ABSTRACT
This paper discusses the relevant of a star – delta control system, and the design procedures. It also highlights the various electrical components that makes this control system possible and obtainable.
INTRODUCTION
The electrical current consumed when starting an ac motor and the consequent effect on nearby equipment has made electrical researchers to come up with a soft starting method of ac motor called star – delta starter. Star – delta starter is a method of starting an ac motor usually of high-power rating with star connection and after a preset time interval switches to a delta connection automatically without human intervention.
With the star – delta starting method power generating equipment is less stressed and with greater efficiency and reliability. The cost of the soft starter will depend on the number of starts required per hour, run-up time, Voltage, power rating, and protection devices required.
METHOD
This is a starting method that reduces the starting current and starting torque. The components normally consist of three contactors, an overload relay and a timer for setting the time in the star-position (starting position).
The motor must be delta connected during a normal run, in order to be able to use this starting method. The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start.
BEHAVIOUR OF A STAR –DELTA CONNECTED MOTOR TO LOADS
This starting method only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position. When starting up, the load torque is low at the beginning of the start and increases with the square of the speed.
When reaching approximately 80-85% of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks.
EFFECT OF A STAR DELTA STARTER ON TORQUE APPLICATION
In some cases the current peak can reach a value that is even bigger than for a direct on line (D.O.L) start. Applications with a load torque higher than 50% of the motor rated torque will not be able to start using the star-delta starter. See power quality analyzer workbench and the AutoCAD design in the nest slides.
With our 10hp motor;
Power in Kw is 746 x 10 = 7.46Kw
Three phase voltage = 380volts
Power factor i.e. 𝑐𝑜𝑠∅ = 0.85
Current = power rating/√3𝑉𝑐𝑜𝑠∅
I = 7460/380x1.72x0.85
I = 13.43A
Received starting current is 30% of nominal current.
Therefore starting current = 0.3xIn
= 0.3x13.43
= 4.43A
Suppose in our 10hp motor ;
Windings resistance = 25.0Ω
Then star current in the star connected end is
Current = voltage/resistance
I = 380/(25 + 25)
I =7.6A
30% of 7.6A = 2.28
Also, the delta current in the delta connected is 380/25 = 15.20A
CURRENT CIRCUIT REPRESENTATION OF A STAR CONNECTION
CURRENT CIRCUIT REPRESENTATION OF A DELTA CONNECTION
Circuit breaker fuse = 1.25x15.20
= 19.0A
Therefore, if 19A breaker is not available, the next sizeable breaker fuse in the electrical table should be selected for optimum delivery.
Overload currents should be set at
1.25x2.28 = 2.85 for the star contactor end and 1.25x15.20 = 19.0A for the delta contactor end.
CONCLUSION
The Star-Delta starting method yields the least power quality problems compare to direct on line (D.O.L). In this design the power quality problem such as current dragging, voltage fluctuation, power equipment inefficiency has been subsided.
RECOMMENDATION
From the above design on the starting of ac motor as regards to power system management. I wish to recommend the design of soft starting method for heavy ac motors. Also, recommending for all electrical workshops to be equipped with electrical workbench and other laboratory equipment for better electrical research and development.
REFERENCES
William D. Stevenson, Jr. Elements of
Power System Analysis Third Edition, McGraw-Hill, New York (1975). ISBN 0-07-061285-4, p. 2
"Three-phase power systems : Polyphase Ac
Circuits - Electronics Textbook". Allaboutcircuits.com.
Retrieved 2015-05-13.
Cotton, H, Electrical Technology, 6th
Ed., Pitman, London, 1950, p. 268
Hawkins
Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4,
Ch. 46: Alternating Currents, p. 1026, fig. 1260.
Hawkins
Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4,
Ch. 46: Alternating Currents, p. 1026, fig. 1261.