How to Choose Between Continuous and Intermittent Duty Gear Reduction Motors

A mixer drive runs 10 minutes on, 50 minutes off, repeated 12 times a day. The nominal load is 3 kW. The buyer selects a 3 kW gear motor rated for S1 (continuous duty), because “the motor power matches the load.” The motor is physically larger than necessary — and more expensive — because a smaller motor could handle the same load in intermittent duty, where the cooling periods allow higher torque during the “on” time.

Now consider the opposite: a conveyor drive runs 24 hours a day, 7 days a week, with a nominal load of 2.2 kW. The buyer selects a 2.2 kW gear motor rated for S3-40% (intermittent duty, 40% on-time), because the motor was in stock and the power matches. Within a month, the motor is running hot, the gearbox oil is degrading, and the insulation is aging — because the motor was designed for intermittent operation with cooling breaks, not for continuous duty at full load.

Both failures come from the same root cause: the duty cycle was not treated as a selection parameter. This article explains how operating hours and start frequency affect gear motor selection, what the IEC 60034-1 duty types mean, and why the same nominal load can require a different motor and gearbox depending on how the drive operates over time.

What Duty Cycle Means for Gear Motor Selection

The Core Concept: Thermal Equilibrium

When a motor runs under load, it generates heat from electrical losses (copper losses in the windings, iron losses in the core) and mechanical losses (bearing friction, fan load). The gearbox generates heat from gear mesh friction, bearing friction, and oil churning. This heat is dissipated through the motor frame, the gearbox housing, and the cooling fan to the surrounding air.

If the motor runs long enough, the temperature stabilizes — the heat generation rate equals the heat dissipation rate. This is called thermal equilibrium, and IEC 60034-1 defines it formally: the state where the temperature rise changes by no more than 1 K per half hour.

In continuous duty (S1), the motor reaches thermal equilibrium. The rated power is the power it can deliver at thermal equilibrium without exceeding its insulation class temperature limit.

In intermittent duty (S3, S4), the motor does not reach thermal equilibrium — it runs for a period, then stops and cools. Because it never reaches the maximum temperature that continuous operation would produce, it can handle a higher load during its “on” period than an S1-rated motor of the same frame size.

This is why the same motor frame size can be rated for different power levels depending on the duty type. A motor that is rated for 2.2 kW in S1 (continuous) may be rated for 3 kW in S3-40% (intermittent, 40% on-time) — because the cooling periods keep the temperature lower.

Why Start Frequency Matters

Every time a motor starts, it draws starting current (also called inrush current or locked-rotor current) that can be much higher than the rated current. Starting current can be much higher than running current depending on motor type, starting method and controller. Use the motor/controller data for the actual drive. This current surge generates a large amount of heat in the windings in a very short time.

For occasional starts, this heat is manageable — the motor cools down during normal operation. But for frequent starts, the repeated current surges create a cumulative thermal effect. The motor may never reach thermal equilibrium during its “on” periods, but the repeated heating from starts can push the winding temperature above the insulation limit.

This is the distinction between S3 and S4 duty in IEC 60034-1:

• S3 (intermittent periodic duty): The starting process does not significantly affect temperature rise. This applies when starts are infrequent enough that the thermal contribution of each start is small compared to the running heat.
• S4 (intermittent periodic duty with starting): Starting is explicitly included as a significant thermal component. This applies when starts are frequent enough that the cumulative heating from start currents is a major contributor to the total temperature.

Classification depends on the actual cycle and motor data, not a universal starts-per-hour threshold. If starting contributes materially to temperature rise, evaluate an S4-type duty or the manufacturer’s equivalent intermittent-starting duty data.

Not sure whether your application is S3 or S4? Send the number of starts per hour and whether starts are under full load. We can help classify the duty type and verify whether the proposed motor’s thermal capacity is adequate — or whether a larger frame is needed to handle the cumulative start current heating.

IEC 60034-1 Duty Types: What They Mean

IEC 60034-1 defines ten standard duty types (S1 through S10). The most relevant for gear motor selection are:

Standard load duty cycle values (the percentage of time under load within one cycle) for S3–S5 are: 15%, 25%, 40%, 60%.

Default assumption: If the duty type is not specified, IEC 60034-1 defaults to S1 (continuous duty). This is the most conservative assumption — the motor is selected as if it will run continuously at the rated load, reaching maximum temperature.

S7 through S10 cover more specialized patterns: continuous duty with electric braking, duty with related load/speed changes, and duty with non-periodic load/speed variations. These are less common in standard gear motor applications but may apply to specific machinery with complex operating cycles.

How Duty Type Affects Motor and Gearbox Selection

Motor Selection

For S1 (continuous duty), the motor must be sized so that its continuous rated power is sufficient for the load, and the thermal equilibrium temperature is within the insulation class limit. This is the most conservative selection.

For S2 (short-time duty), the motor can deliver more power than its S1 rating for the defined duration, because it will not reach thermal equilibrium. For illustration only (these are hypothetical values to show the trend, not specifications): a motor rated 2.2 kW in S1 might deliver roughly 2.6 kW in S2-60 min, 2.9 kW in S2-30 min, or 3.1 kW in S2-10 min. Actual factors vary significantly by manufacturer and motor design — consult the specific motor data sheet.

For S3 (intermittent duty), the motor can deliver more power during its “on” period than an S1 motor of the same frame size, because the cooling periods keep the average temperature lower. The allowable power increase depends on the duty cycle percentage (ed%): a lower on-time percentage allows a higher power increase.

For S4 (intermittent with frequent starting), the selection must account for both the running load and the thermal contribution of frequent starts. This typically requires a larger motor than S3 at the same duty cycle percentage, because the start current heating adds to the running heat.

Gearbox Selection

The gearbox is less directly affected by the duty type than the motor — the gears and bearings are rated for mechanical load, not thermal load. However, the duty type indirectly affects the gearbox in two ways:

1. Thermal effect on lubricant: In continuous duty, the gearbox oil reaches a steady-state temperature. If this temperature is high (due to high efficiency loss, high ambient temperature, or inadequate cooling), the lubricant may degrade faster. In intermittent duty, the oil has cooling periods, which may extend lubricant life.
2. Start frequency effect on mechanical fatigue: Frequent starts (especially under full load) introduce torque transients that fatigue the gear teeth and bearings. An S4 application with 30 starts per hour imposes more mechanical stress than an S1 application running continuously at the same load — even though the average torque may be the same.

Three Typical Duty Profiles

Profile 1: Continuous Duty (S1) — Conveyor Line

Description: A conveyor in a production line runs 16 hours a day, 5 days a week. The load is steady (uniform material flow). Starts are infrequent (2–3 per day, to change shifts or clear jams). Starts are under partial load (the belt is empty when starting).

Selection implications:

• Motor must be sized for S1 (continuous) at the rated load
• Gearbox must be thermally adequate for continuous operation (oil temperature must stabilize within acceptable limits)
• Start frequency is low — S3 or S4 considerations do not apply
• Service factor can be moderate (uniform load, no shock)

Key parameter: The motor’s S1 rated power must be ≥ the required mechanical power. The gearbox must be rated for continuous operation at the output torque.

Profile 2: Intermittent Duty (S3) — Batch Mixer

Description: A mixer runs in batches: 10 minutes on, 50 minutes off, repeated 6 times per shift. The load is moderate (mixing viscous material). Starts are under partial load (the mixer starts empty and fills during operation).

Selection implications:

• The operating-time fraction is 16.7% (10 min on / 60 min total). Whether this corresponds to an allowable S3 rating for the selected motor must be verified from the motor manufacturer’s duty data
• A smaller motor may be sufficient — potentially reducing cost and size
• Gearbox oil has cooling periods — thermal stress is lower
• Start frequency is moderate (6 per hour) — whether this qualifies as S3 or S4 depends on the motor’s thermal data

Key parameter: The duty cycle percentage (ed%) and the “on” period duration determine how much power increase is allowable. Consult the manufacturer’s data for the specific motor and duty cycle.

Profile 3: Frequent Starting (S4) — Indexing Conveyor

Description: An indexing conveyor runs in short cycles — a few seconds of motion, then a brief stop, repeated continuously throughout the shift. The load is moderate. Starts are under partial load, but the frequency is very high.

Selection implications:

• The start current contribution to temperature is significant — this is likely S4, not S3
• The motor must be selected for the thermal stress of frequent starting, not just the running load
• A motor that is adequate for S3 at the same duty cycle percentage may overheat in S4 due to the cumulative start current heating
• The gearbox must be rated for the mechanical fatigue of frequent starts (torque transients at each start)

Key parameter: The number of starts per hour is the critical specification. Whether the application qualifies as S3 or S4 depends on the motor’s thermal data — not on a fixed starts-per-hour threshold.

Application involves frequent starting and stopping? Provide the number of starts per hour and whether starts are under full load. Applications with high start frequency may need S4-rated selection rather than S3 — and the motor size may need to increase to handle the cumulative thermal stress of start currents. We can help verify this before you order.

Why Average Load Is Not Enough

A common mistake is to calculate the “average” load over a period and select the motor based on that average. But the motor and gearbox do not experience the average — they experience the actual load at each moment, plus the thermal and mechanical effects of the duty pattern.

Example: An application has a peak load of 5 kW for 2 minutes, then a rest of 8 minutes. The average load is 1 kW. But the motor must be able to deliver 5 kW during the peak without overheating — and the gearbox must be able to transmit the peak torque without mechanical failure. Selecting based on the 1 kW average would under-size both the motor and the gearbox.

The correct approach: Specify the peak load, the duration of the peak, the rest period, and the cycle frequency. Let the supplier determine the appropriate duty type and select the motor and gearbox for the actual load pattern.

What to Specify for Correct Duty-Based Selection

Have a duty cycle but not sure how it affects motor selection? Send your run time, stop time, starts per hour, full-load start (yes/no), and reversing requirements. We can determine the IEC duty type, check whether the proposed motor’s thermal capacity is adequate for the actual operating pattern, and confirm whether a smaller motor could be used for intermittent duty — or whether a larger motor is needed for frequent starting.