EXPERIMENT 6
6.1Object: Load Test on a D.C. Compound Motor
A compound motor has both series and shunt fields on the same pole and the speed can be changed at greater flexibility. The electrical equivalent circuit of a D.C. compound motor is shown in Figure 6.1; the connected instruments are YY−Y, the series winding, and ZZ−Z, the shunt winding. After reading the nameplate of the motor, we decide the range of the instruments.
Figure 6.1
6.2The Load Test
A load test is simple when we require the speed and torque of the motor for various steps of loading. We increase the torque T = T1 − T2 by a belt and measure the RPM by a tachometer.
EXPERIMENTAL RESULTS
Plot the graph between torque and speed. Note that the torque must be located on the X-axis and speed N on the Y-axis, because speed is the dependent variable and torque is the independent variable.
Figure 6.2
From the characteristic equation of the machine we have:
| eb = Vt − Ia (Ra + Rfs) | (6.1) |
Refer to Figure 6.2 to obtain the characteristic equation of the motor.
| KϕN = Vt − IaR | (6.2) |
Figure 6.3
The speed will depend on Ia and ϕ will also produce a flux in the series winding.
6.3Input and Output Relations
The efficiency will be
6.4Current and Torque Relations
The current and torque relations are shown in Figure 6.5. The torque will be linearly proportional to the current.
| T = K1Ia |
This can be found from all the machines:
| 2π NT = eaIa | (6.5) | |
| or | ||
| T = KIa | (6.6) |
Figure 6.4
Figure 6.5
6.5Discussion
The load characteristics are shown in Figures 6.1, 6.4, and 6.5. All three are called the load curves, in a broad sense. But the speed-torque curve is referred to as the load curve for the motor.
6.6Questions and Answers on the Experiment
Q1. How will you change the flux of the series winding in the compound motor?
Ans. By using a diverter.
Q2. The differentially compounded motor has a tendency to start in the opposite direction. Why?
Ans. The series winding flux will be in the opposite direction from the main field direction and sets earlier than the shunt field flux.
Q3. What are the advantages of a compound motor?
Ans. It can run as a series motor or shunt motor. Speed can be increased or decreased from the normal speed.
Q4. Where are the series and shunt windings?
Ans. On the main poles.
Q5. A cumulatively compounded motor is running at a loaded condition and the series winding is short-circuited; discuss the speed variation.
Ans. Speed will rise due to flux reduction.
Q6. A differentially compounded motor is running at a loaded condition and the series winding is short-circuited; discuss the speed variation.
Ans. The speed will fall due to the rise of flux in the air gap.