| 摘要 |
Relay calculations take into account thermal image values and heating and cooling time constants based on type-testing. For relays applied to motor protection, starting current data and locked-rotor effects, if present, are also included. Both alarm and trip outputs are opt. made available, the latter operating and associated circuit-breaker; in both cases the critical current characteristics take the form of inverse-time curves. For effective protection, the curves calculateed for the plant, e.g. motor, thermal characteristics under hot (Cc2m) and cold (Cf2m) conditions should be slightly above the corresponding curves (Cc2,Cf) for relay operation. The ''cold'' relay curve (Cf) in particular may need to be lowered to meet this requirement, by including in the calculations an allowance for a positive (i.e. non-zero) initial thermal image value for the plant. However, the lowering effect of this on the relay curves must not cause theem to intersect, on motor applications, starting current curves at e.g. nominal (Cd1) or reduced (Cd2) voltage. Preferably, the relay is also provided with an ambient temperature input, used to correct the thermal image value employed. |
