In industrial and HVAC (heating, ventilation, air conditioning) ventilation systems, tachometer outputs and alarm outputs play distinct and complementary roles in facilitating control and safety. Understanding their function is essential for effective fan monitoring and/or control, especially when using sophisticated speed control methods such as PWM or 0-10V.

 
The tachometer output: the measurement signal

 
The tachometer output is a measurement signal, usually a series of pulses per revolution, which measures the fan's rotational speed in real time. It is a signal delivering an analog voltage or a proportional pulse signal that allows the controller to know exactly how fast the fan is spinning.

 

Use for monitoring

The tachometer output serves as a feedback signal in several applications where rotational speed must be monitored without requiring direct regulation by PWM or 0-10V.

• Fault diagnosis: Management systems can monitor the tachometer signal to detect anomalies. A sudden drop in speed or complete stoppage indicates an electrical or mechanical fault, such as a seized bearing or blocked blade. This allows proactive maintenance before the problem causes more serious damage.
• Airflow balancing control: In complex ventilation systems with multiple fans, the tachometer helps balance the airflow. By measuring the speed of each fan, the controller can manually or automatically adjust the air resistance so that all fans run at a similar speed, ensuring uniform airflow throughout the system.
• Safety and compliance: In some environments, fan speed must be verified for safety reasons (e.g., in smoke extraction systems). The tachometer signal provides proof of operation, often recorded by a building management system (BMS), for audits and regulatory compliance.

 

Examples

• Fixed-speed fans: A fan may be designed to operate at a constant speed. The tachometer output is used solely to confirm that the fan is rotating at that speed. If the tachometer signal indicates a lower speed or zero speed, an alarm may be triggered.
• AC-powered fans: Industrial AC fans, often single-speed, use the tachometer for routine monitoring. The speed is generally stable, but fluctuations may indicate a problem with the motor or power supply.
• Redundancy systems (N+1): In critical systems (servers, clean rooms), multiple fans operate in parallel. The tachometer on each fan verifies that all fans are operating properly. If one fan fails (zero tachometer signal), the other fans can compensate, or an alarm is triggered to signal the failure.
• Manually controlled systems: An operator can adjust the speed of a fan with a rheostat or switch. The tachometer signal is displayed on a dashboard, allowing the operator to view the actual speed and confirm that their adjustment has had the desired effect.

 

Use with speed commands

This signal is crucial for the control loop. Without the tachometer, a control system could only send a command without knowing whether it is being executed correctly. Here's how it interacts with control methods:

• PWM (Pulse Width Modulation) control: A controller sends a PWM signal to modulate the fan power. The tachometer signal returns the actual rotation speed, allowing the controller to adjust its PWM signal to reach the target speed. For example, if the controller sends a 50% PWM signal to achieve 1500 rpm but the tachometer only reports 1300 rpm, the controller can increase the PWM to 55% to compensate.
• 0-10V control: Similar to PWM, the controller sends an analog voltage (between 0 and 10 volts) to set the speed. The tachometer provides the feedback needed to adjust this voltage in case of deviation from the desired speed.

 
In short, the tachometer output is the controller's “eye,” allowing it to regulate speed accurately and dynamically.

 

Alarm output: the safety signal

The alarm output is a binary safety signal (OK/Fault). Its sole purpose is to indicate that a problem has been detected. This signal is independent of the speed control itself, but is often based on speed information.

Alarm triggering

The alarm is usually triggered when:

• The speed measured by the tachometer falls below a minimum threshold (e.g., 1600 rpm), indicating a blockage or malfunction. 
• The fan is completely stopped (zero tachometer signal).
• Power is applied but the fan is not rotating.

The alarm signal alerts the system that corrective action is required (e.g., shut down the machine, alert an operator, or switch to a backup fan) to prevent overheating or damage to the system.

 

Examples

• Server cooling systems: In a data center, a fan alarm triggers a critical alert to prevent equipment from overheating. If a server fan stops, the alarm output alerts the computer room management system, which can then:

- Notify the technical team.
- Increase the speed of the remaining fans to compensate.
- Initiate an emergency shutdown of the servers to prevent permanent damage.

• Laboratory or fume extraction hoods: People's safety depends on the proper functioning of the extraction system. If a hood fan fails, the alarm signal triggers a visual and audible alert in the laboratory, immediately informing the operator that they must stop work to avoid inhaling hazardous chemicals.
• Industrial ventilation systems: In a factory, the proper functioning of fans is crucial for air quality and worker safety. An alarm can signal a blocked fan in a dust extraction system. The signal is then sent to the production control system, which can:

- Shut down the production line to prevent the accumulation of flammable dust.
- Display a message on a control panel to guide the operator to the problem area.

• Air conditioning and HVAC (Heating, Ventilation, Air Conditioning) systems: An alarm on an air handling unit fan may indicate a major problem affecting the comfort and air quality of a building. The signal is often connected to a building management system (BMS), which can:

- Alert the maintenance team and facilitate their intervention on the targeted fan.
- Activate a backup system if a redundant fan is available.
 
The alarm output is therefore a simple but vital safety device designed to protect equipment, people, and facility compliance.
 
In summary, the tachometer output is dedicated to fine and continuous regulation, while the alarm output is a simple but vital warning signal for system safety.

ETRI fans with a tachometer output end in 13xxx for DC motors and 31xxx for AC motors.
ETRI DC fans with an alarm end in 14xxx. An alarm is possible on AC motors via an alarm relay or a centrifugal safety device.