Load Characteristics of Motors

When selecting a motor, special attention should be paid to the motor load characteristics to ensure that the motor will meet the needs of the application. The International Electrotechnical Commission IEC 60064-1 (rotating motors) defines ten load characteristics, grouped by continuous, short-term or cyclic items, denoted as S1 to S10.The load characteristics include the frequency and duration of typical operations, including start-up, no-load or full-load operation, electric braking and rest.

S1 Continuous duty
Motors are operated at a constant load for a sufficient period of time to equilibrate for temperature.

Abbreviation S1
S2 Short-time operating Motor operates at a constant load, but not long enough to reach temperature equilibrium. The rest period is long enough for the motor to reach ambient temperature.

The abbreviation S2 is followed by an indication of the duration of the shift. For example: S2 60 minutes
S3 Intermittent cyclic operation It has continuous, identical running and stationary cycles with a constant load. Temperature equilibrium is never reached. The starting current has little effect on the temperature rise.

Abbreviation S3 followed by a cycle duration factor.
For example: S3 25%
S4 Intermittent cyclic operation with start It has continuous, identical start, run and stationary cycles with constant load. Temperature equilibrium is not reached, but start-up current affects temperature rise.

The abbreviation S4 is followed by a cycle duration factor, the rotational inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ) both refer to the motor shaft.
For example S4 25% Iм = 0.15Kgm² Iɩᴑᴀᴅ = 0.7Kgm²
S5 Intermittent Cyclic Work with Electric Brake The cycles of continuous, identical start-up, constant load operation and no-load operation. There are no breaks.

The abbreviation S5 is followed by a cycle duration factor, the rotational inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ) refer to the motor shaft.
For example S5 30% Iм = 0.2Kgm² Iɩᴑᴀᴅ = 0.8Kgm²
S6 Continuous operation at intermittent loads Constant load operation and no-load operation run sequentially, in the same cycle. There is no rest period.

The abbreviation S6 is followed by a cycle duration factor.
Example S6 40%
S7 Continuous operation at electric braking Sequential identical cycling of start-up, constant load operation and electric braking. There is no rest period.

The abbreviation S7 is followed by the rotational inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ), both of which refer to the motor shaft.
For example S7 Iм=0.4Kgm² Iɩᴑᴀᴅ=7.5Kgm²
S8 Continuous operation of load and speed periodic changes The same duty cycle of the sequence runs at a constant load and a given speed, then at other constant loads and speeds. There are no breaks.

The abbreviation S8 is followed by the rotational inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ), both of which refer to the motor shaft, as well as to the load, speed, and cycle duration coefficients for each speed condition.
For example S8 Iм=0.5Kgm² Iɩᴑᴀᴅ=6Kgm²
16kW 740r/min 30%
40kW 1460r/min 30%
25kW 980r/min 40%
S9 Non-Cyclic Loads and Speeds Varying loads Loads and speeds vary cyclically within the permissible operating range. Frequent overloads may occur.

Abbreviation S9
S10 A discrete constant loads and speed loads. A number discrete of load/speed combinations, these combinations that are held long enough to reach thermal equilibrium.

The abbreviation S10 is followed by the corresponding load and its duration per unit amount p/∆t and the relative expected thermal life of the insulation system per unit amount TL. The reference value of the thermal life expectancy is the thermal life expectancy under continuous operation load and the allowable temperature rise limit based on the load type S1. During times of power failure and standstill, the load shall be denoted by the letter r. The value of TL shall be rounded to the nearest multiple of 0.05.
For example S10 p/∆t = 1.1/0.4;1/0.3;0.9/0.2;r/0.1
TL = 0.6
For this type of duty cycle, a constant load that appropriately selected and based on the duty cycle type S1 should be used as a reference value for the discrete load “Pref”. Load characteristics are really the various duty cycles of the motor at specific time intervals and constant loads, which vary in terms of start and stop times and braking system applications. Continuous duty cycles are ideally suited for machines that need to perform load-driven operations for long time period, while other types of duty cycles have their own unique waveforms and cycle duration factors. Therefore, load characteristics are an important parameter that must be considered when selecting a motor.