Start Capacitor and Hard Start Kit
What a start capacitor does during the first moments of a motor start, and when a hard start kit is appropriate.
Safety
What it is
A start capacitor is a high capacitance, short duty device that gives a single phase motor extra starting torque. Unlike a run capacitor it stays in the circuit only for the first fraction of a second, then a potential relay or a current sensing device removes it.
Hard start kit
A start capacitor paired with a switching device, added to a compressor that struggles to start. Sold as a matched potential relay and capacitor, or as a self contained solid state module.
Why it matters
A compressor that cannot break away draws locked rotor current until the internal overload opens. Repeated attempts heat the windings and shorten the life of the compressor. A properly selected start assist reduces the time spent at locked rotor current.
A hard start kit is not a cure for a mechanical problem
How it works
- 1On a call for cooling the contactor closes and the compressor attempts to start.
- 2The start capacitor is in series with the start winding, shifting the phase further and creating more starting torque.
- 3As the motor comes up to speed the start winding generates a back electromotive force.
- 4The potential relay senses that voltage and opens its contacts, removing the start capacitor.
- 5The run capacitor remains in the circuit for normal operation.
Start capacitors are rated in a microfarad range rather than a single value, along with a voltage rating. Most include a bleed resistor across the terminals to discharge the capacitor between starts.
How to diagnose it
Listen and clamp the compressor on a start attempt
Current at locked rotor for more than a moment, with a hum and no start, points to a starting problem
Verify supply voltage while the compressor attempts to start
Significant sag indicates a supply or connection problem rather than a capacitor
Test the run capacitor first
A weak run capacitor is the most common cause of hard starting
Measure the start capacitor with a capacitance meter after discharging
A reading outside the printed range, or an open bleed resistor, means replace
Check the potential relay coil and contacts
Welded contacts leave the start capacitor in circuit and destroy it quickly
Discharge before touching
Tools involved
- Multimeter with capacitance function
- Clamp ammeter with inrush capability
- Insulated tools and eye protection
- Discharge resistor
Symptoms
Practice & Mastery
- Start Capacitor and Hard Start Kit Troubleshooting Scenarios2
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