How to deal with overload faults of the motor?
Overload relay (thermal or electronic)
Overload is the most common fault in a motor. If the current drawn by the motor increases or a thermal effect occurs, it can cause an overload. The insulation category of the relevant standard sets the ambient temperature at which the motor heats up to 40 °C. If it exceeds this range, it will lead to premature aging of the insulation material and shorten the service life of the motor.
However, it should be noted that if the overheating overload is short and infrequent, it will not immediately cause damage. In this case, it is not necessary to stop the operation of the motor, but it is best to resume normal operating conditions as soon as possible.
The importance of taking the correct protective measures against overload is obvious: it prevents the motor from operating under overheating conditions, thus ensuring its normal service life.
The continuity of the operation is ensured by the protection of the sudden stop of the motor and the restart of the person and the device in the safest state after the trip.
When determining the motor operating specifications (power, current) and selecting the appropriate overload protection function, consider, for example, the actual operating conditions of the motor (temperature, altitude, and standard load). The derating value is given by the motor manufacturer.
Depending on the level of protection required, overload protection can be provided via relays:
Overload, thermal (bimetal) or electronic relays provide the lowest level of protection for:
Overload, by controlling the current drawn by each phase
Phase imbalance or phase loss, through difference device
Positive Temperature Coefficient (PTC) Thermistor Detection Relay
Over torque relay
Multi-function relay.
Overload relay (thermal or electronic)
Please note:
The protective relay does not disconnect the circuit. It opens a breaking device with the required breaking capacity in the event of a faulty current, usually a contactor.
Therefore, a fault contact (NC) is mounted in series with the contactor coil in the protective relay.
These relays protect the motor from overload, but they must be able to withstand temporary overloads during start-up, ensuring tripping only when the starting time is too long.
Different applications have different starting times, ranging from a few seconds (no-load starting, low-resistance torque, etc.) to tens of seconds (high-resistance torque, high inertia of the drive load, etc.).
In order to adapt the relay to different starting times, the IEC 60947-4-1 standard specifies several types of overload relays, each defining a corresponding trip time.
The size of the relay should be selected based on the rated current of the motor and the estimated starting time.
The graph on the left shows the usage constraints associated with the time and current settings (multiples of Ir).
In addition to some electronic components, these relays also have a thermal memory device (as manufactured by the manufacturer) that can be connected as follows:
In series with the load, in the case of high power, it can be connected to the inverter in series with the load.
