thermal overload trip time calculation

Overload or Thermal Protection (ANSI 49)

Introduction

Overload or thermal protection protects all types of motor applications against overload currents.

Operating Principle

Overload or thermal protection is I 2 t IDMT (Inverse Definite Minimum Time):

o It incorporates the motor thermal image function.

o It can be configured as the Ir pickup and as the trip class (Class).

Tripping curve:

Ir Pickup Setting Value

The overload or thermal protection pickup (Ir) is set by using a multi-position dial.

The default Ir pickup setting value is 0.4 x In (minimum dial value).

The overload or thermal protection tripping range is 1.05–1.20 x Ir according to IEC/EN 60947-4-1 standard.

The following table shows the preset values of the adjustment dial Ir in amperes for each current rating In:

Trip Class Setting Value

The trip class (Class) is set by using an adjustment dial:

o Class 10 (default value)

The trip class corresponds to the value of the tripping time for a current of 7.2 x Ir according to IEC/EN 60947-4-1 standard.

The following table shows the value of the tripping time depending on the current in the load for all three classes:

The precision range is -20%, + 0%

Motor Thermal Image

The model representing heat rise and cooling in a motor load is constructed according to the algorithm for calculating the thermal demand, taking account of the iron and copper losses.

The following figure represents the limit curves for the iron and copper components calculated for class 20:

Thermal Memory

The trip unit uses a thermal memory function to protect the motor from overheating in case of low amplitude repetitive faults.

Electronic protection without thermal memory function does not protect against repetitive faults because the duration of each overload above the pickup setting is too short to cause tripping. However, each overload causes a temperature rise in the installation. The cumulative effect of successive overloads can overheat the system. The thermal memory function remembers and integrates the thermal heating caused by each pickup setting overrun. The thermal memory function remembers the thermal heating values for 20 minutes before or after tripping.

Example:  Comparison of the heat rise calculation without thermal image (diagram A ) and with thermal image (diagram B ):

With thermal image, the trip unit adds the thermal effect of successive current pulses. Tripping occurs based on the actual thermal state of the motor.

Cooling Fan

The thermal image of the motor is calculated taking account of the fact that the motor is self-cooled (fan mounted on the shaft end).

DOCA0161EN-01

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• Introduction
• Motor Failure and Protection
• Protection of Motor Branch-Circuits
• Withstand Ratings
• Melting Alloy Overload Relays
• Automatic Reset
• Ambient-Temperature Compensated Bimetallic Overload Relays
• Thermal Overload Relay Trip Characteristics
• Phase Loss and Phase Unbalance
• Separate Mounting—Melting Alloy Overload Relays
• Replacement Melting Alloy Overload Relays for Class 8536 Starters
• Special Features for Melting Alloy Overload Relays
• NEMA Style Melting Alloy Overload Relays Dimensions
• Class 9998 Contact Units
• Class 9999 Isolated Alarm Contacts
• Class 9998 Jumper Strap Kits
• Other Features
• Class 9065 Motor Logic SSOLR Selection Tables
• NEMA Style Dimensional Diagrams
• Isolated Auxiliary Contacts
• DIN Adapter
• Lug-Lug and Lug-Extender Kits
• Remote Reset Module
• Adapted Bimetallic or SSOLR Mounting Bracket Adapter—NEMA Sizes 00–1
• TeSys™ D Solid-State Overload Relays for Type S Starters
• Communication Protocols
• Protection Functions
• Meter Functions
• Motor Control Functions
• Motor Control Modes
• Statistical and Diagnostic Functions
• Standards and Certifications
• Configuration
• LTMR Controllers
• Expansion Module
• HMI Modules
• Transformers, Senors, and Probes
• SoMove™ Software for Configuration
• Accessories for Connecting the Components
• Dimensional Diagrams (mm)
• External Reset Mechanisms — Class 9066
• Nameplate vs. NEC Full-Load Current
• Service Factor
• Motor Branch-Circuit Design
• Thermal Overload Relays
• Motor Logic Solid-State Overload Relays
• Application
• Procedure for Thermal Unit Selection
• Selecting Slow Trip Thermal Units
• Selecting Thermal Units

Procedure for Calculating the Trip Current Rating

• Calculating the Trip Current for Ambient Temperatures Other than 40 °C
• Thermal Unit Selection on an Approximate Basis—Based on Horsepower and Voltage
• Mounting the Thermal Units

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Use the selection table for the specific controller involved.

Find the minimum motor full-load current listed for the thermal unit.

Multiply that current by 1.25 (1.15 for Class 8198). The result is the trip current rating.

Determine the thermal unit selection and trip current rating for thermal units in a Class 8536 Type SCG3 Size 1 magnetic starter used to control a three-phase, 1.15 service factor motor with a full-load current of 17.0 Amperes, where the motor and controller are both located in a 40 o C (104 o F) ambient temperature.

From thermal unit Table 13 , the proper selection is B32.

The minimum motor full-load current is 16. 0 Amperes.

Trip current rating is 16.0 x 1.25= 20.0 Amperes.

Protection Level: Is the relationship between trip current rating and full-load current. Protection level, in percent, is the trip current rating divided by the motor full-load current times 100. In the example above, the protection level for the B32 thermal unit is: 20.0/17.0 x 100=118%.

National Electrical Code, Section 430-32, allows a maximum protection level of 125% for the motor in the above example.

Minimum Trip Current: Also called ultimate current, may vary from the trip current rating value, since ratings are established under standardized test conditions. Factors which influence variations include:

The number of thermal units installed

Enclosure size

Proximity to heat producing devices

Size of conductors installed

Ambient (room) temperature, and others

Except for ambient temperature-compensated overload relays, an ambient temperature higher than 40 o C would lower the trip current, and a lower temperature would increase it. This variation is not a factor in selecting thermal units for the average application, since most motor ratings are based on an ambient temperature of 40 °C, and motor capacity varies with temperature in about the same proportion as the change in trip current. Temperature-compensated relays maintain a nearly constant trip current over a wide range of ambient temperature, and are intended for use where the relay, because of its location, cannot sense changes in the motor ambient temperature.

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