05-01-2016, 04:24 PM
Aim: To pre-determine the efficiency of a D.C. shunt machine considering it as a generator or as a motor by performing Swinburne’s test on it.
Name plate details:
D.C. Shunt Motor
Power = H.P
Speed = rpm
Armature voltage = volts
Field voltage = volts
Armature current = amps
Field current = amps
THEORY
This is an indirect test and is applicable only to shunt or compound machine where the field flux is held fairly constant. The machine is run on no-load. The input current, armature current and the supply voltage are measured while the motor runs at rated speed. The armature and field resistance are determined at the room temperature (t1 oc). A temperature rise of about 400 c may be assumed and the corresponding resistance at the full-load temperature (t2 0 c) calculated
1 + a0 t1
R (t2) = R (t1)
1 + a 0 t2
Where a0 = temperature coefficient of copper = 0.00427 ( pure Copper)
Then Ra (t1), Rsh (t1), Ra (t2), and Rsh (t2) are recorded.
No – load power input = V Io Watts
Armature Copper loss = Ia2 Ra Watts
Where Ia is the no-load current in the armature.
V2
Shunt field copper loss = Watts
R sh ( t1)
In take to motor = V. I0
V2
V. Io – Iao 2 Ra (t1) - Watts = Friction loss + Iron loss
Rsh (t1)
V2
Field copper loss on load = Watts
Rah (t2)
V2
Friction loss + Iron Loss + Watts = Constant losses, Wc]
Rsh (t2)
At any armature current Ia ,
Armature Copper loss = Ia 2 Ra (t2)
Total Losses Wf = Wf + Ia 2 Ra (t2)
V
Shunt field current = Ish =
R sh (t2)
When the machine operates as a motor line current Im = Ia + Ish
When the machine operates as a generator line current Ig = Ia –Ish
Intake to motor = V Im
Out put as generator = V Ig
As motor, Efficiency = V Im- Wt
X 100%
V lm
V Im
As generator, Efficiency = X 100%
V lm + Wt
PROCEDURE :
• Keep the field voltage control autotransformer at maximum position.
• Keep the dc drive potentiometers at zero position.
• Connect the circuit as shown in the circuit diagram.
• Run the machine as a motor at no-load.
• Adjust the voltage applied to the armature to get rated voltage.
• Adjust the field autotransformer till the motor attains the rated speed and measure the armature voltage and current.
• Measure the field voltage and current.
• Measure the field resistance and armature resistance.
Observations :
At no-load (self excited dc shunt motor ) :
Speed =
Armature voltage =
Field voltage =
Armature current =
Field current =
Armature resistance =
Field resistance =
Sample Calculations :
Armature copper losses = Ia2 Ra
Filed copper losses = If2 Rf
Mechanical losses + iron losses = Armature input – Armature copper losses.
Total constant losses = Mechanical losses + iron losses + field copper losses
Efficiency as a motor :
Let us assume that the current drawn by the armature =
Input to the motor = V * I L =
Total losses = constant losses + armature copper losses =
Output = Input – total losses
Output
Efficiency m = --------- x 100
Input
Efficiency as a generator :
Let us assume that the current delivered by the armature =
Output =VIg
Total losses = constant losses + armature copper losses =
Input = Output + Losses =
Output
Efficiency m = -------- x 100
Input
Conclusion :
• The power required to conduct the test is very less as compared to the direct loading test.
• Constant losses are calculated from this method are used to compute the efficiency of dc machine as a generator and as a motor without actually loading it.
• Hence, this is an economic method.