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In electric motor selection the question of "how many kW" is often the first that comes to mind; but the way the motor operates, that is the duty type, is at least as decisive as the power. Will the motor run continuously at constant load, or will it run by frequently starting and stopping, braking, and varying its speed and load? The IEC 60034-1 standard defines these operating modes from S1 to S10. In this article we focus on the S7 (continuous duty with braking), S8 (periodic duty with variable speed/load) and S9 (non-periodic / irregular load and speed) duty types, and cover their selection in the context of thermal equivalent power, brake motors and variable speed with a VFD. Ignoring the correct duty type leads either to choosing an unnecessarily large (expensive) motor or to a motor that continuously overheats and fails early.
What Is Duty Type and Why Does It Matter?
The duty type is a standard classification that defines how the motor's load and speed profile changes over time. A motor's heating depends on how long and at what level the load is applied. The same motor heats differently under a long-duration low load than under a short-duration high load. The duty type allows the manufacturer to size the motor within the correct thermal limits. The basic duty types S1 to S6 are more common; S7, S8 and S9 are for more complex applications involving braking, variable speed and irregular load. We collected the basic duty types (S1-S6) in a separate guide: electric motor duty type S1-S6 selection.
Brief Summary of Duty Types
| Duty type | Name | Characteristic |
|---|---|---|
| S1 | Continuous duty | Continuous running at constant load |
| S2 | Short-time duty | Runs for a set time then stops until fully cooled |
| S3 | Intermittent periodic duty | Load-stop cycle, starting heat negligible |
| S4 | Intermittent duty with starting | Heat of frequent starts matters |
| S5 | Intermittent duty with braking | Starting + electric braking |
| S6 | Continuous periodic duty | Load-no-load cycle, never stopping |
| S7 | Continuous duty with braking | Starting + braking, no stop |
| S8 | Periodic duty with variable speed/load | Speed and load vary periodically together |
| S9 | Irregular load and speed duty | Non-periodic, unpredictable variation |
S7: Continuous Duty with Braking
The S7 duty type defines a profile where the motor runs continuously without stopping, but the cycle includes starting and electric braking. Its difference from S6 is that, instead of no-load running, S7 has starting and braking in every cycle; the motor never stops but continuously accelerates and decelerates. This continuous start-brake produces extra heat because both the starting current and the braking energy turn into heat.
Typical S7 applications: some machine-tool drives, machines whose direction/speed changes frequently, drives that continuously position. When selecting a motor for S7, the number of starts per cycle, the braking method and the moment of inertia (GD² / J) are critical inputs. High inertia means more energy, and therefore more heat, at each start and brake. In braking applications a mechanical brake motor is frequently used; we covered brake motor selection in our article on IE4 brake motor: conveyor and crane supply.
S8: Periodic Duty with Variable Speed and Load
The S8 duty type defines an operating mode where, within a given period, both the motor's speed and its load change together and repeatedly (periodically). For example the motor may first run at low speed/high torque, then at high speed/low torque, and this cycle repeats. Pole-changing (multi-speed) motors or drives that move between different operating points with a VFD fall into this class.
In S8 the thermal calculation is more complex because both the load and the cooling (speed-dependent fan cooling) differ at each operating point. At low speed the motor's own fan cools less; at high speed it cools more. So in S8 applications, especially where high torque is needed at low speed, an external (forced) cooling fan may be needed. We detailed the cooling need for continuous torque at low speed with a VFD in our article on external forced cooling fan: low speed VFD.
S9: Irregular (Non-Periodic) Load and Speed Duty
The S9 duty type is the most complex operating mode, where load and speed change in an irregular, non-repeating (non-periodic) manner. There is no defined repeating cycle here; the load can change momentarily by a lot, even rising into overload regions from time to time. Cranes and hoisting systems, rolling-mill drives, and some mining and test-bench applications are examples of S9.
In S9 the motor can rise briefly above its rated power frequently; therefore both thermal capacity and overload (pull-out torque) margin matter. The motor must be large enough to carry the average load yet have enough margin not to stall at momentary peak loads. S9 selection is usually done together with the motor manufacturer through a detailed load profile analysis.
Thermal Equivalent Power: How Is a Motor Sized Under Variable Load?
In variable-load duties such as S6, S7, S8 and S9, the key to sizing the motor is the concept of thermal equivalent power (RMS power / equivalent load). The logic is this: the motor's heating is proportional to the square of the current it draws (I²R losses). So we reduce the variable load profile to a single "equivalent continuous load" that produces the same heat. In practice the load of each segment in the profile is squared and weighted by its duration, divided by the total time, and the square root is taken (RMS). The resulting value shows the minimum continuous power the motor must withstand.
But thermal equivalent power alone is not enough; two additional checks are needed:
- Peak torque / overload check: The highest momentary load in the profile must not exceed the motor's pull-out torque. A motor that looks thermally adequate may stall at a momentary peak load.
- Cooling factor: Since fan cooling decreases at low speed, a cooling correction must be included in the RMS calculation. Otherwise a motor that looks adequate "on paper" overheats in the field.
We covered the relationship between duty-cycle percentage and heating in intermittent duties (S3/S4) in our article on S3/S4 intermittent duty: percent on and heating; and the insulation thermal class (F/H) and temperature rise in our article on insulation thermal class F/H and temperature rise.
Brake Motors and Variable Speed with a VFD
In braking duties such as S5 and S7, there are two basic braking approaches: mechanical braking (spring-applied/electromagnetic brake) and electrical braking (DC braking or regenerative braking). A mechanical brake motor is mandatory in applications where the shaft position must be held when power is cut (crane, lift, conveyor holding). In VFD-driven systems the braking energy can be dissipated as heat in a brake resistor or returned to the grid with a regenerative drive. For facilities that brake frequently and want to recover energy, the regenerative four-quadrant drive solution is valuable.
A VFD is the most flexible way to provide variable speed in S8 and S9 duties. But when running with a VFD, the motor's cooling problem at low speed, the extra heating from the drive output waveform and correct parametering are important. The correct match between drive and motor is critical to safely meet the torque-speed profile the duty type demands.
The Effect of Inertia (J) and Number of Starts on the Duty Type
In duties with frequent starts such as S4, S5, S7 and S8, the moment of inertia (J, or in the older notation GD²) of the motor and the driven load is very decisive. At each start the motor spends energy to accelerate the load from standstill to running speed; a significant part of this energy turns into heat in the windings. The larger the load inertia, the longer the start takes and the more the winding heats. So in an application that starts hundreds of times per hour, even if the continuous power looks adequate, the motor can overheat due to starting heat.
In practice manufacturers state the permitted starts per hour in the motor catalogue, and this number decreases with load inertia and duty type. For high-inertia loads (large fans, flywheels, centrifuges) either a larger motor is chosen, or the starting current and heat are limited with a soft starter / VFD. In high-start-count S7 applications, the motor's thermal model must be built around this cycle; otherwise insulation life is far shorter than expected.
The Relationship Between Duty Type and Insulation Class
Duty type selection is directly related to the insulation thermal class (F, H). Since the motor heats more in variable and braking duties, the insulation system must withstand this temperature. Class F insulation withstands up to 155 °C, class H up to 180 °C. In demanding duty types, using class F insulation with a class B temperature rise (i.e. leaving some thermal margin) is a common and sound approach; it extends the motor's life. A widely accepted rule is that insulation life roughly halves for every 10 °C rise. Therefore in heavy duty types, thermal margin is an inseparable part of the purchasing decision.
Practical Checklist for Duty Type Selection
- Will the motor run continuously, intermittently, or with braking?
- How many starts/brakes per cycle? What is the moment of inertia (J)?
- Is the load profile repeating (periodic) or irregular?
- Does the highest momentary peak load exceed the motor's pull-out torque?
- Will it be variable speed with a VFD? Is cooling sufficient at low speed?
- Has the thermal equivalent (RMS) power been calculated and the cooling correction included?
Frequently Asked Questions
What is the difference between S6 and S7?
In both the motor never stops, but in S6 the cycle goes between "load and no-load"; in S7 the cycle includes "start - load - braking". So S7 contains electrical braking and re-acceleration in every cycle. This continuous acceleration-deceleration produces extra heat, so for the same continuous power an S7 motor is thermally more stressed than an S6 one, and the selection is made accordingly.
How should I size a motor for an S9 duty?
Because S9 is irregular, it does not end with a single formula. First you must measure/log the real load-speed profile, then calculate the thermal equivalent (RMS) power, then verify that the highest momentary peak load does not exceed the motor's pull-out torque. Since cooling decreases at low speed, an external fan may be needed. For this reason S9 selection is usually done together with the manufacturer, based on a measured profile.
What happens if I choose the wrong duty type?
Two kinds of error occur. If you treat the duty type as lighter than it is, the motor continuously overheats, insulation life shortens and early failure comes. Conversely, if you assume a heavier duty type than needed and choose an over-sized motor, both the investment cost rises and the motor runs at low load continuously, lowering efficiency and power factor. The correct duty type provides a balanced and economical choice between these two extremes.
Contact Us for Stock and Fast Delivery
At HEM Motor we determine the right motor for braking, variable-speed and irregular-load duty types such as S7, S8 and S9 by evaluating your load profile and the thermal equivalent power together. By taking into account critical points such as the brake motor, VFD compatibility and cooling at low speed, we recommend a motor that is neither over-sized nor inadequate, but sized exactly to your application. To get a quote with manufacturer stock advantage and fast delivery, contact us and let us analyse your application's operating profile together.
