Overload units have previously been described in this article, the use of the type incorporating directly heated elements are not considered for incorporation into starters as the electronic units are more accurate, have greater repeatability accuracy, have wider setting ranges, generate less heat into the starter and are now of similar cost.
The voltage rating of the overload unit where it is connected in the power circuit and not driven by current transformers must be equal to or greater than the electricity supply voltage.
Directly connected overload units must have type 2 coordination when used in combination with the selected fuses and contactors.
Overloads must be selected so that the motor class and full load current is within their setting ranges.
Where a motor has multiple windings or is of the Dahlander type with tapped windings then each winding requires a separate overload unit, again the motor class and full load current must be within their setting ranges.
Starters for motors with an output rating of 45kWo or more should incorporate into the overload unit PTCR (positive temperature co-efficient resistors, more commonly known as thermistor) protection. Whereas the overload unit determines the motor temperature by mathematical modelling it cannot take into account ambient conditions at the motor’s location nor can it take into account the motor’s physical condition; it’s cooling system efficiency will alter if the motor is dirty, dusty or has impediments to it’s cooling air flow. In the modern world with all it’s electronic devices the electricity supply is dirty, that is there are many harmonics in the electricity supply, harmonics generate heat in a motor which may not be detected in the overload units, those harmonics may be exacerbated by soft start and variable frequency units. This is acceptable for smaller motors, for larger machines the incorporation of PTCR temperature sensors allows accurate temperature monitoring of the motor and will trip it on its’ temperature rising above a preset limit.
Overload units must have two electrically separate manually rest contacts, a normally closed which opens when an overload occurs and a normally open which closes on overload. The normally closed contact is used in the fault circuit and the normally open in the SCADA circuit.