Understanding Relays in Window Air Conditioners

Window air conditioners contain various components that work together to cool a room. One essential component is the compressor motor, which needs help to start efficiently, especially under load. This assistance is provided by a starting mechanism, often involving a starting relay.

A relay is essentially an electrically operated switch. It uses an electromagnet to mechanically operate a switch, opening or closing a circuit. In air conditioning units, a starting relay is used to momentarily connect a 'start winding' or 'auxiliary winding' in the compressor motor circuit during startup. This winding provides the necessary torque to get the motor rotating. Once the motor reaches a certain speed, the relay disconnects the start winding from the circuit, leaving only the 'run winding' active.

Why Electromagnetic Relays in Window AC?

Among the options provided, the Electromagnetic relay is a commonly used type of starting relay in various motor applications, including many window type air conditioners. Here's why:

• Simple Operation: An electromagnetic relay uses a coil that, when energized, creates a magnetic field. This field pulls an armature, closing or opening contacts.
• Reliability: They are known for their robust nature and reliable operation over many cycles, which is important for a frequently starting compressor.
• Cost-Effectiveness: Electromagnetic relays are relatively inexpensive compared to some other types of starting mechanisms for the power levels required by typical window AC compressors.
• Current or Voltage Sensing: While some electromagnetic relays are simple control relays, starting relays are specifically designed to sense either the high starting current (current relay) or the back EMF voltage built up as the motor speeds up (potential relay, which is a specific type). The question asks for the *type* of relay, and "Electromagnetic relay" is a broad category that includes the types used for motor starting in ACs.

A common implementation for starting in window ACs is a current-sensing electromagnetic relay. This type has a coil in series with the run winding. During startup, the high current through the run winding energizes the relay coil, closing contacts that connect the start winding. As the motor speeds up, the current in the run winding drops significantly, the magnetic field collapses, and the relay contacts open, disconnecting the start winding.

Exploring Other Relay Types

Let's look at the other options:

• Potential relay: This is a specific type of electromagnetic relay often used for larger motors. It senses the voltage across the start winding (which increases as the motor speeds up due to back EMF) to disconnect the start winding. While also electromagnetic, "Electromagnetic relay" is a broader category encompassing the current-sensing type also common in window ACs.
• PTC / PTCR: These stand for Positive Temperature Coefficient Thermistor or Resistor. These are solid-state devices that have resistance that increases dramatically with temperature. During startup, current flows through the PTC/PTCR and the start winding. The current heats the PTC/PTCR, its resistance increases, which limits the current to the start winding, effectively disconnecting it. PTC/PTCRs are also used in some smaller air conditioning units or refrigerators but are fundamentally different from electromechanical relays.

Given the options and the common components in window ACs, the Electromagnetic relay is a widely applicable description for the type of relay used for starting the compressor.

Conclusion

The primary function of the starting relay in a window type air conditioner is to provide initial torque to the compressor motor by engaging the start winding and then disconnecting it once the motor reaches operating speed. The Electromagnetic relay is a common and reliable device used for this purpose in many window AC units, working based on the principle of electromagnetism to operate a switch.

Revision Table: Window AC Relay Types

Here's a quick summary for revision:

• A starting relay helps the compressor motor start.
• It connects the start winding briefly.
• It disconnects the start winding when the motor is running.
• Electromagnetic relays (like current relays) are widely used in window ACs for this task.
• Potential relays are also electromagnetic but sense voltage.
• PTC/PTCR are solid-state alternatives.

Additional Information: AC Compressor Starting

Air conditioner compressor motors are typically hermetic single-phase induction motors. These motors need a starting mechanism because a single-phase magnetic field cannot produce a strong rotating field at standstill. The start winding, which is electrically displaced from the main run winding, creates a temporary rotating magnetic field. Once the motor reaches about 75% of its synchronous speed, the start winding is no longer needed and must be disconnected. Failure of the starting relay can prevent the compressor from starting or cause it to trip on overload.