retardation test

DATE:

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:

1) 2) 3) 4) 5) 6) 7) 8)

DC shunt motor(220V DC,2.75 A,7 HP,940 r.p.m) Rheostat: 145, 2.8A (1 no); 50, 8.5A (2 nos.) Voltmeter: 0-300V DC Ammeter:0-10A DC Tachometer  Stopwatch DPST Connecting wires

THEORY: Moment of inertia can be calculated if dimensions and weights of various parts of the load and motor are known. It can also be measured experimentally by retardation test.

In retardation test, the drive is run at a speed slightly higher than rated speed and then the supply to it is cut off. Drive continues to run due to kinetic energy stored in it and decelerates due to rotational mechanical losses. Variation of speed with time is recorded. At any speed ωm, power consumed in supplying rotational losses is given by P = Rate of change of kinetic energy =

 # H $  = H     $ 

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=1.3 N

Va (volts)

CALCULATIONS:

Formula: 1) E=Va - IaRa

$ "

2) ωm=

3) P=EIa

Ia (ampere)

T (sec)

Ea (volts)

P (watts)

ωm

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 H $ =  2 " PROCEDURE:

1) 2) 3) 4) 5) 6) 7) 8)

Connect the circuit as shown in fig. Initially keep the armature resistance at maximum and field at minimum. Adjust armature and field so that motor should run at rated speed. Disconnect the motor from supply and field ON. Measure the time as the motor speed starts decreasing in steps. Now, again start the motor by connecting its supply as given in step 2 and 3. Measure the motor terminal voltage. As well as armature current. Now decrease the armature voltage. With the help of rheostat in armature circuit without any change in field in steps. 9) Measure voltage and current. 10) Measure armature resistance of motor .

CONCULSION: