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The main difference is when the capacitor is used and what it is optimized for:
In short:
| Feature | Capacitor Start Motor | Capacitor Run Motor |
|---|---|---|
| Capacitor use | Only during starting | Continuously during running |
| Capacitor size | Large capacitance | Smaller capacitance |
| Capacitor type | Usually electrolytic, intermittent-duty | Usually polypropylene/oil-filled film, continuous-duty |
| Starting torque | High | Low to moderate |
| Running efficiency | Moderate to good | Better |
| Switch needed | Yes, usually centrifugal switch or relay | Usually no |
| Typical use | Compressors, pumps, heavy starting loads | Fans, blowers, light pumps, HVAC motors |
Single-phase induction motors do not naturally produce a strong rotating magnetic field at standstill. Without assistance, a single-phase motor may simply hum and fail to start. To solve this, many single-phase motors use:
The capacitor shifts the current in the auxiliary winding relative to the main winding. This phase shift creates a rotating magnetic field and gives the motor starting torque.
The difference between capacitor start and capacitor run motors is how the capacitor and auxiliary winding are used.
A capacitor start motor is also commonly called a capacitor-start induction-run motor.
During startup:
The start capacitor is usually a high-capacitance electrolytic capacitor. Typical values may be tens to several hundred microfarads, depending on motor size.
Important point: a start capacitor is not designed for continuous operation. It is intended to remain energized only for a short time, usually a few seconds. If the start switch fails and leaves the start capacitor connected, the capacitor can overheat, vent, rupture, or fail violently.
Capacitor start motors are used when the load requires high torque at standstill, for example:
Their main advantage is high starting torque. Their disadvantages are the need for a switching mechanism and somewhat poorer running power factor compared with capacitor-run designs.
A capacitor run motor is often called a permanent split capacitor motor, or PSC motor.
In this design:
The run capacitor is selected to give good running performance rather than maximum starting torque. It improves:
The run capacitor is usually a metallized polypropylene film capacitor or oil-filled film capacitor. It has a lower capacitance than a start capacitor but is built for continuous AC operation. Typical values are often from a few microfarads to several tens of microfarads, depending on motor rating.
Capacitor run motors are common in applications where the motor does not need to start against a heavy load, such as:
Their main advantage is smooth, efficient, reliable running. Their disadvantage is lower starting torque compared with capacitor start motors.
| Parameter | Capacitor Start Motor | Capacitor Run Motor |
|---|---|---|
| Common name | Capacitor-start induction-run motor | Permanent split capacitor motor |
| Main purpose of capacitor | Starting torque | Running efficiency and power factor |
| Capacitor connection | Connected only during startup | Permanently connected |
| Switching device | Required | Usually not required |
| Capacitor duty | Intermittent duty | Continuous duty |
| Capacitor construction | Usually AC electrolytic | Usually metallized polypropylene film |
| Capacitance | Higher | Lower |
| Starting torque | High, often 200–400% of full-load torque | Lower, often around 50–150% depending on design |
| Running power factor | Moderate | Better |
| Running smoothness | Moderate | Better |
| Reliability | Switch can wear or fail | Fewer moving electrical parts |
| Typical applications | Compressors, pumps, loaded starts | Fans, blowers, light loads |
There is also a combined type called a capacitor start/capacitor run motor.
This design uses two capacitors:
During startup, both capacitors may contribute to the auxiliary winding current. Once the motor reaches speed, the start capacitor is disconnected, but the run capacitor stays connected.
This gives the motor:
These motors are often used in higher-performance single-phase applications such as compressors, larger pumps, and industrial single-phase equipment.
A useful way to think about the difference is:
They are not normally interchangeable.
A start capacitor is not designed for continuous duty. If used as a run capacitor, it can overheat and fail.
A run capacitor usually has much lower capacitance. If installed in place of a start capacitor, it may not provide enough starting torque. The motor may hum, stall, draw locked-rotor current, or overheat.
When identifying or troubleshooting these motors:
Possible failed start capacitor or start switch:
Possible failed run capacitor:
A capacitor should be isolated from the circuit and measured with a capacitance meter. Compare the measured value with the value printed on the capacitor case. Many motor capacitors are considered suspect if they are more than about 5–10% away from their rated capacitance, though the exact tolerance depends on the capacitor marking.
Always disconnect power and discharge the capacitor safely before handling it.
A capacitor start motor uses a large capacitor briefly to create high starting torque, then disconnects it. It is best for heavy starting loads.
A capacitor run motor keeps a smaller capacitor connected continuously to improve efficiency, power factor, and smoothness. It is best for fans, blowers, and lighter loads.
For applications needing both high starting torque and efficient running, a capacitor start/capacitor run motor uses both a start capacitor and a run capacitor.
