Thermal Overload Protection Formula

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.

Thermal overload protection formula.

Check the motor nameplate for the ambient temperature rating and the insulation class ratings. C 104 deg. Therefore this type of relay is used in conjunction with fuses to provide overload and short circuit protection to the circuit. 1 ib in iz ib anticipated operating current of the circuit iz current carrying capacity of conductor or cable or motor.

For understanding motor thermal overload protection in induction motor we can discuss the operating principle of three phase induction motor. However thermal overload protection can be used to trip a circuit breaker as well. Temperature rise is proportional to the current squared. In either case the presence of thermal overload can be detected and removed before cable failure occurs.

Thermal overload protection is normally used in an alarm mode to notify system operators of the potential for cable damage. 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. Electric motors are commonly rated for operation in an ambient temperature of 40 deg. This is the principle behind the thermal replica model of a motor used for overload protection.

The temperature t at any instant is given by. 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. The fundamental equation 6 can be expressed in a simpler form. There is one cylindrical stator and a three phase winding is symmetrically distributed in the inner periphery of the stator.

U dt du i2 τ 12 the solution of the first order equation is. 35 ampere x 125 43 7 ampere 44 ampere. 40 ec and service factor. The thermal overload relay does not provide short circuit protection as it takes sufficient time to open the contacts.

Tmax final steady state temperature τ heating time constant. 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. Basically to setting the overload value we refer to this formula.