Overload or Thermal Protection (ANSI 49)
Introduction
Overload or thermal protection protects all types of motor applications against overload currents.
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The long-time protection is set by two dials according to the starting characteristics of the application. The pickup setting Ir for trip unit long-time protection is expressed in amperes:
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Operating Principle
Overload or thermal protection is I2t IDMT (Inverse Definite Minimum Time):
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It incorporates the motor thermal image function.
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It can be configured as the Ir pickup and as the trip class (Class).
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Item |
Parameter |
Description |
|---|---|---|
|
A |
Ir |
Overload or thermal protection pickup |
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B |
Class |
Overload or thermal protection trip class (according to IEC/EN 60947-4-1 standard) |
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 unit rating In (A) |
150 A |
220 A |
320 A |
500 A |
|---|---|---|---|---|
|
Pickup Ir (A) |
70 |
100 |
160 |
250 |
|
80 |
120 |
180 |
280 |
|
|
90 |
140 |
200 |
320 |
|
|
100 |
155 |
220 |
350 |
|
|
110 |
170 |
240 |
380 |
|
|
120 |
185 |
260 |
400 |
|
|
130 |
200 |
280 |
440 |
|
|
140 |
210 |
300 |
470 |
|
|
150 |
220 |
320 |
500 |
Trip Class Setting Value
The trip class (Class) is set by using an adjustment dial:
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Class 5
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Class 10 (default value)
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Class 20
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:
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Current in the load |
Tripping time (in seconds) |
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|---|---|---|---|
|
Class 5 |
Class 10 |
Class 20 |
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|
1.5 x Ir |
96-120 |
192-240 |
320-400 |
|
6 x Ir |
5.2-6.5 |
10.8-13.5 |
20.8-26 |
|
7.2 x Ir |
4-5 |
8-10 |
16-20 |
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:
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A Limit temperature curve for copper B Limit temperature curve for iron C Tripping curve (low envelope) |
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):
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Diagram A  |
Diagram B  |
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0 Load control (cyclical) 1 Motor temperature 2 Thermal level calculated without thermal image (diagram A), with thermal image (diagram B) 3 Overload or thermal protection level |
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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).