Retardation Test
Retardation Test
Retardation Test
Experiment no:
RETARDATION TEST
CIRCUIT DIAGRAM:
Retardation test
AIM: To measure the moment of inertia (J) of a separately excited dc motor using
Retardation test.
APPARATUS:
From retardation test dωm/dt at rated speed is obtained. Now drive is reconnected to
the supply and run at rated speed and rotational mechanical power input to the drive
is measured. This is approximately equal to P. Now J can be calculated from above
equation. Main problem in this is that rotational mechanical losses cannot be
measured accurately because core losses and rotational mechanical losses cannot
be separated. In view of this, retardation test on a dc separately excited motor or a
synchronous motor is carried out with field on. Now core loss is included in the
rotational loss, which is now obtained as a difference of armature power input and
armature copper loss. In case of a wound rotor induction motor, retardation test can
be carried out by keeping the stator supply and opening the rotor winding
connection.
OBSERVATIONs:
a)Free running
N(rpm) t(secs)
b)Ra=1.3Ω
N Va Ia T Ea P ωm
(volts) (ampere) (sec) (volts) (watts)
CALCULATIONS:
Formula:
1) E=Va - IaRa
2) ωm=
3) P=EIa
J can be determined more accurately by obtaining speed time curve from the
retardation test as above and also rotational losses v/s speed plot. Using these two
plots, rotational losses v/s time plot can be obtained, e.g. for time t1, ωm1 is found
from the retardation plot. Then for this speed rotational loss P1 is obtained from the
plot of rotational loss v/s speed and plotted against t1. Area A enclosed between the
rotational loss v/s t plot and the time axis(shaded area), is the kinetic energy
dissipated during retardation test. If initial speed of the drive during retardation test
was ωm0 then
1
=
2
PROCEDURE:
CONCULSION: