Thermal Overload Protection Formula

However thermal overload protection can be used to trip a circuit breaker as well.

Thermal overload protection formula.

40 ec and service factor. Temperature rise is proportional to the current squared. Due to such symmetrical distribution when three phase power supply. Electric motors are commonly rated for operation in an ambient temperature of 40 deg.

1 ib in iz ib anticipated operating current of the circuit iz current carrying capacity of conductor or cable or motor. U dt du i2 τ 12 the solution of the first order equation is. R r θ dt dθ r r 1 s i c m r 2 8 s τ c m r 9 let r r u θ 10 and dt d r r 1 dt du θ 11 therefore the first order thermal model equation becomes the simple form. The thermal overload relay does not provide short circuit protection as it takes sufficient time to open the contacts.

C 104 deg. τ t. 35 ampere x 125 43 7 ampere 44 ampere. The temperature t at any instant is given by.

In either case the presence of thermal overload can be detected and removed before cable failure occurs. Basically to setting the overload value we refer to this formula. Thermal overload protection is normally used in an alarm mode to notify system operators of the potential for cable damage. This is the principle behind the thermal replica model of a motor used for overload protection.

Calculated sizing for overload device to provide overload protection to a 3 phase 200 volt 15 hp motor with full load ampere is 35 ampere ambient temp. Therefore this type of relay is used in conjunction with fuses to provide overload and short circuit protection to the circuit. Check the motor nameplate for the ambient temperature rating and the insulation class ratings. For understanding motor thermal overload protection in induction motor we can discuss the operating principle of three phase induction motor.

Tmax final steady state temperature τ heating time constant. Answer ambient temp 40 ec. If the motor in the example from step 2 had a service factor of 1 15 then its overload would be acceptable and the motor could be operated without damage.