Retardation Test or Running Down On D.C Machines
Retardation Test [Running Down Test] is used to find stray losses in D.C machines.this is one of the best method to find the mechanical (friction and windage) and iron losses of the machine.we can find efficiency by knowing armature and shunt copper losses at any given load current.
Theory points to be noted of retardation test:
1. d.c. machine is run as a motor at a speed just above the normal.
2. then the supply to the armature is cut off while the field is normally excited.
3. the speed is allowed to fall to some value just below normal. The time taken for this fall of speed is noted.
Circuit Diagram Running Down Test:
The armature consequently slows down and its kinetic energy is used in supplying the rotational or stray losses which includes iron, friction and winding loss.
If I is the amount of inertia of the armature ans is the angular velocity.
Kinetic energy of armature = 0.5 Iω2
Rotational losses, W = Rate of change of kinetic energy
Moment of Inertia (I) of the Armature:
In retardation test, the rotational losses are given by:
In order to find W, the value of I must be known.to find I value we perform two methods.
W = 0.011 IN dN/dt.
Methods to find I value:
(a) Using Flywheel
(b) Without using Flywheel
Using Flywheel:
It is a fly-wheel method in which the value of I is calculated. First, retardation test is performed with armature alone and dN/dt1 is determined. Next, a flywheel of known moment of inertia I1 is keyed on to the shaft of the machine. For the same change in speed, dN/dt2 is noted. Since the addition of fly-wheel will not materially affect the rotational losses in the two cases,Since the values of I1, t1 and t2 are known, the moment of inertia I of the
armature can be determined.
Without using Flywheel:
In this method, I is eliminated from the expression by an experiment. First,
retardation test is performed with armature alone. The rotational losses are given by;
W = 0.011 IN dN/dt1
Next the motor is loaded with a known amount of power W' with a brake. For the same change in speed, dN/dt2 is noted. Then, Retardation test on dc machines.
- Since the values of W', t1 and t2 are known, the value of W can be determined.
- Here dN/dt1 is rate of change in speed without extra load whereas dN/dt2 is rate change in speed with extra electrical load which provides retarding torque.
Tags:Retardation Test On D.C Machines Wikipedia,Retardation or Running down test on d.c machines,lab manual,retardation test on dc shunt motor theory
