Efficiency By Direct Loading Method:
In this direct loading method, the d.c. machine is loaded and output and input are measured to find the efficiency. For this purpose, two simple methods can be used.
1)Brake Test
2)Other Method
(i)Brake test on D.C Machines
In this Brake test on D.C Machines, a brake is applied to a water-cooled pulley mounted on the motor shaft as shown in Fig. (6.1). One end of the rope is fixed to the floor via a spring balance S and a known mass is suspended at the other end. If the spring balance reading is S kg-Wt and the suspended mass has a weight of W kg-Wt, then,
Net pull on the rope = (W – S) kg.Wt = (W – S)* 9.81 newtons
If r is the radius of the pulley in metres, then the shaft torque Tsh developed by the motor is
Tsh = (W – S)*9.81*r N – m
If the speed of the pulley is N r.p.m., then,
Output power =(2*3.14*N*Tsh)/60
=(2*3.14*N*(W-S)*9.81*r)/60
Let V = Supply voltage in volts
I = Current taken by the motor in amperes
Input to motor = V I watts
Efficiency= (2*3.14*N*(W-S)*9.81*r)/(60*V*I)
The efficiency of dc machines obtained from brake test is as above.
Brake Test Disadvantages:
- Spring balance readings are not stable ,they always fluctuate
- Some of the output power is wasted
- Frictional torque at a particular setting doesn’t be similar.
(ii)
In another method, the motor drives a calibrated generator i.e. one whose
efficiency is known at all loads.The output of the generator is measured with the help of an ammeter and voltmeter.
Output of motor = Generator output/Generator efficiency
Let V = Supply voltage is volts
I = Current taken by the motor is amperes
Input to motor = VI
Thus efficiency of the motor can be determined.
Because of several disadvantages , direct loading method is used only for determining the efficiency of small machines.