In capacitor-start and capacitor-run designs, the capacitor plays a specific role in creating the phase shift needed for motor operation. A motor start capacitor provides a short burst of high starting torque and is then disconnected, while a motor run capacitor remains in the circuit during startup and normal operation. Confusing the two can prevent a motor from starting properly or cause a capacitor to fail prematurely. This guide explains the key differences and how to choose the right capacitor.

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Figure 1. Motor Start Capacitor vs. Motor Run Capacitor

Motor Start vs. Run Capacitors: The Key Differences

How Capacitor-Based Single-Phase Motors Use Capacitors

Single-phase induction motors do not produce a rotating magnetic field from the main winding alone. Instead, the main winding produces a pulsating magnetic field that does not provide sufficient starting torque by itself.

In capacitor-based designs, an auxiliary winding is used with a capacitor to create a phase difference between the winding currents. This produces a rotating magnetic field effect that provides starting torque and supports motor operation.

Depending on the motor design, the capacitor may be used only during startup, remain connected during operation, or be used in both stages. Motors that use a start capacitor also require a switch or relay to disconnect it after startup.

Figure 2. Run capacitor in a permanent split capacitor (PSC) motor

What Is a Motor Start Capacitor

A motor start capacitor provides the additional starting torque needed by certain single-phase induction motors. It is connected during startup and is disconnected from the circuit after the motor accelerates to a sufficient speed.

How It Works

At startup, the capacitor is connected in series with the auxiliary winding and shifts its current out of phase with the main winding current. The two windings then produce a near-rotating magnetic field, giving the rotor enough starting torque to accelerate and overcome the load.

As the motor speeds up, a centrifugal switch, start relay, or other switching device disconnects the start capacitor from the circuit. It is built for intermittent duty and should not stay connected during normal operation, as continuous use can cause overheating and early failure.

How to Identify a Motor Start Capacitor

A motor start capacitor may have a black plastic cylindrical case, but appearance alone is not a reliable way to determine its type. Always check the capacitor label, motor wiring, nameplate, service documentation, or manufacturer specifications.

Common identifying information may include:

• Read the capacitor label for its capacitance range, AC voltage rating, and start-duty designation

• Some models include a bleed resistor across the terminals

• Wired through a centrifugal switch or start relay, not connected continuously

• Labeled “Motor Start” or “MS”

Figure 3. A Motor Start Capacitor

Typical Specifications and Applications

As outlined in the quick comparison chart, start capacitors are rated for intermittent duty and feature wide tolerances (often ±20%). They are non-polarized, AC-rated electrolytic capacitors designed for high-demand applications such as air compressors, refrigeration compressors, heavy-duty pumps, and bench grinders. An electrolytic start capacitor is not built for continuous AC duty. If it stays in the circuit after startup, it can overheat and fail.

What Is a Motor Run Capacitor

In a PSC motor, a run capacitor remains connected startup and normal operation. The Specified run capacitance helps the motor operate as designed; an incorrect value can impair torque and increase heating.

How It Works

In a typical PSC motor, the run capacitor remains connected to the auxiliary winding during startup and normal operation. Its specified capacitance provides the phase relationship between the winding currents required for the motor to operate as designed.

The correct run capacitance helps maintain the motor's designed operating characteristics. An incorrect value can impair torque and increase heating.

How to Identify a Motor Run Capacitor

A run capacitor may have a metal oval or round case, often with a silver-colored finish, but appearance alone cannot reliably determine whether a capacitor is a run type. Always check the capacitor label, motor wiring, nameplate, service documentation, or manufacturer specifications.

Common identifying information may include:

• Label shows a single capacitance value with a tolerance such as ±5%, and a higher voltage rating (370 or 440 VAC)

• May be a dual run capacitor with two µF values and C / HERM / FAN terminals

• Stays connected to the motor during startup and normal operation

Figure 4. A Motor Run Capacitor

Typical Specifications

Unlike motor start capacitors, motor run capacitors are built for continuous duty using self-healing metallized polypropylene film. They require tight tolerances (typically ±5–6%) to maintain motor balance. Common applications include HVAC condenser and blower motors, fans, and PSC motors.

Motor Types and Capacitor Configurations

The right capacitor depends on the motor type. Check the equipment or motor documentation and the original capacitor label to identify the required configuration.

Capacitor Types by Motor Type

If the nameplate is unreadable, look at the wiring. A centrifugal switch or start relay in series with a capacitor points to a start capacitor. A capacitor that stays connected all the time is a run capacitor.

Dual Run Capacitors: How to Identify and Replace Them

Dual run capacitors are common in HVAC systems. They combine two run capacitors in a single case: one for the compressor and one for the fan motor.

How to identify one

• The label shows two capacitance values, such as 45/5 µF, and one voltage rating, such as 440 VAC.

• The case has three terminals marked C (common), HERM (hermetic compressor), and FAN.

How to replace one

• Photograph the wiring before disconnecting anything.

• Match both µF values, the voltage rating (equal or higher), and the physical size.

• Reconnect each wire to the same terminal marking: C to C, HERM to HERM, FAN to FAN.

Using two single run capacitors is possible in some cases, but it requires careful wiring and space. A matching dual capacitor is usually the simplest replacement when the original configuration uses one.

A Note on Hard-Start Kits

A hard-start kit adds a start capacitor and a relay to a motor that normally uses only a run capacitor, such as an HVAC compressor with starting trouble. It can improve starting performance, but it should not be used to replace proper troubleshooting when a motor develops a new starting problem.

How to Choose the Right Capacitor

Capacitance (µF) and Tolerance

Match the original value. For run capacitors, stay within the tolerance on the label, typically ±5–6%. For start capacitors, match the labeled range. Larger is not automatically better. An oversized run capacitor can raise motor current and heat, and an undersized one reduces torque and efficiency.

Voltage Rating (VAC)

Choose a voltage rating equal to or higher than the original. For example, a 440 VAC run capacitor may replace a 370 VAC run capacitor when the required capacitance, duty, terminals, dimensions, temperature rating, and equipment specifications also match. A higher voltage rating alone does not establish compatibility.

Duty Type: Intermittent vs. Continuous

This is the most important check. A start capacitor is rated for intermittent duty and a run capacitor for continuous duty. They are not interchangeable, even if the µF and voltage happen to look right.

Case Size and Terminal Type

Check the diameter, height, and mounting style. Terminals may be quick-connect, screw, or wire leads. A capacitor that doesn't fit the housing or connectors creates problems even if the electrical specs match.

Operating Temperature and Life Rating

Look at the maximum operating temperature, commonly 70 °C or 85 °C, and the rated life. For hot environments such as outdoor condensers, choose a higher temperature rating.

Certifications and Compliance

Check the certifications and compliance requirements applicable to the product and market, such as UL or other recognized approvals, as well as applicable RoHS requirements.

Signs of a Failing Capacitor and Basic Testing

Common Symptoms

Motor start capacitor failure:

• Motor hums but does not start

• Slow or weak starting

• Tripped breaker or blown fuse on startup

Motor run capacitor failure:

• Motor overheating

• Reduced output or airflow

• Unusual noise or unstable speed

• Higher energy consumption

Basic Testing

Visual Inspection: Before using any tools, examine the capacitor. If you see bulging, leakage, cracks, or other obvious physical damage, replace the capacitor rather than relying on electrical testing alone.

Set the Multimeter: If the capacitor looks physically intact, ensure it is fully discharged. Turn your multimeter to the capacitance setting (often marked with a “–|(–” symbol).

Measure and Compare: Disconnect the capacitor from the circuit before measuring its capacitance. Place the meter's probes on the capacitor's terminals. Compare the reading on the screen to the µF value and tolerance printed on the label.

• Example: A 45 µF run capacitor with a ±5% tolerance must read between 42.75 and 47.25 µF.

• Interpreting Results: If the measured capacitance is outside the specified tolerance, the capacitor may need replacement. An “OL” reading may indicate an open or out-of-range condition, depending on the meter and test setup.

Replace a Motor Capacitor Safely

Capacitors can hold a charge even after power is disconnected, so treat them as live until discharged.

Step 1. Turn off and verify the power

Switch off the breaker, then use a voltage tester to confirm the circuit is dead. Don't rely on the breaker position alone.

Step 2. Discharge the capacitor

Remove the service cover and place a properly rated discharge resistor across the terminals for a few seconds. Connect a properly rated discharge resistor across the terminals, then use a properly rated multimeter to verify that the voltage has fallen to a safe level. For a dual capacitor, discharge between C and HERM, and between C and FAN. Never short the terminals with a screwdriver.

Step 3. Photograph the wiring

Take a clear photo of all wires and terminals before disconnecting anything.

Step 4. Remove the old capacitor

Pull the wires off one at a time, holding the connector rather than the wire, then loosen the bracket.

Step 5. Match the replacement

Check that the replacement has the correct capacitance, duty type, AC voltage rating, temperature rating, physical dimensions, and terminal configuration specified for the motor.

Step 6. Install and reconnect

Secure the new capacitor and reconnect each wire to the same terminal as before, using your photo as a reference.

Step 7. Check related parts and test

Inspect the contactor or start relay for burn marks or loose connections, since a worn part can cause repeated failures. Then restore power and confirm the motor starts smoothly and runs quietly.

If you're not comfortable working with electrical equipment, or the system is a high-voltage or commercial installation, call a licensed electrician.

Conclusion

Motor start capacitors deliver a brief, powerful boost and drop out of the circuit, while motor run capacitors stay connected and keep the motor efficient. They are not interchangeable, and the right choice depends on your motor type, the original label, and the operating conditions.

When in doubt, match the nameplate, choose an equal or higher voltage rating, and never swap types.