Transcript DC Motors
DC MOTORS SEE 3433 ELECTRICAL MACHINES DC MOTOR - Shunt motors - Separately excited - Series (we’ll look briefly) DC MOTORS DC MOTOR + - + - DC motor DC MOTOR + Va - Load torque opposing the motor torque + Vf + - + Tm Tload DC motor Mechanical Load: fans, blowers, Compressors, DC MOTOR + V - + V + - + Tm Tload DC motor F = mg DC MOTOR Hoist DC MOTOR - Some applications require the control the speed - Some applications require the control the torque - In order to control the torque or speed we need to know the T- characteristics of the motor and the mechanical load Intersections between the two characteristics will determine the operating point DC MOTOR Shunt motor If Vt = IaRa + Ea It It = Ia + If Rcf + Ia Ra Vt Ea = k Te = kIa Rcw Te Tload Mechanical load k = Vt - IaRa Vt IaR a k Vt Ra T 2 e k (k) Three possible methods of speed control: Field flux Armature voltage Vt Armature resistance Ra Vt Ra T 2 e k (k) Vt kT Vt R T a e2 k T k T Varying Vt TL Vt ↓ Te Requires variable DC supply Vt kT Vt R T a e2 k T k T Varying Vt TL Vt ↓ Te Requires variable DC supply Vt (k T ) R a Te k T Varying Vt TL Constant TL Te Requires variable DC supply Vt (k T ) Vt R a Te k T Vt (k T) IaRa Vt,rated Varying Vt Constant TL Ia R a base Introduction Vt R T a e2 k T k T Varying Ra Vt kT TL Ra ↑ Te Simple control Losses in external resistor Vt kT Vt R T a e2 k T k T Varying TL ↓ Te Not possible for PM motor Maximum torque capability reduces Method of speed control in DC motor drives Armature voltage control : retain maximum torque capability Field flux control (i.e. flux reduced) : reduce maximum torque capability For wide range of speed control 0 to base armature voltage, above base field flux reduction Armature voltage control Field flux control Te Maximum Torque capability base Te Maximum Torque capability base P Te Constant torque Constant power Pmax base 0 to base armature voltage, P = EaIa,max = kaIa,max above base field flux reduction Pmax = EaIa,max = kabaseIa,max 1/ 0 to base armature voltage, above base field flux reduction 0 to base armature voltage, If Rcf above base field flux reduction It + Ia Ra Rcw Vt BUT there are problems ! 0 to base armature voltage, If Rcf above base field flux reduction It + Ia Ra Rcw Vt Controlling Vt will also affect If Controlling If via Rcf caused losses I2R 0 to base armature voltage, above base field flux reduction Separately Excited DC motor DC supply for armature What if we have an AC supply ? DC supply for field 3-phase AC source Separately Excited DC motor AC to DC converter + Vdc + Vdc - - Armature voltage control AC to DC converter Field voltage control