DC Series Machine Flashcards
(14 cards)
what is a DC series machine
field winding is in series with armature
same current flows through armature and field windings
Ia = If
what happens when load increases in DC series machine
when load increases, Ia increases, If increases, torque increases
when load is removed (no R), I, BF drops and speed increases a lot (no e to resist V)
what happens to the formula for V(in) (form shee)
Ra causes resistance but field circuit has R too RF
Vin = Ia*(Ra+Rf) + e
how is BF related to If
magnetic field is created by the current flowing through the field winding so
BF∝IF
how is back emf related to BF and ω
when a rotor spins the conductors cut through the magnetic field creating a voltage called back emf (e)
this is proportioanl to magnetic field strength and speed
e∝BF⋅ω
how is back emf related to Ia (form sheet)
Since 𝐵𝐹∝𝐼𝐴
this becomes e∝IA⋅ω and
this is 𝑒 = 𝐾𝐼𝐴𝜔
K = machine parameter
using all of the prev formulas what is the new T (form sheet)
T = K(Ia)^2
using all of the prev formulas what is the new V
𝑉𝑖𝑛 = 𝐼𝐴(𝑅𝐴 + 𝑅𝐹) + 𝐾Iaomega
what is the speed current relationship form shee
𝜔 = (𝑉𝑖𝑛/𝐾*𝐼𝐴) − ((𝑅𝐴 + 𝑅𝐹) / 𝐾
what is torque-speed relationship formula(form sheet)
rearraging spspeed current to make Ia subject and then putting that in as Ia in torque form
𝑇 = (𝐾(𝑉𝑖𝑛)^2) / ([𝐾𝜔 + (𝑅𝐴 + 𝑅𝐹)]^2)
T-IA characteristics
torque is proportional to the square of Ia .eg. if current increasing by 2 then T increases by 4
omega-Ia chareacterists
when there is no mech load (0 Ia) speed goes to infinity, when more mech load is applied (IA>0) speed drops sharply
speed decreases nonlinearly as armature current increases due to rising magnetic field
torque - speed characteristics
Torque drops sharply as speed increases — high torque at low speed, good for heavy starts.
applications for dc series machine
low power elec apppliances
motor for lifts and trains
