CHAPTER 4: SETPOINTS PROTECTION869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–127HOT/COLD SAFE STALL RATIORange: 0.01 to 1.00 in steps of 0.01Default: 1The motor manufacturer sometimes provide thermal limit information for a hot/coldmotor. The algorithm uses this data if this setting is programmed. The value entered forthe setting dictates the level at which Thermal Capacity Used settles for current that isbelow the motor overload factor (OL) times FLA. When the motor is running at a levelthat is below this limit Thermal Capacity Used rises or falls to a value based on Ieq(thermal model biased motor load current). Thermal Capacity Used either rises at thefixed rate of 5% per minute or falls as dictated by the running cool time constant.Eq. 19where:• TCUend = Thermal Capacity Used, if Ieq remains steady state.• Ieq = equivalent motor heating current (also defined as Thermal Model Biased MotorLoad Current)• OL = Overload factor specified by the Motor Overload Factor setting.• FLA = Motor rated current specified by the Motor Full Load Amps setting.• hot/cold = Hot/Cold Safe Stall Ratio setting.RTD BIASRange: Disabled, EnabledDefault: DisabledWhen Enabled, this feature acts as an additional check of the motor overheatingthrough the current based thermal model. The current based thermal model estimatesmotor heating from the thermal overload curves and cooling time constants. Thethermal overload curves are based solely on measured current, assuming a normal 40°Cambient temperature and normal motor cooling. This feature provides additionalprotection in cases where there is an unusually high ambient temperature, or motorcooling is malfunctioning, or motor temperature increases due to other unexpectedfactors, or the overload curve was selected incorrectly. Therefore, if the motor stator hasembedded RTDs, the RTD Bias feature is used to augment the thermal model calculationof Thermal Capacity Used.This feature uses the hottest stator RTD temperature value toestimate the RTD Thermal Capacity Used and compare this value to the ThermalCapacity Used calculated by the current based thermal model (overload curve and cooltimes). The larger of the two values is used from that point onward. Since RTDs have arelatively slow response, RTD biasing is useful for slow motor heating. Other portions ofthe thermal model are required during starting and heavy overload conditions whenmotor heating is relatively fastNOTENOTE:1. The RTD Bias feature is active only if the optional RTD Input module has been installed.2. Each stator RTD must be first configured as STATOR application under Setpoints/Monitoring/RTD Temperature/RTD 1(X). RTDs configured as Stator type are used by thethermal model for determining the RTD Bias.3. The RTD bias feature alone cannot generate a trip. Even if the RTD bias feature forcesthe RTD bias thermal capacity used to 100%, the load current must be above theoverload pickup (OL x FLA) setting to set the output.
