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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
Apparatus
Name Range Quantity
DC Voltmeter 0 – 300V 1 No.
DC Ammeter 0 – 20V 1 No.
DC Ammeter 0 – o2 A 1 No.
Speed Indicator (Tachometer) 0 – 2000 rpm 1 No.
Variac 1-Phase, 2A 1 No
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