Motor protection is the one of important criteria that we must consider when design the application to avoid serious damage or injury.
Normally we used thermal overload relay to protect electric motor from overload current problem.But if we used soft starter to driven our motor application,the suitable types of protection is using the motor thermal model.
This types provide more precise protection for electric motor and it more accurate and efficient.It only available for high end motor protection relay for conventional starter.
This post i would like to share some information about thermal model protection for soft starter.I explain generally about characteristic,operation and advantage for this thermal protection.
Basic operation of Thermal protection
For this thermal protection it operated depend on constantly of motor temperature,motor design and actual operation system.It can trigger and account for the differential of heating and cooling value when motor is starting,running or stopped.
This system is accurate and allows the motor to operated until maximum capacity without nuisance tripping.It uses an advanced second order thermal model,which models iron and copper losses separately.It provides the full protection for the motor from any damage.
Standard overload relay VS Thermal model protection
For standard thermal overload relays,it are less precise.It not calculated for iron loss or differential of cooling rates at different stages of motor operation.
It also cannot be adjusted to match the characteristics of the individual motor because the mass of the bi-metal strips is fixed setting by manufacture.Basically the bi-metal strips are also affected by their own ambient temperature, which may be different from the motor’s ambient temperature.
For Thermal model protection for soft starter is awesome to inverse time-current and I2T electronic overloads, which do not calculated for iron loss or for different cooling rates at different stages of motor operation.
It only operated for limited adjustment and the trip curves do not closely match motor heating. Inverse time-current protection also does not allow for motor temperature before the overload.