An alarm relay is a device that converts a tachometer signal into an alarm signal. It is particularly useful when the equipment cannot provide an alarm directly, or when the customer is unable to interpret the tachometer signal from the motor.

Why use an alarm relay?

An alarm relay is essential in several situations:

• For AC motors: it is impossible to extract an alarm signal directly.
• For DC motors: the customer cannot use or convert the tachometer signal into an alarm.

The relay therefore ensures reliable monitoring of the motor speed and generates an alarm if the speed falls below a critical threshold.

Setting the trigger speed

Each alarm relay model allows you to set the frequency (and therefore the motor speed) at which the alarm is triggered.

RA104 - 25 Hz

Tacho 1* pulse per revolution: equivalent to 1500 rpm

Tacho 2* pulses per revolution: equivalent to 750 rpm

RA105 - 15 Hz

Tachy 1* pulse per revolution: equivalent to 900 rpm

Other configurations are possible depending on the application.

*Depends on the fan reference or customer request

The different types of alarm relays

1. DC-powered alarm relays

1-channel relay (RA105)

• Converts the signal from a single device
• Ideal for simple, individual monitoring

Multi-channel relay (RA112)

• Allows up to 3 devices to be connected
• The relay triggers an alarm if one of the fans falls below the programmed threshold

               Advantages:

• Simplified wiring
• Saves space in the cabinet
• Potential cost savings

2. AC-powered alarm relays (RA107)

For customers who do not have a DC power supply, the RA107 model, which is powered directly by AC mains voltage, is the ideal solution for AC fans.

Alternative: centrifugal safety switch for AC motors

When the customer does not want to use a relay or does not have a DC power supply, there is a simple and reliable alternative: the centrifugal safety switch.

This is a mechanical device, without electronics, based on the centrifugal force generated by the rotation of the rotor.

How centrifugal safety switch works

The device has weights mounted on articulated arms, attached to the motor shaft.

When the shaft rotates, centrifugal force pushes the weights outward.

If the speed exceeds the predefined threshold, the weights activate an electrical switch (dry contact) to signal that the speed is sufficient.

If the speed drops below the threshold, the weights return to their initial position and the contact switches back.

This solution is particularly appreciated for its reliability, lack of electronics, and ease of integration.