are less expensive than starting of DC motors.
Three phase induction motors are very sensitive and get damaged, when they are subjected to Single-phasing. For three phase induction motor, it is necessary that all the three phases of supply are present. While it is on load if any one of the fuse goes out, or goes missing, the motor will continue to run with two phases only, but it will start drawing a huge current for the same load. This high current may run the motor unless switched off immediately.
Failure of any of the phases makes the appliance prone to erratic functioning and may even lead to failure. Hence it is of paramount importance to monitor the availability of the three-phase supply and switch off the appliance in the event of failure of one or two phases. The power to the appliance should resume with the availability of all phases of the supply with certain time delay in order to avoid surges and momentary fluctuations.
A single phasing preventer avoids such a mishap with this circuit, the motor will not run unless all the three phases are present. In this context we need to design a preventer which prevents these mishaps and protects the costly motor under such conditions. The single phase preventer is very less expensive and protects reliably the motor which is very costly.
What Is Single-Phasing?
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. About 14% of the motor failures are due to single phasing.
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.
Diagram of a WYE/DELTA transformation with one primary phase open.
The motor is protected by two overload devices. Note that one phase to the motor is carrying two times that of the other two phases. Without an overload device in the phase that is carrying two times the current in the other two phases, the motor will burn out.
Hazards of Single Phasing for a Three-Phase Motor
When one phase of a secondary opens, the current to a motor in the two remaining phases theoretically increase to 1.73 (173%) times the normal current draw of the motor. The increase can be as much as 2 times (200%) because of power factor changes
The increase in current is in only phase of the motor in case of delta connected load and in star connected load the increase in current occurs in two phases.
Jayabratha Saha
Sohel Ali Mandal
Sourav Mondal
Rajesh Kumar
Pushraj
Avik Mahapatra
[attachment=13190]
INTRODUCTION
In India there are so many industries in different fields. For example steel sector, Oil sector, Irrigation etc. All industries have many drives and equipment’s like conveyor belts, pumps, Mills etc.
All the drives of industries use electrical motors. Most of the electrical motors are designed for three phase, 50Hz (in India) supply. The starting of three phase motors are less expensive than starting of DC motors.
Three phase induction motors are very sensitive and get damaged, when they are subjected to Single-phasing. For three phase induction motor, it is necessary that all the three phases of supply are present. While it is on load if any one of the fuse goes out, or goes missing, the motor will continue to run with two phases only, but it will start drawing a huge current for the same load. This high current may run the motor unless switched off immediately.
Failure of any of the phases makes the appliance prone to erratic functioning and may even lead to failure. Hence it is of paramount importance to monitor the availability of the three-phase supply and switch off the appliance in the event of failure of one or two phases. The power to the appliance should resume with the availability of all phases of the supply with certain time delay in order to avoid surges and momentary fluctuations.
A single phasing preventer avoids such a mishap with this circuit, the motor will not run unless all the three phases are present. In this context we need to design a preventer which prevents these mishaps and protects the costly motor under such conditions. The single phase preventer is very less expensive and protects reliably the motor which is very costly.
What Is Single-Phasing?
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. About 14% of the motor failures are due to single phasing.
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.
Diagram of a WYE/DELTA transformation with one primary phase open.
The motor is protected by two overload devices. Note that one phase to the motor is carrying two times that of the other two phases. Without an overload device in the phase that is carrying two times the current in the other two phases, the motor will burn out.
Hazards of Single Phasing for a Three-Phase Motor
When one phase of a secondary opens, the current to a motor in the two remaining phases theoretically increase to 1.73 (173%) times the normal current draw of the motor. The increase can be as much as 2 times (200%) because of power factor changes
The increase in current is in only phase of the motor in case of delta connected load and in star connected load the increase in current occurs in two phases.
