Monday, November 8, 2010

Duty Cycles of Rotating Machines

Unless otherwise specified, the rating of the motor will be regarded as its continuous maximum rating (CMR), defined by duty S1. But a machine is not always required to operate at a constant load. Sometimes it must operate at varying loads, with a sequence of identical operations, involving starts, stops, braking, speed control and reversals, with intermittent idle running and de-energized periods etc. (e.g. a hoist, a crane, a lift or other applications.) Using a CMR motor for such applications, with a rating corresponding to the maximum short-time loading will mean an idle capacity during no-load running or de-energized periods and a constant drain on energy, in addition to a higher cost of installation. To economize on the size of machine for such applications, IEC 60034-1 has defined a few duty cycles, as shown in diagrams below. These may be considered while selecting an economical size of machine and yet meet the variable load demands safely. Such motors may be running overloaded during actual loading but for shorter durations not sufficient to exceed the permissible temperature rise limits. They dissipate excessive heat during idle running or de-energized periods to reach a thermal equilibrium at the end of the load cycle. These duties are described in the following sections.
The main categories are

1. Continuous duty, CMR (S1)
The operation of a motor at a rated load may be for an unlimited period to reach thermal equilibrium (See Figure) and possible applications are pumps, blowers, fans and compressors.

2. Short Time Duty (S2)
In this case the operation of the motor is at a constant load during a given time just adequate to attain the maximum permissible temperature rise, followed by a rest and de-energized periods of long durations to re-establish equality of motor temperature with the cooling medium (See Figure). The motor should restart for the next cycle only when it has attained its ambient condition. The recommended values for short-time duty are 10, 30,60 and 90 minutes. The type designation for a particular rating, say for 30 minutes, will be specified as S2 - 30 minutes. Likely applications are operation of lock gates, sirens, windlasses (hoisting) and capstans. 

3. Intermittent periodic duty (S3) 
This is a sequence of identical duty cycles each consisting of a period of operation at constant load and a rest and de-energized periods. The period of energisation may attain the maximum permissible temperature rise. The period of rest and de-energization is sufficient to attain thermal equilibrium during each duty cycle (See Figure). In this duty the starting current Ist does not significantly affect the temperature rise. Unless otherwise specified, the duration of each duty cycle should be 10 minutes. The recommended values for the cyclic duration factor CDF are 15%, 25%, 40% and 60%. The type designation for a particular rating, say for 40%. will be specified as S3 - 40%. 


4. Intermittent periodic duty with start (S4)  


This is a sequence of identical duty cycles. each consisting of a period of start, a period of operation at constant load and a rest and de-energized periods. The starting, operating, rest and de-energized periods are just adequate to attain thermal equilibrium during one duty cycle. In this duty the motor is stopped, either by natural deceleration, after it has been disconnected from the supply source, or by mechanical brakes, which do not cause additional heating to the winding. 

5. Intermittent periodic duty with start and brake (S5)    

This is a sequence of identical duty cycles, each consisting of a period of start, a period of operation at constant load, a period of braking and a rest and de-energized periods. The starting, operating, braking, rest and de-energized periods are just adequate to attain thermal equilibrium during one duty cycle. In this duty braking is rapid and is carried out electrically.

6. Continuous duty with intermittent periodic loading (S6)  
This is a sequence of identical duty cycles, each consisting of a period of operation at constant load and a period of operation at no-load. The repeat load and no-load periods are just adequate to attain thermal equilibrium during one duty cycle. There is no rest and de-energizing period. Unless otherwise specified, the duration of the duty cycle will be 10 minutes.

7. Continuous duty with start and brake (S7)    
This is a sequence of identical duty cycles, each consisting of a period of start, a period of operation at constant load and a period of electric braking. The start, operating and braking periods are just adequate to attain thermal equilibrium during one duty cycle. There is no rest and de-energizing periods.

8. Continuous duty with periodic speed changes (S8)  
This is a sequence of identical duty cycles, each consisting of a period of operation at constant load, corresponding to a determined speed of rotation, followed immediately by a period of operation at another load, corresponding to another speed of rotation, say, by change of number of poles. The operating periods are just adequate to attain thermal equilibrium during one duty cycle. There is no rest and de-energizing period. For this duty type, the abbreviation is followed by the indication of the number of duty cycles per hour, the FI and the load at the various speeds. Likely applications are where the motor is required to run at different speeds.

9. Non-periodic duty (S9)  
This is a type of duty in which load and speed both vary non-periodically, unlike the periodic duty cycles noted above. The motor now supplies variable load demands at varying speeds and varying overloads, but within the permissible temperature rise limits. It is a duty similar to duty cycle S8, axcept that sometimes the overloads may exceed the full load but are within the thermal withstand limit of the motor.

10. Duty with discrete constant loads (S10)     
This is a type of duty in which load and speed both vary non-periodically, unlike the periodic duty cycles noted above. The motor now supplies variable load demands at varying speeds and varying overloads, but within the permissible temperature rise limits. It is a duty similar to duty cycle S8, axcept that sometimes the overloads may exceed the full load but are within the thermal withstand This is a type of duty consisting of a number of varying loads, not more than four in each cycle. Each load is performed for sufficient duration to allow the machine to attain its thermal equilibrium. It is, however, permitted that each load cycle may not be identical, provided that each discrete loading during one particular load cycle is performed for a sufficient duration to attain thermal equilibrium. The temperature attained during each discrete loading is within permissible limits or within such limits that if it exceeds the permissible limit, the thermal life expectancy of the machine is not affected. For example, performing one discrete loading P2, as in Figure, the temperature reached may exceed the permissible limit for a short duration (t2), but the final temperature at the end of the cycle is still such that the next duty cycle can be performed. The short duration excess temperature, reached while performing the load duty  P2, will not however, be detrimental to the thermal life expectancy of the machine. 
 
  
 

 

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