Understanding Motor Duty Rating 

By on

One of the comments on my Motor Starting Series was asking for something on duty cycles.  Here it is. 

As a purchaser of a motor, you have responsibility to let the manufacturer know the anticipated duty of the motor.  To assist in the communication of this information, the standard IEC 60034-1 (Rotating electrical machines) defines several duty characteristics, denoted S1 to S10:

S1  Continuous duty The motor operates at a continuous load for sufficient time to enable machine to reach thermal equilibrium. 
S2 Short Time duty Operation at a load for a time not sufficient to reach thermal equilibrium, followed by enough time for the motor to cool down.
S3  Intermittent periodic duty
Series of identical duty cycles each a constant load for a period, followed by a rest period. Thermal equilibrium is not reached during the cycle.
S4 Intermittent periodic duty with starting Similar to S3, but there is a significant starting time within the periodic operation.
S5 Intermittent periodic duty with electric braking Sequence of identical duty cycles - starting, operation, braking and rest.  Again thermal equilibrium is not reached.
S6 Continuous operation periodic duty Identical duty cycles with a period at load followed by a period at no load.  Difference between S1 is that the motor runs at no-load, without actual stopping.
S7 Continuous operation periodic duty with electric braking As per S6, but with a significant starting and electric breaking periods.  Again motor operates at no-load for  period instead of stopped. 
S8 Continuous operation periodic duty with related load/speed changes Series of identical repeating duty cycles, where within each cycle the motor operates at several different load levels and speed. There is not stopped time and thermal equilibrium is not reached.
S9 Duty with non-periodic load and speed variations     Load and speed vary periodically within the permissible operating range. Frequent overloading may occur.
S10 Duty with discrete constant loads and speeds Duty with discrete number of load/speed combinations, with these maintained long enough to reach thermal equilibrium.
     

 

Thermal Equilibrium is the state reached when the temperature rise of the machine does not vary by more than 2K per hour.   If you don't specify the duty cycle, the manufacturer will likely assume S1. Click on the image to see a larger version, illustrating the duty cycles.

If anyone has anything to add, please do so below. 



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

  1. sabari nath's avatar sabari nath says:
    5/24/2013 7:51 AM

    Can you help me understand the relation between Motor duty cyce and Starts per hour?


Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us



Resistors

Resistors are electronic components that oppose the flow of current.  Manufactured in various types and ranges they have a wide application to electronics...

Induction Motor Calculator

Just added a page to the tools, which will allow you to calculate the synchronous speed, slip and rated torque for an induction motor. Not a particularly...

Voltage Levels – Confused?

I was having a conversation the other day about voltage levels.  While everyone was in agreement that low voltage was 1000 V and less, there was more confusion...

Windows Live Writer and myElectrical

When making adding a Note to our site we have a great online WYSIWYG editor and things are pretty simple.  However, if you prefer you can write, manage...

Introduction to Lighting

When looking at the design of a lighting scheme it is useful to have an understanding on the nature of light itself and some of the basic theory associated...

Frame Leakage Protection

While not as popular as it once was, frame leakage protection does still have some use in some circumstances.  In essence frame leakage is an earth fault...

Post Authorship

In 2011, with the introduction of it’s Panda search ranking algorithms, Google introduced tools for determining the original author of posts.  The intention...

Periodic Electrical Installation Inspection – What to Inspect?

This is the second post in a series of two on periodic electrical inspections. In the first post, I discussed how often inspections should be carried out...

Electromagnetic Fields - Exposure Limits

Exposure to time varying magnetic fields, from power frequencies to the gigahertz range can have harmful consequences.  A lot of research has been conducted...

Difference Between Live and Dead Tank Circuit Breakers

A quick post in connection with an email question: Live Tank - the circuit breaker the switching unit is located in an insulator bushing which is live...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note