26-10-2016, 12:14 PM
PROTECTION OF 3-PHASE INDUCTION MOTOR AGAINST FAULTS DUE TO SINGLE PHASING, UNDER VOLTAGE AND VOLTAGE UNBALANCE
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ABSTRACT-A System is designed and implemented to make protection for faults due to single phasing, voltage unbalance and under voltage in a 3-phase Induction motor. In the system, three potential transformers with transformation ratio 220/12 are connected to each phase of three phase supply. The low ac supply taken from transformer secondary windings are converted into DC values by use of rectifier circuit. Control process is implemented using simple electronic circuit, which include three power MOSFETs connected in series with a normally open contactor. The DC supply is given to the gate terminal of each MOSFET which would turn on only when the gate source voltage is greater than 7.5V (which is achieved by 110V ac supply). When a voltage unbalance, under voltage or single phasing occurs, the gate source voltage across MOSFET is reduced to less than 7.5V which in turn opens the circuit of the contactor and thus cuts the power supply of the Induction Motor.
INTRODUCTION
The squirrel cage AC Induction Motors are the main workhorse of the Industry due to its ruggedness, versatility and low manufacturing cost. Although Induction Motors are simple and reliable machines, their annual failure rate is conservatively estimated at 3%per annum. In modern plants, thousands of electric motors work together in manufacturing process. An unscheduled shut down occurs if only one of these motor fails. If it is possible to detect a developing fault, then it is possible to schedule an orderly shutdown of the complete process. Motor failures can be classified in a number of ways. The majority of the motor troubles are categorized into 3 groups
(a)Single Phasing effects
(b)Unbalanced voltage effects
© Under voltage effects
Down time in a factory can be extremely expensive and in most cases it exceeds the cost of replacement itself. The proper protection of motor is required to minimize damage to the motor and associated equipment, to enhance personal safety and productivity. Motor protection equipment is typically aimed to protect the motor against overheating.
If single phasing occurs when an 'Induction Motor is running , the motor draws large current from the other two phase to run on the same load thus the winding gets overheated and eventually get damaged.
When the three phase voltage is unbalanced, the unbalance has detrimental effects on 3-phase induction Motors including overheating, shaft vibration noise and additional losses and hence reduction in its life time and performance. An abnormally low supply voltage causes the motor to run well below the synchronous speed, drawing a current which is even though not as high as the starting current, quickly overheats stator windings and rotor of the induction motor which in turn leads to insulation deterioration and stator winding failure. The report describes a method to protect induction Motor from faults due to voltage unbalance, single-phasing and under voltage.
FAULTS DUE TO VOLTAGE UNBALANCE,
SINGLE-PHASING AND UNDER VOLTAGE IN THE
SUPPLY OF THREE PHASE INDUCTION MOTOR.
1. Single-phasing effects:
Single phasing can occur as a result of a fuse blowing or a protection device opening on one phase of the motor. Other possibilities include feeder or step down transformer fuses blowing .The loss of one phase or leg of a 3-phase line causes serious problems for Induction motor. If single phasing occurs when the motor is running, the torque produced by the remaining two positive rotating fields continues to rotate the motor and develop the torque demanded by the load. The negatively rotating field (due to negative sequence currents)i.e. the field associated with the lost phase produces currents in the inductive loads resulting in voltages at the faulted leg of the 3-phase supply. These voltages may be nearly equal to the phase voltage that was lost. Three-phase motor may continue to run, but they are not capable of starting on single-phasing. Even though the motor will continue to operate in this condition, the motor will heat up quickly, and it is essential that the motor should be removed from service. Though the overload devices on the energized phases isolate the motor, the motor is not isolated from the lost phase; later attempting a restart on that single-phasing supply will cause the motor to draw locked rotor current.
2. Unbalance Voltage effects:
Voltage unbalance of a 3-phase system is expressed as a percentage value, and is often defined as the maximum deviation from the average of the 3-phase voltages divided by the average of the 3-phase voltages .When the voltages are unbalanced, a much higher current is induced in the rotor because it has much lower impedance to the negative sequence voltage component. The percentage increase in temperature of the winding is approximately two times the square of the voltage unbalance. These higher temperatures soon result in degradation of the motor insulation and shorten motor life. This additional rotor heat can exist for a considerable time period and since the rotor and shaft are continuous metallic structure, the heat transfer to the shaft ends can result in bearing failure.
The effects of single phasing are similar to the unbalanced voltages, since the single phasing represents the worst case of an unbalanced voltage condition. An additional effect is the remaining phase windings experience excessive overheating, thereby creating a greater potential for stator winding failure.
3. Under voltage effects:
Under voltage protection for Induction motor is provided for both sustained and transitory under voltages. With three phase motors, the under voltages are assumed to be the balanced type. If the motor has the voltage magnitude at the motor terminals is 50% or less, then increased current will be required. This increase in current will cause increase in heating of both stator windings and the rotor. Short duration transitory under voltages may result from motor starting or system short circuits. Short duration under voltages usually does not harm induction motor that are allowed to remain on-line. To prevent the above mentioned faults due to unbalanced voltage, single phasing and under voltage in the supply, a motor protection scheme is designed and implemented using a simple electronic circuit with help of power MOSFETs.
MOTOR PROTECTION SCHEME
The below circuit diagram shows the overall protection mechanism of 3-phase Induction motor .The motor supply is given through a 3-phase variac. The 3-phase ac supply is than given to 3-single phase transformers(220/12) through 3-single phase variacs (to test for voltage unbalance condition) with switches (for single phasing condition). The ac supply through transformer is than given to a rectifier circuit which converts it into DC .The DC voltage generated at each phase is given to the gate terminal of each power MOSFET which are connected in series with a normally open contactor. The contactor operates on DC supply (greater than 200V) which is generated through rectification of one of the 3-phase ac supply. When the contactor is on it connects the 3-phase supply to the induction motor.
PROTECTION MECHANISM:
The circuit is to protect the motor from following three faults:
a) Single- Phasing or Phase failure
b) Unbalanced Voltage
c) Under Voltage
(a). Single phasing protection:
In the event of phase failure at load condition, the contactor should trip (disconnect) automatically. In the circuit, we generate single phasing condition by switching off one of the 3-phase ac supply with help of switches which would than reduce the gate source voltage of the corresponding power MOSFET to zero volts, thus the path of the contactor coil gets disconnected from DC
supply, which would turn off the contactor as a result of which the 3-phase supply gets disconnected from the motor, and the motor goes off. The motor will not turn on until all the supplies are on. This would protect the motor from single phasing.
(b)Unbalanced Voltage protection:
Percentage of unbalanced voltage between the phases has to be monitored and if it increases above 50% motor has to be disconnected from main supply. Now, we design a potentiometer to obtain half the voltage of the rectified output voltage at gate terminal of MOSFET, by connecting a 10kΩ resistor in between gate of power MOSFET and rectified dc supply and the same resistance across gate and ground as shown in the circuit diagram. At 50% of the rated voltage (110V ac) we obtain a rectified dc output of 7.5V which makes the gate source voltage equal to 3.75V which is not sufficient to turn on the MOSFET so the motor would not start. Thus, whenever any phase is lower than 50% of the rated voltage motor would turn off.
© Under Voltage protection:
In under voltage protection we follow the similar concept as in case of unbalanced voltage protection. In case of under voltage all the phases goes below 50% of the rated voltage simultaneously thus, turning off all three MOSFETs resulting in turning off of the motor.
Conclusion:
This paper presents a reliable system for protecting Induction motor against the faults created by unbalanced voltage, under voltage and single phasing. The implementation of the design is performed using simple electronic circuit without use of any microcontroller. Hardware tests were made using 3-phase induction motor and the results were satisfactory. This system is flexible in the range Settings, so it can be applied to different motors in every situation.