Thermal Overload Protection Formula

The fundamental equation 6 can be expressed in a simpler form.

Thermal overload protection formula.

However thermal overload protection can be used to trip a circuit breaker as well. 35 ampere x 125 43 7 ampere 44 ampere. 40 ec and service factor. τ t.

U dt du i2 τ 12 the solution of the first order equation is. Temperature rise is proportional to the current squared. Thermal overload protection is normally used in an alarm mode to notify system operators of the potential for cable damage. Basically to setting the overload value we refer to this formula.

The temperature t at any instant is given by. 1 ib in iz ib anticipated operating current of the circuit iz current carrying capacity of conductor or cable or motor. 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. Therefore this type of relay is used in conjunction with fuses to provide overload and short circuit protection to the circuit.

There is one cylindrical stator and a three phase winding is symmetrically distributed in the inner periphery of the stator. This is the principle behind the thermal replica model of a motor used for overload protection. Due to such symmetrical distribution when three phase power supply. C 104 deg.

Tmax final steady state temperature τ heating time constant. Check the motor nameplate for the ambient temperature rating and the insulation class ratings. Answer ambient temp 40 ec. In either case the presence of thermal overload can be detected and removed before cable failure occurs.

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. 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.