Operating Modes in Electric Motors
Hello everyone,
In this week’s article, we would like to explain the operating modes of electric motors used in hydraulic systems.
In hydraulic systems, electric motors drive the pump which can be considered the heart of the system to pressurize the hydraulic fluid. The flow produced by the pump enable cylinders to move, hydraulic motors to rotate, and ultimately allow the system to perform its function.
At this point, it is not enough for an electric motor to simply “rotate.” How long and under what conditions it operates is critically important. Not every application requires continuous operation. Some systems operate intermittently, some for short durations, and others under continuous load.
These different operating characteristics introduce the concept of operating modes in electric motors.
What Is an Operating Mode?
An operating mode is the classification of an electric motor based on its load duration, stopping period, and thermal behavior.
According to international standards, these modes are defined by codes such as S1, S2, and S3.
This classification helps to:
- Prevent motor overheating
- Ensure proper motor selection
- Improve energy efficiency
- Extend motor service life
The three most commonly used operating modes are S1, S2, and S3.
S1 Operating Mode (Continuous Duty)
In S1 mode, the motor operates continuously under constant load and reaches thermal equilibrium.
This means:
- The motor runs continuously once started
- It heats up and stabilizes at a certain temperature
- It does not have a cooling period because it does not stop
Typical Applications:
- Continuously operating hydraulic power units
- Conveyor systems
- Fan and pump applications
For example, a hydraulic pump motor operating 24 hours a day is typically designed for S1 duty.
S2 Operating Mode (Short-Time Duty)
In S2 mode:
- The motor runs for a specified period
- It stops before reaching thermal equilibrium
- It is allowed to cool down completely before the next cycle
For example, if the motor nameplate states “S2 – 30 min”:
- The motor must not operate longer than 30 minutes
- It must then remain off long enough to cool completely
Typical Applications:
- Press systems that operate occasionally
- Workshop machinery
- Applications requiring short-duration load
The key point here is that the motor is not designed for continuous operation.
S3 Operating Mode (Intermittent Periodic Duty)
In S3 mode:
- The motor operates for a certain period
- It stops for a short interval
- This cycle repeats continuously
However, the critical difference is that the stopping period is not long enough for the motor to cool completely.
This operating mode is typically expressed as a duty cycle percentage.
For example:
“S3 – 40%” means:
- The motor operates during 40% of the cycle
- It remains off during 60% of the cycle
Typical Applications:
- Elevator systems
- Packaging machinery
- Hydraulic systems with frequent start-stop cycles
In such applications, the motor frequently starts and stops without fully cooling down.
Understanding the Difference Between S1, S2, and S3 with a Simple Example
Let’s consider a hydraulic power unit:
🔹 S1
The machine starts in the morning and runs continuously until the end of the shift.
Motor does not stop → Continuous load → Thermal equilibrium is reached.
🔹 S2
The machine runs for 20 minutes and then remains off for 1–2 hours.
The motor stops before overheating → It cools completely before restarting.
🔹 S3
The machine runs for 2 minutes and stops for 3 minutes.
This cycle repeats throughout the day.
The motor partially cools but never fully.
Why Is Proper Motor Selection Critical?
Selecting a motor with the wrong operating mode may result in:
- Overheating
- Reduced winding lifespan
- Lower energy efficiency
- Unexpected failures
Therefore, motor selection should not be based solely on kW rating. The operating duration and load characteristics of the application must also be carefully evaluated.
Our Approach at Hidros Group
At Hidros Group, we provide motor solutions tailored to different operating scenarios of our customers.
Depending on the project requirements, we can supply:
- S1 duty motors for continuous operation
- S2 duty motors for short-time applications
- S3 duty motors for intermittent and frequent start-stop systems
For every application, our engineering team evaluates load analysis, operating duration, environmental conditions, and system requirements to determine the most suitable operating mode.
In conclusion, selecting the correct operating mode in electric motors is one of the fundamental elements of hydraulic system performance. Choosing the right motor for the application is critical not only for technical reliability but also for long-term investment sustainability.