What is the motor duty system?

Today, in our lives motors are almost everywhere, from aerospace, ships and automobiles to washing machines, fans, vacuum cleaners, electric robots and so on. Motors are used in almost all application scenarios and are controlled by drives. Almost different kinds of motors run at different operation times. Some motors are always on, while others run for a short time during the duty cycle or alternate between cycles. Therefore, based on this, the concept of the motor duty system is introduced.

The working system of the motor actually refers to the working state of the motor, such as continuous operation or short-term operation or intermittent operation. In order to understand the working system of the motor more intuitively, we can divide the working cycle of the motor into time periods such as starting, load, braking, no-load, rest, etc., as well as the duration and sequence of the stages (the actual load includes starting, load, brake, no-load, here for the convenience of explanation).

In IEC60034-1 the motor working system is classified into 10 kinds of motor duty and the characteristics of them are descibied respectively, including start, electric brake, no-load and stop by energy cut, as well as duration time in stes and sequential order.   

#1 Continuous duty (S1)

The first and simplest type of motor duty is continuous duty, which can be represented by S1. In this type of duty cycle, the motor runs at a constant load long enough to reach thermal equilibrium. Under the S1 duty system, it is assumed that the starting of the motor can be considered as having a negligible effect on the temperature of the motor. This duty system shows that the motor can run for a long enough time and the temperature reaches a stable value. Motors with S1 duty system are often used in factories to control drives, compressors, conveyors, etc.

#2 Short-time duty (S2)

Motors run under constant load for a given time, which is not long enough to achieve thermal stability and then are shut down for a long enough time to get the motor to cool down again to a temperature within 2K of the temperature of the cooling medium. In this duty system, the motor running time is very short and the heating time is much lower than the cooling time. Therefore, the motor is warmed up to ambient temperature before running again. Motors with this duty cycle are commonly used in cranes, household appliance drives, valve controllers, etc.

#3 Intermittent periodic duty (S3)

Motors run on a series of identical duty cycles that do not reach thermal equilibrium, each cycle including a constant load operating time and a shutdown interval (shutdown time by energy cut). The duty duration ratio is the ratio of constant load runtime to duty cycle. The starting current of each cycle in this duty system has no significant effect on the temperature rise. However, the cycle period must last for 10 minutes, and the duty duration ratemust be one of four types: 15%, 25%, 40%, and 60%. Motors with this duty system are commonly found in stamping and drilling machines.

#4 Continuous operation with intermittent load (S6)

Motors perate on a series of identical duty cycles, each cycle including a period of constant load operation and a period of no-load operation, but without shut down. That is, operating cycle = work (load + no load), referring to S3, commonly used in pressing machines, cutting machines etc.

#5 Continuous operation with electric braking (S7)

The motor duty system is a specific method to control the motor movement, a work instruction for motors to avoid overheat of motors and burnout, and also a user requirement. At the same time, it is also a guideline for you to select a suitable motor according to the motor working system of the motor. However, in practical applications, the working systems of many motors are diverse, one or more working systems are needed at the same time to meet different application scenarios.