- Permanent Split Capacitor Motor Definition: A permanent split capacitor motor is a type of split-phase induction motor that continuously connects a capacitor, enhancing efficiency and stability.
- Capacitor Functionality: The capacitor in these motors ensures a phase difference between the main and auxiliary windings, crucial for smooth operation and consistent torque.
- Torque Generation: As the motor’s load changes, adjustments in the rotor’s torque occur automatically to maintain consistent speed, demonstrating the motor’s adaptive performance.
- Speed Adjustment Methods: Motor speed can be finely controlled through changes in input voltage or frequency, enabling flexible operation across different applications.
- Practical Uses: These motors are integral to various applications, powering devices such as fans in air conditioning systems and compressors in refrigerators.
What is a Permanent Split Capacitor Motor?
A permanent split capacitor motor, or PSC motor, is a single-phase induction motor whose run capacitor remains connected in series with the auxiliary winding during starting and running.
This single-phase induction motor has a stator with main and auxiliary windings plus a squirrel-cage rotor.
Unlike a capacitor-start motor, a PSC motor uses one continuous-duty capacitor and does not switch the auxiliary winding out after acceleration.
Because the run capacitor remains connected, the motor needs no centrifugal switch for that function.
The two stator windings and their phase-shifted currents create a rotating field. They approximate a two-phase system at the design point, but the phase balance and torque vary with load.
How Does a Permanent Split Capacitor Motor Work?
A PSC motor runs from the single-phase AC voltage and frequency shown on its nameplate.
The stator contains two windings:
- Main winding
- Auxiliary winding, also called the starting winding
The main winding connects directly to the supply. The auxiliary winding connects to the same supply through capacitor C.
The diagram shows this connection.
The capacitor changes the auxiliary branch impedance so its current is out of phase with the main-winding current.
The windings are positioned about 90 electrical degrees apart. The actual current phase displacement depends on the winding impedances, capacitance, speed and load.
When the supply is applied, Im current flows in the main winding. The capacitor shifts auxiliary current Ia relative to it; it does not create a simple time delay.
The figure compares the main and auxiliary current waveforms.
Together, the winding currents produce a rotating magnetic field. That field induces rotor current and produces starting torque.
As rotor speed rises, induced voltages and winding impedances set the running currents. The auxiliary branch remains energised throughout normal operation.
At rated operation, the auxiliary winding still carries current. Its winding and run capacitor are designed for continuous duty when used at their specified ratings.
When load increases, rotor speed falls slightly and slip rises. The larger slip changes the rotor current produced by the applied voltage.
The added rotor current produces more torque until motor torque matches the load. Speed still decreases with load according to the motor’s speed-torque curve.
Permanent Split Capacitor Motor Speed Control
Voltage control can adjust the speed of a PSC motor on fan and blower loads when the motor and controller are rated for it. Lower voltage also reduces available torque and can increase heating or cause a stall.
An autotransformer can provide stepped or adjustable voltage for a compatible motor.
At low voltage, starting torque falls sharply and speed becomes more sensitive to load.
A variable-frequency drive can also change speed because synchronous speed follows supply frequency . The drive must also manage voltage and remain compatible with the PSC winding and capacitor design.
An electronic drive normally uses a rectifier to create a DC bus and an inverter stage to produce AC at the commanded frequency.
A lower commanded frequency generally lowers speed, while a higher frequency raises synchronous speed. There is no universal 20% to 110% range; use the limits and control method specified for the motor, capacitor and load.
Advantages
A PSC design offers these advantages when matched to its load:
- No centrifugal start switch is required, removing one mechanical switching component.
- It can be more efficient and draw less line current than a comparable shaded-pole motor.
- The continuous run capacitor can improve running power factor; the actual value depends on the motor design and load.
- The continuously energised auxiliary winding supports smooth running torque for fan, blower and small centrifugal-pump loads.
Disadvantages
A PSC motor also has these limits:
- The run capacitor must be rated for continuous AC motor duty. Polarised electrolytic start capacitors are unsuitable; modern run capacitors commonly use metallised plastic film.
- The single run-capacitor design has low starting torque compared with capacitor-start arrangements.
Applications of Permanent Split Capacitor Motor
PSC motors suit loads that need modest starting torque and steady running:
- Some light-start compressor and small centrifugal-pump designs use PSC motors; hard-start compressors usually need a capacitor-start arrangement or another motor type.
- Office machinery can use PSC motors for quiet, continuous-duty drives.
- Fans and blowers in heaters, air conditioners, ventilators and range hoods are common applications.
