Thermal Overload Protection Formula

Check the motor nameplate for the ambient temperature rating and the insulation class ratings.

Thermal overload protection formula.

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. Temperature rise is proportional to the current squared. However thermal overload protection can be used to trip a circuit breaker as well. 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.

For understanding motor thermal overload protection in induction motor we can discuss the operating principle of three phase induction motor. 40 ec and service factor. C 104 deg. U dt du i2 τ 12 the solution of the first order equation is.

The temperature t at any instant is given by. Tmax final steady state temperature τ heating time constant. Thermal overload protection is normally used in an alarm mode to notify system operators of the potential for cable damage. Answer ambient temp 40 ec.

35 ampere x 125 43 7 ampere 44 ampere. The thermal overload relay does not provide short circuit protection as it takes sufficient time to open the contacts. Basically to setting the overload value we refer to this 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.

Therefore this type of relay is used in conjunction with fuses to provide overload and short circuit protection to the circuit. This is the principle behind the thermal replica model of a motor used for overload protection. 1 ib in iz ib anticipated operating current of the circuit iz current carrying capacity of conductor or cable or motor. Electric motors are commonly rated for operation in an ambient temperature of 40 deg.

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