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Which is most economical method of starting a single phase motor?
Resistance start
Induction start
Capacitor start
Split phase
Capacitor start
A capacitor-start single-phase induction motor is considered the most economical method of starting among high-torque single-phase motors because it uses a low-cost electrolytic capacitor connected temporarily during startup. It provides a high starting torque per unit cost compared to other phase-splitting methods, making it commercially and practically the most cost-effective solution.
A capacitor-start single-phase induction motor is considered the most economical method of starting among high-torque single-phase motors because it uses a low-cost electrolytic capacitor connected temporarily during startup. It provides a high starting torque per unit cost compared to other phase-splitting methods, making it commercially and practically the most cost-effective solution.
╧ХтЙИ90┬░ тАФ Phase angle difference between main and auxiliary winding currents
TsтАЛтИЭImтАЛIaтАЛsin╧Х тАФ Starting torque equation of a split-phase motor
The starting winding is connected in series with an electrolytic capacitor and a centrifugal switch across the main supply. The capacitor creates a large phase shift (nearly 90┬░) between the auxiliary winding current (IaтАЛ) and main winding current (ImтАЛ), producing a strong rotating magnetic field. Once the motor attains about 75% to 80% of synchronous speed, the centrifugal switch opens and disconnects the capacitor and auxiliary winding.
Capacitor start motors use short-duty electrolytic capacitors rated for high capacitance and low voltage.
Provides a high starting torque, typically around 300% to 450% of full-load torque.
Economical because it delivers strong starting performance without the added cost of a permanent run capacitor.
High starting torque at a relatively low overall manufacturing cost.
Capacitor is only active during starting, so a smaller short-duty rating is sufficient.
Better power factor during starting compared to pure resistance split-phase motors.
Mechanical centrifugal switch is prone to wear and tear.
Electrolytic starting capacitor can fail if the motor undergoes frequent start-stop cycles.
Compressors and refrigeration units
Pumps and conveyor belts
Machine tools and heavy-duty blowers
Option A (Resistance start): Cheapest in construction, but produces very low starting torque with high starting current, making it economically inefficient for loads requiring high starting effort.
Option B (Induction start): An improper technical term in this context; single-phase induction motors are inherently non-self-starting without auxiliary phase-splitting arrangements.
Option D (Split phase): General term referring to phase-splitting techniques, but simple resistance split-phase has poor starting performance compared to capacitor-start.
C is correct тАФ Capacitor start provides the optimal balance of high starting torque and low component cost, making it the most economical starting method for heavy single-phase loads.
Always distinguish between Capacitor-Start (high starting torque, electrolytic capacitor, switch disconnects it) and Capacitor-Run (higher operating power factor, continuous paper/polypropylene capacitor).