I'm doing a lab report for a shunt DC motor evaluation. I seem to be stuck on the efficiency vs torque of the motor.
I have plotted a graph of measured \$\frac{P_{out}}{P_{in}}\cdot 100\%\$, which shows that the motor becomes less effcient with more load. (Increments from 0.1Nm to 1.1Nm then stalls.)
This is fine as it shows me that the motor is running aroud 50% efficiency at 0.1Nm and 18% at 1Nm.
My issue is when I try to use some more involved formulas the graph seem to flip around using the recorded values and shows that the motor is most efficient just before stall condition up to 95% so I'm unsure which to use.
The formulas I have been given are
$$ P_{out}= 2\pi T \frac{N}{60} \\P_{in} = V_f I_f + V_a I_a$$
Calculate these then put into the previous formula \$\frac{P_{out}}{P_{in}}\cdot 100\%\$.
Any ideas why the results are flipping?
To add here are my results from the Lab I have calculated the Pin and Pout using Ohms law V.I from the armature values Pin and Field values Pout i have also added the equation 5,6,7 calculations on the right hand side
Here are my calculations using the Eq 5,6,7 from above excell doing the calculations
Hopefully this all makes sense thanks
Best Answer
Eq5, Eq6 and Eq7 are correct for a motor with a separately excited field. If the motor has a shunt field, the field and armature voltages would be equal. Since they are not quite equal, the motor is treated like it has a separately excited field.
The Excel spreadsheet must be set up for some other conditions. Since nothing you have posted indicates that anything else is applicable, Eq5, Eq6 and Eq7 should be used.
The RPM increasing with load would seem to indicate that something is going on that is not explained. However I don't see a reason that should have much effect the efficiency very much.