Explanation of Thermal
Overload Relay Operation
How a thermal overload relay operates;
A thermal overload relay protects the motor
from overcurrent by utilizing heat-generated resistance in the internal
heater.
When an overcurrent occurs, the heater generates heat,
which bends the internal bimetal strip in one direction.
This bending activates the trip mechanism,
thereby disconnecting the circuit and protecting the motor from damage due to
excessive current.
1. Classification of
Non-Differential Type (2H vs. 3H)
The Non-Differential Type is categorized into
two types:
- 2H (2 Bimetal/Heater Type)
- 3H (3 Bimetal/Heater Type)
Differences Between 2H and
3H
- The 2H type lacks a bimetal/heater element in
the S-phase, meaning it cannot detect overcurrent in the S-phase.
- However, it can detect overcurrent in the R
and T phases, or when all three phases (R, S, T) experience
overcurrent.
- The 3H type has bimetal/heater elements
for all three phases, allowing it to detect and protect against
overcurrent in all phases.
2. Difference Between
Non-Differential and Differential Types
The key difference between Non-Differential Type
and Differential Type is whether phase loss (open-phase) protection is
included.
- Phase loss (open-phase) can cause more severe
damage to a motor than regular overcurrent.
- Because of this, the Differential Type is
designed to react more quickly to overcurrent by incorporating a differential
amplification mechanism (please refer to the technical document
diagram for details on the working principle).
Differential Type (3K)
Features
- The 3K type includes bimetal/heater elements
for all three phases, allowing it to provide standard overcurrent
protection while also offering enhanced phase loss detection.
3. Difference Between Trip
Classes 10A (2H, 3H, 3K) and 20 (3D)
The difference between Trip Class 10A and 20 is
based on the response time when overcurrent occurs.
- Class 10A (2H, 3H, 3K) → Activates within 2 to 10 seconds when an
overcurrent of 720% occurs.
- Class 20 (3D) → Activates
within 6 to 20 seconds when an overcurrent of 720% occurs.
Most general-purpose motors use Class 10A.
However, for motors with high inertia (long startup time), using Class
10A may cause the relay to trip during startup.
In such cases, Class 20 is used instead to
allow for longer startup times before tripping.
4. Performance Ranking
(From Least to Most Advanced)
The following is the performance ranking of these
overload relays, listed in order of increasing functionality and protection:
- Non-Differential Type (2H) → Basic overcurrent protection, but lacks S-phase
detection.
- Non-Differential Type (3H) → Overcurrent protection in all three phases.
- Differential Type (3K) → Includes phase loss protection with enhanced overcurrent
response.
- Differential Type (3D) → Similar to 3K but with a longer trip time (Class 20)
for high-inertia motors.
There is a lot more details and diagrams to understand
our TORs better in our catalog. The information regarding our relays start on
page 208 of Catalog [Metasol_MS]_EN_C04002-36-202504.pdf which can be located on this website: https://www.ls-electric.com/support/download-center