Start Fail. The fire pump does not start. Troubleshooting

Greetings! Today our fire pump does not start; the “Start Fail” signal is displayed on the motor controller. The Reset button does not reset the error.

We open the starter panel, open the fire pump start-up and operation circuit, and look for the “Start Fail” signal. The motor controller produces this signal in many cases.

Let's look at some of them, and also look at this specific example.

Motor controller

General circuit diagram of the start and operation of the fire pump electric motor

Inside the starter panel

Motor controller rear panel

This may be a problem with the RY1 starter relay or with contactors 88M, 88D, 88Y. There may also be a 19T timer, which is involved in the star-delta motor starting circuit. In general, anything that works in the motor starting circuit can affect the output of the "Start Fail" signal.

Start Fail on the diagram

The first thing that is checked in this case is thermal protection. If the thermal relay is tripped, the Reset button on the controller will not work until until protection is restored.

What is a thermal relay for an electric motor?

A thermal relay is a protective device used to safeguard electric motors from overheating due to excessive current. It functions by monitoring the current flow and tripping the circuit when the current exceeds a predetermined limit for a specific duration. This prevents potential damage to the motor caused by prolonged overloading or phase failure.

Key Components and Working Principle:

1. Bimetallic Strips:

   • Composition: Two different metals with distinct thermal expansion coefficients bonded together.
   • Function: When heated by the motor current, the strip bends due to the different expansion rates, eventually triggering the relay.

2. Heater Elements:

   • Function: Connected in series with the motor, these elements heat up in proportion to the current passing through them, indirectly heating the bimetallic strip.

3. Contacts:

   • NO (Normally Open): Open when the relay is in a normal state and close when the relay trips.
   • NC (Normally Closed): Closed when the relay is in a normal state and open when the relay trips.
   • These contacts are used to break the motor circuit and initiate protective measures.

4. Adjustment Dial:

   • Allows setting the current rating according to the motor’s full load current.

Types of Thermal Relays:

1. Bimetallic Thermal Relays:
   • Utilize bimetallic strips and are common for general applications.
2. Electronic Thermal Relays:
   • Use electronic sensors and microcontrollers for more precise protection.

Applications:

• Overload Protection: Protects motors from sustained overload conditions by disconnecting the supply if the motor draws current above a set threshold.
• Phase Failure Protection: Detects single-phase conditions and trips the motor to prevent damage due to uneven loading.

Installation and Maintenance:

• Proper Sizing: Ensure the relay is rated appropriately for the motor’s full load current.
• Periodic Testing: Regularly test the relay to ensure its proper functioning.
• Environment Consideration: Install in environments free from excessive dust, moisture, and vibration for optimal performance.

Thermal relays are crucial for the safe operation of electric motors. By ensuring that motors are not subjected to damaging conditions, they enhance the longevity and reliability of motor-driven systems.

Thermal protection check (in the photo the contact is closed and gives 0.1 Ohm)

To check (in a de-energized state), use a multimeter to measure the resistance of contact 95-96 of thermal relay 51. In good condition, this contact (in diagram 04 - 08) should be normally closed.

NC contact of thermal relay

Normally open thermal relay contact (for signaling)

In our case, the contact gives an “open”, the resistance is infinity (in the photo the contact is closed, for example). This means the thermal relay has tripped.

The thermal relay has a special red flag and a Reset button for resetting. If the red flag comes out, it means that the relay has been tripped. Thermal relays are different and there are not always such red flags for clarity (they can be white and instead of extending out they can fall inward), so you should always check the NC thermal contacts with a multimeter in such a situation.

Contactor and thermal relay

The Reset button does not reset the thermal relay. You need to make sure that the electric motor is not damaged and the problem is only in the thermal relay. To do this, we check the insulation resistance of the motor relative to the ground; this is usually done in the phases after the thermal relay.

Checking the insulation resistance of the electric motor (circuit de-energized)

You also need to check the motor for an interturn short circuit, if possible (check the insulation resistance between each phase, they must match each other). This procedure is best done in the motor terminal box.

After we are convinced that the motor is "not sitting on the ground" and there is no inter-turn short circuit, we replace the thermal protection (relay) with a new one.

As a result, the motor was tripped due to a malfunction of the relay itself. The “OVER CURRENT” signal did not come out.

Old thermal relay (disassembly)

Old thermal relay (red flag)

After replacing the thermal relay we hang a current clamp on one of the phases to measure the starting and rated operating current, and start the electric motor of the fire pump.

Current clamps

The motor operates without any problems, the currents are in the normal operating range. The problem has been resolved and we can disassemble the old thermal relay.

Controller model: HYOMYUNG HMC - CONSH