Retardation Test : Working, Theory, Example & Its Applications

A DC machine is an electromechanical device, used to change DC electricity into mechanical energy (or) mechanical energy into DC electricity. If the DC machine changes the energy from DC electrical to mechanical then it is called a DC motor. Similarly, if the DC machine changes the energy from mechanical to DC electrical, then it is called a DC generator. DC machine works on the electromagnetic induction principle. There are different tests performed on DC machines to know their performance and efficiency. So, one of the most important tests among them is the retardation test. The DC machine’s efficiency mainly depends upon its losses because when the losses are less, then the efficiency of the DC machine is higher. This article provides brief information on Retardation Test, its theory, and its applications.

What is Retardation Test?

The retardation test or running down test is a very efficient method to discover the iron, friction, and windage losses within dc machines. In this type of test, the stray or rotational losses and efficiency are also measured at any preferred load.

The Retardation test can be performed by simply applying a braking torque to the shaft of the motor & measuring the equivalent armature voltage, speed & current. So the motor will run in the opposite direction to generate a braking effect.

The motor in this test runs in the reverse direction and causes a magnetic field to generate in the reverse direction. So this magnetic field interacts simply with the stray magnetic fields within the motor and causes eddy currents to flow within the iron core & resulting in stray losses. During the retardation test, measuring the voltage & armature current, the stray losses can be measured.

Retardation Test Working Principle

If we consider a DC shunt motor running at no-load condition, the supply to the armature is discontinued however the field remains usually excited, then the motor gradually slows down and stops running finally. The armature’s kinetic energy is used up to conquer windage, iron & friction losses.

If the supply is cut off to the armature & field excitation, then again the motor runs slow & stops finally. At the moment, the armature’s kinetic energy can be used up to conquer the friction & windage losses only. This is estimated because, in the nonexistence of flux, there is no iron loss.

By performing the first test, we can discover the windage, friction, iron losses, and efficiency of the DC machine. But, if we carry out the second test we can also separate windage & friction losses from the iron losses.

Retardation Test Theory

The simplest and best technique to find the D.C. machine’s efficiency. In this technique, we find the mechanical & iron losses of the DC machine. After that, knowing the shunt Cu & armature losses at any electric load, the DC machine efficiency can be measured at that load. The DC machine in this test runs like a motor at just above the normal speed. After that, the armature supply will be cut off when the field is excited normally. The machine speed is allowed to drop to below normal value. The required time for this speed drop of the machine is simply noted. From these examinations, the rotational losses like friction, iron, and windage & the machine’s efficiency can be determined.

The retardation test circuit diagram is shown below. This test is used to get total stray losses like the combination of mechanical losses like windage & friction and iron losses of the DC machine. In this circuit, the A1 and A2 are armature terminals. The Retardation Test on D.C Machines procedure follows as;

The main points in the retardation or running down test are discussed below,

First, need to Turn ON the DC machine normally. After that run the machine slightly above the fixed speed by adjusting its resistance.

Once the fixed speed is achieved, disconnect the power supply to the armature, although keeping the field usually excited.

Now need to stay for some time to drop the machine speed below-rated speed, then note down the machine speed values in rpm & time in the sec with the tachometer.

As a result, the armature slows down & the amount of available kinetic energy within the armature is utilized for supplying the stray or rotational losses which comprise friction, winding & iron losses.

Let ‘N ‘be the normal speed within r.p.m.

‘w’ is normal angular velocity within rad/s = 2p N/60.

Rotational losses (W) = Rate of loss of Kinetic Energy of armature.

(or) W = d/dt (1/2 Iω^2)

Here ‘I’ is the armature’s inertia moment. As ω = 2πN/60.

W = I x (2πN/60)x d/dt (2πN/60) => (2π/60) ^2 IN dN/dt

(or)

W = = 0.011 IN dN/dt

Inertia Moment (I) for the Armature

In the retardation test of the DC machine, the rotational losses can be given as;

W = 0.011 IN dN/dt

Here the ‘I’ value must be known to find ‘W’ but it is hard to determine ‘I’ directly (or) through calculation. So, we perform another test like the fly-wheel method by which either ‘I’ is calculated (or) it is removed from the above equation.

Example:

Suppose the DC machine’s normal speed is 1200 r.p.m. Once the retardation test is achieved, then the required time for the DC machine speed to drop from 1050 – 970 r.p.m. is 10 seconds with the usually excited field. If the inertia moment for the armature is 80 kg m, then,

Rotational losses (W) = 0.011 IN dN/dt.

I = 80 kg m^2, N = 1200 r.p.m

dN = 1050 – 970 = 80 r.p.m, dt = 10 Secns.

W = 0.011 x 80 x 1200 x (80/10).

W = 0.011 x 80 x 1200 x (8) = 8448 watts.

Advantages and Disadvantages

The retardation test advantages include the following.

• The DC machine in this test acts as a motor at above a normal speed.
• This test is useful in finding the DC machine’s efficiency.
• This test needs extremely small power as compared to the motor and generator coupled system’s full load power.
• This test is the simplest and the best method to find out the efficiency of a DC machine.
• This test helps measure the total losses within the motor.
• This is a very convenient test.

The retardation test disadvantages include the following.

• The main drawback of using this test is the precise determination of the speed which is constantly changing.
• This test is made on only a separately-excited DC machine.

Applications

The applications of retardation test include the following.

• Retardation test or running down test is a very efficient way to detect stray losses within dc shunt motors like friction, iron & windage losses.
• This test is used to find the shunt wound DC machine’s efficiency.
• This is the simplest & best method to find out the constant-speed DC machine’s efficiency.
• This test is applicable for shunt generators & motors.
• This test is mainly done to measure rotor inertia.

Thus, this is an overview of the retardation test on dc motor, theory, examples, advantages, disadvantages, and applications. The retardation test is the best method used on the DC shunt motor to find out stray losses which occur within the motor because of eddy currents as well as hysteresis losses within the iron core, and magnetic flux leakage from the stator & rotor. This test helps find the mechanical & iron losses of the DC machine. Here is a question for you, what is Swinburne’s Test?