SYNCHRO |1
Question: What is synchro? Describe the parts of synchro.
Answer:
Synchro: Synchros are low torque devices widely used in control system for
transmitting shaft position information or for making two or more shafts to run
in synchronism.
So, synchro is used to transmit torque over long distances.
Parts of synchro: It has two parts:
i) Synchro transmitter
ii) Synchro receiver
Synchro transmitter: Its construction is similar to a 3-Φ alternator. The
stator has 3-Φ windings each at 120° apart. The rotor is of salient pole and has
single phase (1-Φ) winding. The stator and rotor cores are laminated with
silicon steel. The rotor is of dumb-bell shape. It has concentrated to single phase
supply, V1=Vmsinωt, through a pair of slip rings, it produces an alternating flux
field along the axis of the rotor. This alternating flux induces three unbalanced
1-Φ voltage in the three stator windings by transformer action. The output of the
transmitter is applied to the 3-Φ winding of the synchro receiver.
Prepared By: Hasan Bin Firoz (533)
Energy Conversion-III (EEE-705)
SYNCHRO |2
Synchro receiver: The synchro receiver has a cylindrical type rotor. But
other construction is same as transmitter. The cylindrical rotor gives uniform air
gap. So, it can work as an error detector.
Question: What are the types of synchro?
Answer: There are many types of synchro but the four basic types used for
applications of positioning and error voltage as-
i) Control transmitter
ii) Control receiver (C.R)
iii) Control transformer (C.T)
iv) Control differential (C.D)
where, type (i), (ii) and (iii) are always 1-Φ but type (iv) is 3-Φ type. Control
differential type synchro can be classified into two types:
a) Control differential transmitter
b) Control differential receiver
Question: Describe the working principle of synchro (voltage relation) along
with its transmitter and receiver.
Answer:
Working principle: When 1-Ф supply is given to the rotor, an alternating flux
is produced that induces voltage in the 3-Ф stator. The stator voltage will be
unbalanced due to 1-Ф supply.
Voltage relation: Let, rotor voltage
be Er. The alternating flux induces
three different voltage in the stator
windings s1, s2 and s3 respectively by
transformer action.
If transformation ratio, K = =
then the stator voltage, Es = KEr.
∴ RMS value of induced voltage in
stator winding 2 when the rotor is
displaced by an angle α,
Prepared By: Hasan Bin Firoz (533)
Energy Conversion-III (EEE-705)
SYNCHRO |3
E2S = Ercosα
Since, the axis of stator winding 1 is 120° ahead of stator winding 2,
∴ Induced voltage in stator winding 1 is E1S = Ercos(α 120°)
Similarly, E3S = Ercos(α 240°) = Ercos(α 120°)
∴ Vector sum,
E12 = E1S+E2S
= Ercos(α 120°) Ercosα
= Er cosαcos120° + Er sinαsin120° Ercosα
√
= Ercosα + sinα Ercosα
√
= Er[ ]+ sinα
√
=√ [ ]
=√ [cosαcos150°+sinαsin150°]
√
=√ cos(α 150°) [∵ cos150° = and sin150°= ]
∴ E12 = √ cos(α 150°)
Similarly, E23 = E2S + E3S = √ cos(α 30°)
And, E31 = E3S + E1S = √ cos(α 90°)
Question: Write down the application of synchro.
Answer:
Synchros are used to transmit torque without the use of rigid mechanical
connection. Synchros are basically torque transmission system.
As control transmitter (CX) is rotated by an angle α, control receiver (CR) is
also rotated by an angle α.
Let, CX rotor is displaced by an angle α and k1 is closed then rotor winding is
energized by 1-Φ supply. The 1-Φ supply will produce 1-Φ flux that induces
Prepared By: Hasan Bin Firoz (533)
Energy Conversion-III (EEE-705)
SYNCHRO |4
three voltages in the three stator windings. These three voltages circulate three
currents in the stator windings through the stator windings of CR also.
These current in CR produce fluxes whose axis is fixed by an angle α. Now K 2
is closed then the rotor flux of CR will interact with the CR stator flux and
produce a torque. The torque will rotate the freely moving rotor of CR to a
position which will correspond to CX rotor position, that is α.
When CX and CR in equilibrium position their stator voltages will be exactly
equal and opposite and no current flows in the stator.
If the CX rotor angle α changes to a new value, then CR rotor angle also
changes. Here CX is called Master and CR is called slave.
Prepared By: Hasan Bin Firoz (533)
Energy Conversion-III (EEE-705)
