Presentation Summary • Different types of dc motors •Separately excited dc motor steady state equations •Speed control methodology by armature voltage control •Speed control methodology by flux control •Steady-state operating regions of a drive •Numerical example on steady-state operating region •Dynamic model of separately excited dc motor with constant flux •Numerical example on transfer function of the dynamic model •Block diagram for dc motor speed control
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Presentation Summary • Different types of dc motors •Separately excited dc motor steady state equations •Speed control methodology by armature voltage control •Speed control methodology by flux control •Steady-state operating regions of a drive •Numerical example on steady-state operating region •Dynamic model of separately excited dc motor with constant flux •Numerical example on transfer function of the dynamic model •Block diagram for dc motor speed control
DC motor speed control Two types of dc machines are commonly used for speed control: 1) Separately excited dc machine . 2) Series excited dc machine . For this course we will discuss only about the separately excited dc motor.
Separately excited dc machine Under steady state,
V𝑎 = 𝐼𝑎𝑅𝑎 + 𝐸𝑏 : Armature equation (i)
𝐸𝑏 = 𝐾𝑚𝜔: Back EMF equation(𝐾𝑚 = constant =𝐾𝑒Φ) (ii)
𝑇𝑚 = 𝐾𝑚 𝐼𝑎 : Torque equation (iii)
𝑉𝑎=𝑅𝑎𝑇𝑚
𝐾𝑚+ 𝐾𝑚𝜔 (Using (ii) and (iii) in (i))
𝜔 =𝑉𝑎
𝐾𝑚−
𝑅𝑎
𝐾𝑚2 𝑇𝑚
Here 𝑉𝑎 = armature voltage in V, 𝐼𝑎 = armature current in A, , 𝐸𝑏 = back emf in V, 𝑇𝑚
= motor torque in N − m, 𝜔 = speed inrad
s, 𝐾𝑒 = machine constant (unitless),Φ =
flux
polein Wb., 𝑅𝑎 = armature resistance in Ω. These are steady-state values.
Armature voltage control
Speed below base (rated) speed is controlled by controlling Va. . Speed can be varied
by varying Va , keeping flux rated value.
Tm
ω Va =Va1
Va =Va2
Va =Va3
Va1>Va2>Va3F (Km) is constant
Flux control
Speed above base (rated) speed is controlled by changing flux Φ (or
Km) and keeping armature voltage constant at rated value.
Km2
Tm
ω
Km1
Km3
Km1<Km2<Km3
Va is at rated value
Steady-state operating regions of a drive
Base Speed
Torque limit
Power limit
Armature Voltage
Control
Flux
Control
• By controlling Va smoothly one can obtain speed control for any torque. This is
the preferred method of speed control below base speed (i.e.) constant torque,
variable power region.
• Above base speed the speed control is achieved by reducing flux of the machine
(flux weakening) at rated armature voltage. This means reducing Φ (or Km )
(constant power, variable torque region).
•Time constant of the armature circuit is much smaller compared to the armature
in a separately excited dc motor . Hence Va control is faster than F(Km ) control.
Example on operating regions
In a separately excited dc motor, the speed is varied from 0-1500 rpm
(base speed) by varying the terminal voltage from 0-500V, and keeping
the machine flux constant. The rated torque is 300 N-m.
(a) Find the output power at (i) 750 rpm and (ii) 1500 rpm if the torque
is held constant at 300 N-m up to base speed.
(b) Above base speed, the armature voltage is kept constant at 500V and
the flux is weakened to achieve speed control. Find the motor torque at 3000 rpm.