- It is direct test useful to find efficiency of small DC Motor.
Theory
- The output power of the DC Motor is measured by applying brake to a pulley.
- One rope is wound round the pulley and its two ends are attached to spring balances W1 and W2.
- One end of the rope is slack side and other end is tight side it means that we can tight rope from one end.
- The tension of the rope can be adjusted by using swivels until the ammeter at the input side shows full load current.
Let
V = Supply
voltage
I = Full load
current
W1 =
Spring balance reading in kg
W2 =
Spring balance reading in kg
R = Radius of
the pulley in meter
N = Speed of the
motor in RPM
- The shaft torque developed by the motor
Tsh =
F × r
= ( W1 – W2 ) × r kg –
meter
= 9.81 ( W1
– W2 ) × r Newton – meter………( 1 )
- Motor output power Po = Tsh × ω
= Tsh
× ( 2πN / 60 ) watt…….. ( 2 )
= ( 2π / 60 ) ×
9.81 ( W1 – W2 ) × r × N
= 1.02 × ( W1
– W2 ) × r × N
- Motor input power = VI
Efficiency of
motor at full load = ( Output power / Input power ) × 100 %
= { ( 1.02 × ( W1
– W2 ) × r × N ) / VI } × 100%
Observation Table
Supply voltage
V
|
Input current
I
|
Spring weight
W1
|
Spring weight W2
|
Speed
N RPM
|
Radius of rope r
|
Calculation
Efficiency at
full load
Input power
VI ( A )
|
Output power
1.02 × ( W1
– W2 ) × r × N ( B )
|
Efficiency
= ( B / A ) × 100 %
|
Disadvantages
- This test is not useful to find out internal losses and efficiency of large machines because of large loss of energy.
- The output power is wasted.
Precaution
- This test is applicable for all type of DC motor e.g. DC shunt motor, DC series motor and DC Compound motor.
- Some load or brake is already applied before switched on the supply while performing this test on DC series motor because the speed of the DC series motor at no load is dangerous high.
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