Selecting Thermal Overload Relays for Motor Starters

Motor rating depend upon torque-speed requirement for a specific function. As motor rating increases, current in line increases. So, ultimately we need the starter accordingly.

” Motors are electrical machines fulfil with courage & bravery !”

” Motor run normally at no load condition with normal no load current. As load increases, it takes more current from the circuit & tries to pull the load. When load get more than the motor capacity even than Motor tries to produces more torque to pull the load.

In doing so it takes much larger current than its capacity. It continuously takes more & more current until the current get stable at highest level. At this stage, motor damages it-self ; By deteriorating its winding.

That’s why we are saying; Motors are fulfil with courage & bravery !“

Every motor has particular rated current; Up to that rated current, It works safely & precisely under the safe temperature, vibration & noise limits. That’s why Motor should run under the rated current. Selection of thermal overload relay & back up fuses should be according to the rated current.

“Overload relay’s current setting neither be high nor be low ! Relay with high current range is dangerous for the motor on the other hand low range don’t allow the motor to attain it’s full capacity. That’s why right selection of thermal overload relay is very important. “

 

How does thermal overload relay Works?

The thermal overload relay provides from two conditions; 1st is short circuit condition & 2nd is overload condition. Both the conditions works with two different types of releases/safeties.

1. Electromagnetic release– This release works in short circuit conditions Which is current dependent. Sudden rise of current through the line initiate this release. In this Load current flows through electromagnetic coil, sharp rise of current through this coil create magnetic push which is used in tripping the circuit.
2. Thermal bimetallic release– This release works in overload conditions which depend on both current & time. This release works due to thermal bimetallic strips through which load current flows. When high current for particular amount of time flow through these thermal bimetallic strips; Due to high generated temperature; these strips bend which further pushes the tripping circuit.

 

This bending of metallic strips known as tripping which is being connected to mechanical mechanism which further changes the position of contacts either NC OR NO OR Both NO+NC.

As in MK-1 relay by L&T, In case of tripping; NC contact get open which further get reset after pressing the reset button but reset button works only When thermal bimetallic strips get cool down.

 

Normally, Supply to contractor’s coil passes through the NC of overload relay. Whenever current exceeds the limit; overload relay trips; NC contact get opened which results supply to contactor get break off.

 

These overload relays are equipped with ampere setting range through which current setting is adjusted. Generally, the setting current is the rated current of motor which is not the tripping current.

No tripping occurs at 1.05×Isetting current

Tripping occurs at 1.2×Isetting current.

Selection of thermal overload relay also depend upon the type of starter through which Induction motor is running on.

For Induction motor, there are two type of starters.

1. DOL Starter

2. STAR-DELTA Starter

 

In DOL Starter, Power supply to motor is connected directly without any change in frequency, voltage & winding connections That’s why known as Direct online starter. This starter has only one single power line through which entire load current flows. .More about DOL| Since in this starter, entire load current flows through single line that’s why we need overload relay same as per according to the rated current of motor.

Motor’s winding connection changes in this starter. At the time of starting, motor runs in star connection while after that motor runs in Delta connections until motor get stopped.

In STAR connection, load current flows in single line while in Delta; current get divided into 2 lines. .More about Star-Delta| In this starter, Star connection appears only for few moment at the time of starting & rest Delta connection works. In Delta connections, current get divided in two lines. That’s why we need thermal overload relay half of the rated current of motor.

Lets understand this with an Example

Let suppose we have :

Induction motor

Power=7.4hp

RPM= 1440 

Rated Current= 12 Amps approx.

• In DOL Starter we need thermal overload relay ranges from 9 to 14 Amps Because full current flows through the overload relay .
• In STAR-DELTA Starter we need thermal overload relay ranges from 6 to 10 Amps Because half divided current flows through thermal overload relay.
• From short circuit protection, rating of back up circuit breaker must be 2-2.5 times the rated current of motor.

 

SUMMARY

• This post discusses the essential functions of an electrical motor starter, which include starting the motor and providing protection against overload and phase failure.
• Key components of starter comprise a contactor, thermal overload relay, and push button assembly.
• Proper selection of contactor and thermal overload relay ratings is crucial for motor safety, as they depend on motor ratings.
• The thermal overload relay functions through electromagnetic and thermal bimetallic releases to protect against short circuits and overloads.
• This  post covers selection criteria for thermal overload relays for Direct-On-Line (DOL) and Star-Delta starters, emphasizing the importance of correct settings to prevent motor damage during operation.

FAQs: Selecting Thermal Overload Relays for Motor Starters

Q1. What is the function of a thermal overload relay in a motor starter?
A thermal overload relay protects the motor against overload and phase failure conditions and helps prevent overheating of the motor windings.

Q2. Why is correct selection of a thermal overload relay important for motors?
Correct relay selection ensures that the relay trips during motor overload while still allowing the motor to deliver its full load current safely.

Q3. What happens if the overload relay current setting is too high or too low?
A higher setting may fail to protect the motor during overload, and a lower setting may cause the relay to trip earlier, preventing the motor from delivering its rated performance.

Q4. How does a thermal overload relay respond to overload conditions?
The relay uses a bimetallic strip that bends when heated by overload current. This mechanical action opens the control circuit and stops the motor.

Q5. How does the thermal overload relay’s tripping characteristic work?
It follows inverse time characteristics — higher the overload current, faster the relay trips.

Q6. Do thermal overload relays trip exactly at their setting current?
No. The setting typically matches the motor’s rated current, but actual tripping occurs at a multiple of the setting current after a certain time.

Q7. How should a thermal overload relay be selected for a DOL starter?
In a DOL starter, the full motor current flows through the relay, so it must cover the motor’s full rated current range.

Q8. How does relay selection differ for a Star-Delta starter?
In Star-Delta starters, the motor current in star connection is lower, so the relay range can be roughly half of the motor’s rated current range.

Q9. What protection does a starter provide besides starting the motor?
Motor starters provide overload and phase failure protection through contactors, thermal relays, and push button assemblies.

Q10. Why do motors draw more current under mechanical load?
As load increases, the motor demands more torque, which requires more current, leading to heating if sustained overload occurs.