Determination of Efficiency

An efficient DC machine should be developed so that the relations between electrical and mechanical energy can be studied and understood. There are several methods used to determine the efficiency of DC machines: the Direct technique, the Indirect technique, and the Regenerative technique. undefined

Direct Method

• In a direct method, an efficiency is obtained through the measurement of input power and output power directly.
• Input Power (Pi) can be measured by connecting an ammeter and a voltmeter in the feed of the electric power to the machine. Pi = Vi × Ii, where Vi be the input voltage and Ii the input current.
• Mechanical Power (Pm) is defined as the pressure generated when a load is applied to the machine. Po = Vo × Io, where Vo is the output voltage and Io is the current of the output.

Efficiency (η) is then calculated as the ratio of output power to input power:

η = 100% × (Po / Pi).

Indirect Method

• One indirect method of measuring the efficiency of the cause-effect relationship involves measuring the losses in the machine separately and then using the principle of conservation of energy to calculate the efficiency.
• The common negative types of losses in DC machines are copper losses (due to resistance in windings), iron losses (due to hysteresis and eddy currents in the core), and mechanical losses (due to friction and winding).
• Copper losses Pcu can be calculated conducting a no-load test where the machine is supplied rated voltage but no mechanical load, the current is recorded. Pcu = I²R, since I is the no-load current and R is the windings’ resistance.
• The Pfe (iron losses) are determined by using a scenario where the rated voltage is applied to the stator while the rotor is held stationary in the form of a blocked rotor test. The power differently appears and the iron losses take place in this mode.
• Mechanical losses (Pmech) can be calculated through load free operation and reading the input power.

Efficiency is then calculated using the formula:

η = (Wout – Total Losses) / Wout.

Regenerative Method

• A dynamometer principle is considered to be an advanced technique which is exercised by applying load on the machine while allowing it to act as a generator.
• Speaking of the machine working as a generator, it holds some of the mechanical power back to the electrical supply system and therefore it decreases the amount of the power input that is used. This technique exploits the phenomenon of the regenerating braking.
• Efficiency refers to the ratio between electrical power that enters a machine when it acts as motor and electrical power that a machine delivers to the distribution grid when it runs as a generator. The dissimilitude between the varying energies denotes the power taken by the machine.

Efficiency is then calculated using the formula:

η = [(Output Power – Absorbed Power)] / Output Power.

Every technique has its own pros and cons and the decisions about methodologies depend on factors including the accuracy required, the equipment available, and the particular characteristics of the specific DC machine in question.

The direct current (DC) machines have been from the beginning of electrical engineering studies, since the end of the 19th century. They are used in many different industries such as manufacturing and transportation, and they are still applicable even though the current age is digital. The quality performances and dependability of these machines are the most important factors during their whole lifespan. This is why they are subject to extensive testing.

Testing is a tool that helps us to determine the functions of the machine, show if there are any mistakes, and correct them if needed. It also serves to define the instrument, which is an important criterion of its energy conservation and cost-effectiveness. Here we will consider the DC machine’s test type, procedure, and value.