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How to know if you set the correct current on a motor thermal overload relay

Home / Technical Articles / How to know if you set the correct current on a motor thermal overload relay

The full-load current at a given voltage indicated on the nameplate is normative for setting the overload relay. Because of the variable voltages around the world, motors for pumps are made to be used at both 50 Hz and 60 Hz in a wide voltage range.

Therefore, a current range is indicated on the motor’s nameplate. The exact current capacity can be calculated when we know the voltage.

Calculation example

When we know the precise voltage for the installation, the full-load current can be calculated at 254 Δ/440 Y V, 60 Hz. The data is indicated on the nameplate as shown on the illustration on below:

• f = 60 Hz
• U = 220-277 ∆/380 – 480 Y V
• In = 5.70 – 5.00/3.30 – 2.90 A

60 Hz data calculation:

• Ua = actual voltage 254 ∆/440 Y V (actual voltage)
• Umin = 220 ∆/380 Y V (Minimum values in the voltage range)
• Umax = 277 ∆/480 Y V (Maximum values in the voltage range)
The voltage ratio is determined by the following equations:

UΔ = (UA – Umin) / (Umax – Umin)
which is in this case: UΔ = (254 – 220) / (227 – 220) = 0.6

UY = (UA – Umin) / (Umax – Umin)
which is in this case: UY = (440-380) / (480-380) = 0.6

So, UΔ = UY

Calculation of the actual full-load current (I)

Imin = 570/3.30 A
(Current values for Delta and Star at minimum voltages)

Imax = 500/2.90 A
(Current values for Delta and Star at maximum voltages)

Now, it is possible to calculate the full-load current by means of the first formula:

• I for Delta values: 5.70 + (5.00 – 5.70) × 0.6 = 5.28 = 5.30 A
• I for Star values: 3.30 + (2.90 – 3.30) × 0.6 = 3.06 = 3.10 A

The values for the full-load current correspond to the permissible full-load current of the motor at 254 ∆/440 Y V, 60 Hz.

Rule-of-thumb: The external motor overload relay is always set to the nominal current shown on the nameplate.

However if motors are designed with a service factor, which is then shown on the nameplate eg. 1.15, the set current for the overload relay can be raised by 15% compared to full-load current or to the service factor amps (SFA) which is normally indicated on the nameplate.

If the motor is connected in star = 440 V 60 Hz the overload relay then has to be set to 3.1 A.

Reference // The motor book by Grundfos

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

1. Razi abbas
Jun 29, 2020

Do you have diagram for relay

2. aklilu
May 25, 2020

it is a good document.

3. Clement Amegnigan
Dec 02, 2019

I would like to have all the news about electrical engineering

4. Maxwell mbiyu
Nov 23, 2019

It was very nice about. Automation knowledge congratulations

5. Owerri Imo State Nigeria
Nov 22, 2019

15 by 12 meters conference Hall Electrical piping and wiring.

• luckson
Dec 09, 2019

95 Eden North Sunnyside pretoria

6. Subrata Kumar Bera
Nov 22, 2019

The above explanation is good . Normally , O/L replay is connected in one of the phase of delta connection in the starter panel. In case of high HP motor , we provided CT in 3 phase connection for current & voltage checking. In that case can we provide O/L Relay by using CTs in the main income of starter panel.

7. Apolonio Nisperos
Nov 21, 2019

we can also check the actual current and set the overload relay.

8. Apolonio Nisperos
Nov 21, 2019

always refer to the motor nameplate

9. Sanket Patil
Sep 13, 2019

Setting a pick up current for an overload relay is specified in any IEC standard

10. ronaldo cortez
Jul 24, 2019

this only for 3phase how about single phase connection

11. Shafi
Mar 29, 2019

Good explanation . Need specific value of current at which relay should trip the motor with 440V Y and 3.1A setting.
Also plz explain if overload setting is made at nominal current of motor, mentioned on nameplate . Then what is the use of 1.15 or 1.25 times rated current setting. Thanks

12. SLTAN TEKLAY
Dec 08, 2018

13. SAYAK DAS
Oct 29, 2018

THANK YOU SIR FOR THE EXPLANATION .I WANT TO KNOW IF THE FREQUENCY IS 50 HZ THEN CAN I USE THIS FORMULA? PLEASE GIVE ME THE ANSWER
THANK YOU

14. Umar
Sep 20, 2018

I need more diagrams about motor connections

15. Glen Sullivan
Sep 20, 2018

Thank you for a good explanation. I have a couple of people I work with that believe if you have a fast acting overtemp relay, you dont need overloads. If it gets hot, it’s overloaded. Not very fast enough for lost phases or locked rotors. I am not sure if it’s even legal as the standard calls for overload fitted to motors over 370w. Has anyone here got a correct answer?

• Samir Ismail
Dec 02, 2018

Dear Sullivan
Normally the Fast Acting O.T.R. have an embedded sensor(s) in the motor armature coils, suppose for some reasons these sensors are no more working, then your motor is not protected at all.
It’s preferable to use the usual Over Load protection mean as the “Main Protection One” and the F.A.O.T.R. as a “Backup”, in this case you realize a complete protection of your motor against the overload events.

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