In this method full load is applied to the machine and output is directly measured. Though this method is very simple in looking, but in¬volves complication in the measurement of mechanical power input in the case of a generator, and output in the case of a motor. In the case of a generator, the input mechanical power is measured by connecting some form of dynamometer to the prime mover and water resistance load is applied, in the case of a motor, some form of brake is applied to water-cooled pulley for measuring the output. The load is adjusted untill full load current is flowing. The input and output are measured and the efficiency is determined. Since the complete power is wasted during this test, this method is suitable for small machines only. If this test is to be done in a series motor, the brake must be tight before the motor is started, other¬wise the armature may get damaged and fly to pieces. In the case of large motors the motor is coupled to a generator and generator is loaded by means of a resistance.
The testing of large machines by the direct method entails a con¬siderable loss of power; infact, in the case of a very large machine, there may be no facilities for a direct test.
Figure shows the arrangement for this load test. It consists of a brake drum keyed to the shaft of the motor. The brake drum is filled with water to cool it. It is made of cast iron or aluminium. A belt is wrapped around the drum and each end of the belt is attached to a spring balance. The spring balances are suspended from a frame which is fixed rigidly. Hand wheels are provided on the top of the frame. The tension on the belt is adjusted by turning the hand wheels. If the tension on the belt is increased, then the load on the motor is increased. The spring balances indicate the force acting on the brake drum. The net torque in Kg-m is calculated using the expression
No comments:
Post a Comment