Thyristor speed control of separately excited D.
C motor
The bridge rectifier converts a voltage into d.c voltage which is then applied to the
armature of the separately excited d.c motor M.
𝑉−𝐼𝑎 𝑅𝑎
The speed of the motor is given by 𝑁 = .
If is kept constant and also if 𝐼𝑎 𝑅𝑎 is neglected ,then N ∝ V ∝ voltage across the
armature.
The value of this Voltage furnished by rectifier can be changed by varying the firing
angle of the thyristor T with the help of its control circuit.
• As is increased i.e
thyristor firing is delayed
more and its conduction
period is reduced, the
armature voltage is
decreased in turn and this
reduces the motor speed
and vice versa.
• So as increases, V
reduces and hence N
reduces and vice versa.
The free wheeling diode connected across the motor
provides a circulating path a shown by the dotted
path for the energy stored in the inductance of the
armature winding at the time T turns OFF.
Without D ,current will flow through T and the bridge
rectifier ,prohibiting T from turning OFF.
Thyristor speed control of a D.C Series motor
In this circuit ,an RC network is used to control the diac voltage that triggers the
gate of a thyristor.
As the a.c supply is switched ON, thyristor T remains
OFF but the capacitor C is charged through motor
armature and R towards the peak value of the
applied a.c voltage.
The time it takes for the capacitor voltage 𝑉𝐶 to reach
the breakover voltage of diac depends on the setting
of the variable resistor T.
When 𝑉𝐶 becomes equal to the breakover voltage of
diac ,it conducts and a triggering pulse is applied to
the thyristor gate G.
Hence T is turned ON and allows current to pass
through the motor.
Increasing R delays the rise of 𝑉𝐶 and hence the breakover of
the diac so that the thyristor is fired later in each positive half
cycle of the a.c supply. This reduces the conduction angle of
the thyristor which consequently delivers less power to the
motor and hence motor speed is reduced.
If R is reduced ,time constant of the RC network is decreased
which allows 𝑉𝑐 to rise to breakover voltage of diac more
quickly hence making the thyristor fire early in each positive
input half cycle of the supply. Due to the increase in the
conduction angle of the thyristor ,power delivered to the motor
is increased with a subsequent increase in its speed.
Diode D is a free wheeling diode which provides circulating
current path for the energy stored in the inductance of the
armature windings.
Full wave control of a shunt motor
This circuit provides a wide range of speed control for a fractional kW shunt dc motor.
It uses a bridge cct for full wave rectification of the a.c supply.
The shunt field winding is permanently connected across the d.c output of the bridge
circuit.
The armature voltage is supplied through thyristor T and hence the magnitude of this
voltage (motor speed) can be changed by turning 𝑇𝑂𝑁 at different points in each half
cycle with the help of R.
The T only turns OFF at the end of each half cycle.
𝐷3 provides a circulating current path for the energy stored in the armature winding at
the time T turns OFF.
At the beginning each half cycle is OFF state and C starts charging up via motor
armature ,𝐷2 , and speed control variable resistor R ( can not charge through
𝑅1 because of the reverse biased 𝐷1 .
When 𝑉𝐶 𝑏𝑢𝑖𝑙𝑑𝑠 up to breakover voltage of diac ,the diac conducts and applies a
sudden pulse to T thereby turning it ON.
Hence power is supplied to the motor armature for the remainder of that half cycle.
At the end of each cycle ,C is discharged through 𝐷1 , 𝑅1 𝑎𝑛𝑑 𝑠ℎ𝑢𝑚𝑡 𝑓𝑖𝑒𝑙𝑑 𝑤𝑖𝑛𝑑𝑖𝑛𝑔.
The delay angle ∝ depends on the time it takes 𝑉𝐶 𝑡𝑜 become equal to breakover
voltage of the diac which in turn depends on the time constant of the RC circuit and
the voltage available at point A.
By changing R ,𝑉𝐶 can be made to build up either slowly or quickly and thus
change the angle ∝ at will.
In this way ,the average value of the dc voltage is controlled .
Motor speed is also controlled as it is directly proportional to the armature
voltage.
E.g when 𝑉𝐴 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒𝑠 the voltage across R-C charging circuit increases building
𝑉𝐶 𝑚𝑜𝑟𝑒 quickly ,leading to early switch ON on T in each half cycle, as result
power supplied to the armature increases which inturn increases motor speed
thereby compensating for the motor loading.
