What is Continuous Duty?
Continuous duty refers to the operation of an electric motor where the load remains constant for a long duration of time. This type of duty requires the motor to operate under the same conditions for an extended period without any significant changes in the load. For example, a motor driving a conveyor belt or a centrifugal pump is typically under continuous duty.
It is crucial to select a motor with the correct power rating for continuous duty to ensure that the motor operates efficiently and without any damage. If the motor is overrated, it will operate at less than its full load value, resulting in reduced efficiency and power factor. On the other hand, if the motor is underrated, it will overheat and eventually burn out.
Therefore, it is necessary to calculate the exact amount of power required for the application and select a motor that matches that power rating. This requires thermal calculations and consideration of the average power required for the application.
In a continuous duty cycle, the temperature of the motor will continue to increase until it reaches a steady-state value. If the motor is not properly sized for the load, it may operate at a temperature that exceeds its maximum temperature rating, which can lead to motor failure.
Some common examples of applications that require continuous duty motors include fans, centrifugal pumps, blowers, paper mill drives, compressors, and conveyors. These applications require motors that can operate continuously without overheating or damaging the motor.
What is Intermittent Periodic Duty?
Intermittent periodic duty is a type of duty cycle in which a motor is alternately subjected to constant load and standstill duration. This duty cycle is used for machines that require intermittent operation, such as lifts, cutting, escalators, cranes, presses, drilling machines, and other similar equipment.
There are two types of intermittent duty cycles:
1. Short-time duty cycle:
In this type of duty cycle, the motor is loaded for a short period of time and then it is shut down for a very long time. During the off-load interval, the motor cools down to ambient temperature. The duty cycle is expressed as:
N minutes ON / (N+R) minutes = D%
Where N is the operating time of the motor, R is the rest period, and D is the duty cycle.
The maximum temperature attained by the motor during the ON period is denoted by θmax. The motor should be selected based on the thermal calculation and average power.
2. Intermittent periodic duty with insufficient cooling time:
In this type of duty cycle, the motor is loaded for some duration of time, and then it is shut down for a very short time. Due to the short cooling time, the motor's temperature rises with an increase in the time of operation. The duty cycle is expressed as:
N minutes ON / (N+R) minutes = D%
Where N is the operating time of the motor, R is the rest period, and D is the duty cycle.
The motor should be selected based on the thermal calculation and average power.
Examples of intermittent duty cycle applications:
- Lifts: Lifts require a short-time duty cycle because they operate intermittently. The motor is loaded for a short period of time when the lift is moving, and then it is shut down for a long period of time when the lift is stationary.
- Cutting: Cutting machines require a short-time duty cycle because they operate intermittently. The motor is loaded for a short period of time when cutting is being done, and then it is shut down for a long period of time when no cutting is being done.
- Escalators: Escalators require a short-time duty cycle because they operate intermittently. The motor is loaded for a short period of time when the escalator is moving, and then it is shut down for a long period of time when the escalator is stationary.
Difference Between Continuous and Intermittent Duty of Motor:
| Continuous Duty | Intermittent Duty |
|---|---|
| The motor operates continuously for a long time under the same conditions | The motor is loaded for short time and then it is shut down either for a long duration or a very short duration |
| The duty cycle is simple and has high efficiency as it can stabilize the temperature as it runs | The duty cycle is complicated as the motor frequently starts and stops |
| For line-operated motors, these loads have constant torque | For line-operated motors, these loads have the torque that changes suddenly |
| These loads operate continuously | These loads operate by taking intervals |
| Examples: fans, centrifugal pumps, paper-making machines, etc. | Examples: cranes, excavators, mixers, lifts, hoisting machines, etc. |
| Requires cooling mechanisms to keep the motor at a stable temperature | Requires thermal calculations to ensure the motor does not overheat |
| Has a continuous power rating | Has an intermittent power rating |
| Has a higher overall efficiency | Has a lower overall efficiency |
| Used in applications that require a constant power output | Used in applications that require a varying power output |
| Usually found in industrial settings | Found in both industrial and residential settings |