Thermal Overload Protection Formula

1 ib in iz ib anticipated operating current of the circuit iz current carrying capacity of conductor or cable or motor.

Thermal overload protection formula.

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. The thermal overload relay does not provide short circuit protection as it takes sufficient time to open the contacts. Electric motors are commonly rated for operation in an ambient temperature of 40 deg. For understanding motor thermal overload protection in induction motor we can discuss the operating principle of three phase induction motor.

Due to such symmetrical distribution when three phase power supply. 40 ec and service factor. Thermal overload protection is normally used in an alarm mode to notify system operators of the potential for cable damage. 35 ampere x 125 43 7 ampere 44 ampere.

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. Tmax final steady state temperature τ heating time constant. Answer ambient temp 40 ec. C 104 deg.

τ t. There is one cylindrical stator and a three phase winding is symmetrically distributed in the inner periphery of the stator. Protection point 2500 kva 2500 kva ansi 17 6 x i fl x 58 16 6 x i fl x 58 inrush for 0 1 sec 8 x i fl 12 x i fl nec rule 6 x i fl 6 x i fl 6 x i fl time current curves transformers 29. Check the motor nameplate for the ambient temperature rating and the insulation class ratings.

However thermal overload protection can be used to trip a circuit breaker as well. Basically to setting the overload value we refer to this formula. This is the principle behind the thermal replica model of a motor used for overload protection. Temperature rise is proportional to the current squared.

The fundamental equation 6 can be expressed in a simpler form. Therefore this type of relay is used in conjunction with fuses to provide overload and short circuit protection to the circuit. 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.