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    DC Machines

    Uploaded by

    Moses Mberwa
    - DC motors come in two types: series and shunt. Series motors have the armature and field coils connected in series, while shunt motors have them connected in parallel. - Shunt motors maintain nearly constant speed as load increases due to constant field current. They are well-suited for applications requiring constant speed. Series motors experience decreasing speed as load increases due to the armature current relationship. They provide high starting torque. - Example calculations are provided for torque, speed, power and efficiency of sample shunt and series motors. The effects of changing supply voltage and field resistance on motor operation are also examined.

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    Attribution Non-Commercial (BY-NC)
    Download as PDF, TXT or read online from Scribd

    DC Machines

    Uploaded by

    Moses Mberwa
    49 views19 pages
    - DC motors come in two types: series and shunt. Series motors have the armature and field coils connected in series, while shunt motors have them connected in parallel. - Shunt motors maintain nearly constant speed as load increases due to constant field current. They are well-suited for applications requiring constant speed. Series motors experience decreasing speed as load increases due to the armature current relationship. They provide high starting torque. - Example calculations are provided for torque, speed, power and efficiency of sample shunt and series motors. The effects of changing supply voltage and field resistance on motor operation are also examined.

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    dc machines notes

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    dc machines

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    - DC motors come in two types: series and shunt. Series motors have the armature and field coils connected in series, while shunt motors have them connected in parallel. - Shunt motors maintain nearly constant speed as load increases due to constant field current. They are well-suited for applications requiring constant speed. Series motors experience decreasing speed as load increases due to the armature current relationship. They provide high starting torque. - Example calculations are provided for torque, speed, power and efficiency of sample shunt and series motors. The effects of changing supply voltage and field resistance on motor operation are also examined.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    49 views19 pages

    DC Machines

    Uploaded by

    Moses Mberwa
    - DC motors come in two types: series and shunt. Series motors have the armature and field coils connected in series, while shunt motors have them connected in parallel. - Shunt motors maintain nearly constant speed as load increases due to constant field current. They are well-suited for applications requiring constant speed. Series motors experience decreasing speed as load increases due to the armature current relationship. They provide high starting torque. - Example calculations are provided for torque, speed, power and efficiency of sample shunt and series motors. The effects of changing supply voltage and field resistance on motor operation are also examined.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 19

    DC Machines

    Dr Andrew Cruden
    Department of Electronic & Electrical Engineering (Centre for Economic Renewable Power Delivery)

    Objective of Lecture
    Objective: to investigate the torque/speed characteristics of series and shunt DC motors and describe typical applications for both types Review There are two types of DC machine to be considered: series and shunt connected motors Series motors have the armature and field coils connected in series Shunt motors have the armature and field coils connected in parallel
    2

    DC Motor Relationships
    In the motor armature, the effect of the applied voltage, Va, is reduced by the influence of an induced emf, Vemf, which is established through the process of electromagnetic induction in opposition to the applied voltage Therefore the equivalent applied voltage, Veq, equals Veq = Va - Vemf The induced emf is only present when the motor armature is turning. If the motor is stopped then there is no induced emf, as no lines of flux are being cut, and the armature current will be a maximum (as Veq = Va). Note that the induced emf can never be equal to or higher than the applied voltage
    3

    DC Motor Relationships - contd


    As Va is regulated by the induced emf, the armature current is regulated, i.e. if no emf was induced the armature current flowing would be much higher and the speed of motor operation would be much higher The value of induced emf, Vemf rate of change of flux, or Vemf = Ke where is the motor flux, is the rotational speed of the motor in rads/sec and Ke is a constant relating motor parameters such as number of poles etc and is specific for a particular motor The flux, , is established by the field current If
    4

    DC Motor Relationships - contd


    The torque output of the motor is proportional to the force on the conductors and from F = BIaL = (/A)IaL then T = IaKt where is as before, Ia is the armature current and Kt is the torque constant of the motor.

    DC Shunt Motor
    For a shunt connected motor if the applied voltage is constant then the field current, If, and therefore the flux, , is constant E ( = Vemf = Ke) So when a load is applied to the shunt motor the speed decreases, and therefore so does E As E decreases then the armature current, Ia, starts to increase and the torque, T = IfIaKt, starts to increase Va I a Ra Va = IaRa + E = I f Ke

    Ra

    La

    Ia

    Rf Va If E Lf M

    T,

    DC Shunt Motor
    Torque vs Speed Characteristics for a DC Shunt Motor

    Increasing V a

    Speed,

    The motor torque will increase until it matches the applied load torque For typical values of Ia, and Ra, it is normal to assume Vemf ~ Va Hence as Va is varied so does E and Practically, at no-load (T~0), Ia is small and E~Va, and as the load torque increases then Ia increases and causes E to fall slightly relative to Va. Therefore the speed does fall slightly as torque increases but in general for a shunt motor the speed is considered constant w.r.t Torque 7

    Torque, T

    Practical Concerns: Shunt Motor


    Va I a Ra Field Weakening: Examine the earlier equation = I K f e

    The speed could be varied by varying If This is known as field weakening - however there is a danger if the field current is reduced to much - the motor could overspeed!! Some motors sense any decrease in field current and operate a protective relay trip i.e. disconnect the power If would typically be varied by changing the field resistance Rf

    Example: Shunt Motor


    Assume that the shunt motor is supplied from a 100V supply, and has an armature resistance of 1 and a field resistance of 100, torque of 0.949Nm at a speed of 990rpm and Ke = 0.955 V/(A.rad/s). Calculate the power output of this motor, the efficiency at this load and the new speed if the field resistance is changed to 125? 1 Iin Ia
    If M

    E
    100

    100V
    9

    Example: Shunt Motor - contd


    Power Output, P = T = T*(2/60)*n , where T is the motor torque and n is the motor speed in rpm P = 0.949*(2/60)*990 = 98.4 W Power input = Iin*Vin = (Ia+If)*Vin. The field current can be calculated easily from the circuit diagram If = 100V/100 = 1A. E=Ke=IfKe=1*(2/60)*990*0.955= 99V Va=Ia.Ra+E Ia=(100-99)/1= 1A Therefore Power input = (1+1)*100 = 200W The motor efficiency is =Po/Pin*100% = 98.4/200*100 = 49.2%
    10

    Example: Shunt Motor - contd


    From previous it was shown that the torque for a shunt motor is proportional to the armature current, which has not changed, therefore the motor torque is still = 0.949Nm The new field current, If = 100/125 =0.8A From T = IfIaKt, for If = 1A then Kt= 0.949/(1*1) = 0.949Nm/A2 For the new value of If , the corresponding new value of the armature current can be calculated Ia = 0.949/(0.8*0.949) = 1.25A Therefore,
    =
    Va Ia Ra 100 (1.25 * 1) = = 129.25 rad / s = 1234 rpm 0.8 * 0.955 I f Ke
    11

    Example: Shunt Motor - contd


    Say the load torque on the motor was trebled on sudden application of a new load. Investigate the effect on the motor speed for constant field current (with Rf = 100) . New torque, T = 3*0.949 = 2.847Nm = IfIaKt Therefore new Ia = 2.847/(1*0.949) = 3A (This is intuitively correct as torque Ia if the field current is constant) From Va IaRa 100 (3*1) . = = =10157rad / s =970rpm 1* 0955 . If Ke Therefore for a trebling of motor load the speed has changed by ((990-970)/990)*100% = 2% i.e. almost constant speed with changing load Shunt motors are good for conveyors and machine tools which must operate at constant speed with varying loads 12

    Series DC Motor

    R a + Rf L a + Lf Ia

    Va

    T,

    The flux, , of the motor is directly proportional to the field current, If, flowing to establish the magnetic field. For a series motor the current in the field and armature coils is exactly the same T = IfIaKt = Ia2Kt From the circuit diagram, Va = Ia(Ra+Rf) + E, where Va and Ia are average values and E = Vemf = Ke E=IaKe 13

    Series DC Motor - contd


    At no load, T~0, Ia is minimal and E~Va, and hence is a maximum as E=IaKe As the load torque increases, Ia rises to the square power (TIa2) As Ia increases E falls relative to Va, however the speed falls according to E=IaKe Therefore as the torque increases the speed of a series motor decreases quite rapidly (T = constant)

    Torque vs Speed Characteristics of a Series DC Motor Stall Torque


    Torque, T

    Increasing V a

    Speed,

    14

    Practical Concerns: Series Motors


    From the torque vs speed characteristic it is clear that as the torque is decreased the motor speed increases - and will keep on increasing into overspeed (similar to potential field weakening run away for the shunt motor) It is good practice therefore to always have a load connected to a series DC motor before it is turned on to prevent overspeed. (generally this only applies to large DC motors as small motors tend to have enough friction to prevent this naturally)

    15

    Speed Regulation
    Speed regulation determines the ability of the motor armature to maintain its speed when a changing load is applied (Analogous to transformer voltage regulation) Speed regulation is a ratio of no-load to full-load speed
    % speed regulation = nnl n fl n fl 100%

    where nnl is the no-load speed and nfl is the full load speed in rpm The lower the speed regulation the more constant the motor speed will be over a range of applied loads
    16

    Example: Series Motor


    Say the torque a series motor produces is 4Nm with 5A flowing in the armature. Calculate the value of the stall torque if the supply voltage to the motor is 100V and Ra= Rf = 1 ? From previous it was shown that: T = Ia2Kt Kt = T/Ia2 = 4/25 = 0.16 Nm/A2 The armature current can be calculated by analysing the equivalent circuit: Va Vemf 100 0 Ia = = 50 A = Ra + R f 1+1 Note: Vemf = 0 at stall i.e. zero speed Therefore the stall torque Ts = 502*(0.16) = 400 Nm
    17

    Example: Series Motor - contd


    From previous it was shown: Va = Ia(Ra+Rf) + E and E=IaKe Ve Ia * ( Ra + R f ) Ke = Ia This can be rearranged to give: From previous, if the motor were turning at 900rpm, with a current of 5A then the emf constant, Ke, is
    Ke = 100 5 * (1 + 1) = 0.19 (V / A. rad / s ) 900 5*( *2 *) 60

    If the motor now takes 20A calculate the new torque and motor speed? T = Ia2Kt = 202*0.16 = 64Nm
    Va Ia *(Ra + Rf ) 100 20 *(1+1) = = =158rad / s =1501rpm . . IaKe 20 * 019 .
    18

    Example: Series Motor - contd


    The speed regulation of the series motor is difficult to calculate as at full-load the motor is virtually stalling and at no-load it could overspeed. Most applications for DC series motors are traction drives for locomotives or for cranes, where a high starting torque, high accelerating torque and high speed at light load is required These applications need high starting torque to overcome the inertia of the load and the train requires to run at high speed once moving The series motor self-adjusts to protect itself from overloads i.e. it will reduce speed as the load increases and then stall
    19

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