The load profile (S1–S10 per IEC 60034-1) defines how a motor is thermally stressed and therefore directly determines the required motor power and frame size. An undersized motor leads to overload and failures; an oversized motor wastes energy and inflates procurement costs. This article shows you in practice how to select a motor correctly based on its load profile.
Key Takeaway:
The load profile (S1–S10) determines the required motor power and size. A thermal analysis with duty factor, ambient temperature, and duty cycle (DC%) is essential. Use the Affinity Law (P ~ n³) for pumps and fans to quantify energy savings.
Understanding Load Profiles: S1–S10 per IEC 60034-1
IEC 60034-1 defines ten duty types (S1–S10) that describe different load scenarios. These directly affect the required motor power and size:
| S-No. | Duty Type | Description |
|---|---|---|
| S1 | Continuous Duty | Motor runs continuously under constant load. Typical applications: pumps, fans in district heating systems. |
| S2 | Short-Time Duty | Motor runs for a fixed operating time (seconds to minutes), then cools down to ambient temperature. The fixed duration defines this duty type. |
| S3 | Intermittent Duty | Repeated short-time operation with pauses (does not cool to ambient temperature). DC% 15–60%. E.g., hoists, crane systems. |
| S4 | Intermittent Duty with Starting | Like S3, but starting time is significant. Typical for presses, air compressors. |
| S5 | Intermittent Duty with Elec. Braking | Like S3, but with electromagnetic braking and frequent starts. High thermal stress. |
| S6 | Continuous Operation with Intermittent Load | Motor runs continuously, but load varies periodically. E.g., conveyor belt with variable load. |
| S7 | Continuous Duty with Elec. Braking | Like S1, but with frequent braking and starts. High thermal stress from braking energy. |
| S8 | Periodic Duty with Direction Reversal | Motor periodically reverses direction (forward/backward). High stress on drive and brakes. |
| S9 | Duty with Non-Periodic Load and Speed Variations | Irregular load and speed (e.g., rolling mill, wind turbine). Very high thermal demands. |
| S10 | Periodic Duty with Braking | Complex load profiles with multiple phases: acceleration, load, braking, pause. Crane systems, travel drives. |
Important: The higher the duty type (S3 vs. S1), the larger the motor can be sized. An S3 motor with DC=40% can have 50–100% higher power than an S1 motor of the same frame size, since it has cooling breaks.
Load Types: Constant, Linear, Quadratic
The way load changes with speed is a decisive factor in determining required motor power:
Constant Load (M = const.)
Torque remains constant regardless of speed. Power changes linearly: P = M × n.
Examples: Conveyor belt (friction), coil winders, escalators. Implication: With speed reduction (using a VFD), power drops linearly. A motor rated at 30 kW at 1500 rpm requires only 15 kW at 750 rpm. However, the motor must be sized for the highest required torque.
Linear Load (M ~ n)
Torque grows linearly with speed. Power grows quadratically: P ~ n².
Examples: Viscous friction (highly viscous fluids), bearings with damping. Implication: Rare in practice. Energy savings from speed reduction are moderate.
Quadratic Load (M ~ n²)
Torque grows with the square of speed. Power grows with the third power: P ~ n³. This is the Affinity Law.
M ~ n² ⟹ P ~ n³
At 80% speed: P = 0.8³ = 0.512 = 51% of rated power
Examples: Centrifugal pumps (without throttling), fans, blowers. Implication: With a variable frequency drive, massive energy savings (40–60%) are achievable — when a frequency inverter pays off. This is the basis for energy efficiency measures in pump and fan systems.
Practical Tip: Always verify the load profile in the machine drawing or with the machine manufacturer. Incorrect assumptions about load type lead to costly mistakes in motor selection. For dynamic load profiles (S3–S10) with frequent acceleration and braking, the load-to-motor inertia ratio (Jload/Jmotor) is equally important — a value above 10:1 typically requires a higher gear ratio or a motor with greater rotor inertia.
Calculation Example: Chain Conveyor
Task: A horizontal chain conveyor transports boxes at a speed of 2 m/s. Total mass is 500 kg (boxes + chain). Friction coefficient is 0.05. What motor is required?
Step 1: Calculate resistance force
F = m × g × f = 500 kg × 9.81 m/s² × 0.05 = 245 N
Step 2: Determine speed and drum radius
Assumption: drum diameter 200 mm (r = 0.1 m). Peripheral speed is 2 m/s. Speed n = v / (2πr) = 2 / (2π × 0.1) = 3.18 rev/s = 191 rpm. With gearbox (ratio 1:7.9): n_motor = 191 × 7.9 = 1509 rpm ≈ 1500 rpm.
Step 3: Calculate torque
M_drum = F × r = 245 N × 0.1 m = 24.5 N·m
With gearbox and losses: M_motor = M_drum / ratio / η_gearbox ≈ 24.5 / 7.9 / 0.90 ≈ 3.4 N·m
Step 4: Calculate motor power
P = M × ω = 3.4 N·m × 2π × 1500 / 60 = 3.4 × 157.08 = 534 W ≈ 0.75 kW
Step 5: Add safety factor
With safety factor 1.15: P_required = 0.75 × 1.15 = 0.86 kW. Motor selection: 1.1 kW three-phase motor, 1500 rpm, IE3 class, 4-pole.
The accelerated mass moment of inertia of the load is the main factor governing run-up time — the online moment-of-inertia calculator lets you determine this value directly for rotary and translational loads.
This is a typical scenario for continuous duty (S1) with constant load. The motor would deliver 100% rated power in duty type S1.
Thermal Design: Duty Factor & Ambient Temperature
Every motor has a maximum permissible temperature (e.g., 130 °C for Class B per IEC 60034-1). This temperature is reached when the motor runs at its rated power at 40 °C ambient temperature. Deviations must be accounted for:
Duty Factor for Intermittent Operation
A motor in intermittent duty (S3 with DC=40%) can be operated at higher loads since it has cooling breaks. The duty factor accounts for this:
| Duty Cycle (DC%) | 15% | 25% | 40% | 60% |
|---|---|---|---|---|
| Duty Factor (typical) | 1.50–1.60 | 1.25–1.35 | 1.10–1.20 | 1.00–1.10 |
Meaning: For an application with DC=40% and a required torque of 10 N·m, you can select a motor with rated torque 10 / 1.15 ≈ 8.7 N·m (approx. 5% savings). However, caution: duty factors specified by the manufacturer can vary depending on motor type.
Ambient Temperature Correction
Motors are typically designed for 40 °C ambient temperature. Deviations require power adjustments:
- At 50 °C: Reduce motor power by ~10% (or choose a larger motor)
- At 60 °C: Reduce motor power by ~20%
- At 20 °C: Increasing motor power by ~10% is possible (better cooling)
Rule of thumb: For every 10 °C deviation from 40 °C ambient temperature, the permissible motor power changes by approximately 10%. This is a rough approximation; precise values can be found in the motor datasheet.
Motor-Gearbox Combination: Design & Heat Losses
When motor and gearbox are combined, heat losses from the gearbox must be taken into account:
Efficiency of typical gearboxes:
- Spur gear: 96–98% per stage
- Worm gear: 50–90% (depending on ratio)
- Planetary gear: 94–97% per stage
- Bevel gear: 95–97% per stage
The losses result in heat generation. A poorly matched motor and an undersized gearbox can lead to overheating and failures.
Example: A motor with 10 kW drives through a spur gear with 98% efficiency. The losses are 10 kW × (1 - 0.98) = 0.2 kW = 200 W. This must be compensated by gearbox cooling (ventilation, heat dissipation). If the gearbox is too small or insufficiently cooled, the temperature quickly exceeds the permissible range (typically <80 °C oil temperature). For a detailed walkthrough: calculate gearbox efficiency.
Motor Selection Checklist by Load Profile
- Determine load profile: S1–S10? Ask the machine manufacturer or refer to the operating manual.
- Analyze load type: Constant, linear, or quadratic? This determines power at variable speeds.
- Calculate torque and speed: M [N·m] = F [N] × r [m], then P [W] = M × 2πn / 60.
- Add safety factor: Multiply by 1.1–1.25 depending on application safety requirements.
- Check duty factor: For intermittent duty (S3–S5), account for the duty factor.
- Evaluate ambient temperature: At >40 °C ambient temperature, reduce motor power or choose a larger motor.
- Energy efficiency: Choose at minimum IE3, preferably IE4.
- Define mounting style and protection rating: B3, B5, B14? IP54, IP55, IP65?
- Motor-gearbox heat losses: Verify that combined heat losses are absorbed by cooling.
- Contact the manufacturer: For uncertainties or complex requirements, consult an Application Engineer.
TEA Recommendation: Checklist for Motor Selection by Load Profile
Use this structured decision guide:
Required for every motor selection:
- Duty type (S1–S10), duty cycle (DC%)
- Load type (constant, linear, quadratic)
- Torque [N·m], speed [rpm], power [W]
- Ambient temperature, safety factor
- Mounting style, protection rating, energy efficiency class
- Gearbox efficiency, total system heat balance
Optional for optimization:
- Speed control (variable frequency drive) for energy savings?
- Servo motor for high precision or dynamic performance?
- Noise requirements, EMC requirements?
Our Application Engineers are happy to assist you with complete system design. Send us a request with the operating parameters – we will calculate the optimal motor selection with a cost-benefit analysis. The full motor programme — from servo and stepper to asynchronous motors — is available in the TEA motors category.
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