DC Motor Ratings

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DC Motors DC MOTOR OPERATION The amount of starting resistance necessary to limit starting current to a more desirable value is calculated using Equation (6-10. Rs   = (6-10) Et Is Ra where Rs   = starting resistance Et   = terminal voltage Is = desired armature starting current Ra  = armature resistance Example: If  the  full  load  current  of  the  motor  mentioned  previously  is  50 amps,  and  it  is desired to limit starting current to 125% of this value, find the required resistance that must be added in series with the armature. Rs Et Is Ra Rs 260VDC 125%(50  amps) 0.4W Rs 3.76W The starting resistors are used in a DC motor by placing them in the starting circuit of the motor controller  that  is  used  to  start  the  DC  motor.    Starting  resistors  are  normally  of  variable resistances,  with  the  value  of  resistance  in  the  circuit  at  any  time  being  either  manually  or automatically controlled.   The maximum amount of resistance will always be inserted when the motor is first started.   As the speed of the motor increases, counter EMF will begin to increase, decreasing armature current.  The starting resistors may then be cut out, in successive steps, until the motor reaches full running speed. DC Motor Ratings The nameplate ratings of a DC motor refer to the conditions of voltage, current, speed, and power at which the motor is normally operated.  The principal rating is known as the continuous rating, which  is  the  rating described  on  the  nameplate  of  a  motor.   The  continuous power  rating  is  a thermal rating.  At this power, the motor can be operated for long periods of time without a large rise in temperature and beyond the limits of the conductor insulating material, bearings and other components, which are greatly affected by temperature. The speed rating of a DC motor is often given on the nameplate.   This speed is the upper limit at which a motor can be operated without mechanical damage occurring. Rev. 0 Page 13 ES-06


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