Demonstrative Video

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LOAD TEST ON DC SERIES MOTOR

OBJECTIVE FOR LOAD TEST:

To conduct load test on a DC series motor and to find the eeficiency of the motor

NAME PLATE DETAILS:

 

APPARATUS REQUIRED:

CIRCUIT DIAGRAM FOR LOAD TEST:

THEORY:

Performance of a DC machine is largely influenced by the way in which the field winding is excited with DC current. There are two basic ways of exciting a DC machine:

1. Shunt field: It is in parallel with the armature circuit and hence the name shunt field. It has high resistance and carries a small current. It has a large number of turns of thin wire.
2. Series field: The field winding is excited from the armature current by placing it in series with the armature, thus the name series field.
   • The field coil and armature windings are connected in series to the power source. The field coil is wound with a few turns of heavy gauge wire.
   • In this motor, the magnetic field is produced by the current flowing through the armature winding; with the result that the magnetic field is weak when the motor load is light (the armature winding draws a minimum current). The magnetic field is strong when the load is heavy (the armature winding draws a maximum current).
   • The armature voltage is nearly equal to the PS line voltage (just as in the shunt wound motor if we neglect the small drop in the series field).
   • Consequently, the speed of the series wound motor is entirely determined by the load current.
   • The speed is low at heavy loads, and very high at no load. In fact, many series motors will, if operated at no load, run so fast that they destroy themselves.
   • The high forces associated with high speeds cause the rotor to fly apart, often with disastrous results to people and property nearby.
   • The torque of any DC motor depends upon the product of the armature current and the magnetic field. For the series wound motor, this relationship implies that the torque will be very large for high armature currents.
   • The series wound motor is, therefore, well adapted to start large heavy inertia loads, and is particularly useful as a drive motor in electric buses, trains, and heavy-duty traction applications. Compared to the shunt motor, the series DC motor has high starting torque and poor speed regulation.
   • The DC series motor thus behaves differently compared to the DC shunt motor. The shunt winding produces an almost constant speed of operation (i.e., low speed regulation) whereas the series winding leads to a machine with very high speed regulation.

The EMF induced in a DC machine is expressed as \( \mathrm{K}_a \phi n \), where \( n \) is the speed, \( \phi \) is the flux per pole, and \( \mathrm{K}_a \) is a constant for the given machine. From the figure,

\[ \begin{aligned} & E_a = v_t - I_a \left( R_a + R_{se} \right) = k_a \phi_n \\ & n = \left( \frac{V_t}{k_a \phi} \right) - \left( \frac{(R_a + R_{se}) \cdot I_a}{k_a \phi} \right) \\ & \text{As } I_a \rightarrow 0, \phi \rightarrow 0 \quad \text{hence}, \quad n \rightarrow \infty \end{aligned} \]

Thus, an unloaded series motor can over-speed and literally spin itself apart when under-loaded. Hence, a DC series motor should never be allowed to run at no-load condition even accidentally.

\( T = k_a \phi_n \), where \( T \) is the torque. For linear magnetization, \( \phi = k_f I_a \). Therefore, \( T = k_a k_f I_a^2 \).

EM LAB SET-UP:

PRECAUTIONS:

• Motor should be started and stopped with load.
• Brake drum should be cooled with water when the motor is loaded.
• Do not exceed the full load current.
• Do not exceed 1800 rpm speed.

CONNECTION:

• Supply Positive to Ammeter Terminal (Red)
• Supply Positive to Voltmeter Terminal (Red)
• Ammeter Terminal (Black) to 2 Point Starter L
• Starter N to Supply Negative Terminal (Black)
• Starter Terminal A to Series Field Terminal S
• Series Field Terminal SS to Motor A
• Supply Negative Terminal to Voltmeter Terminal (Black)
• Supply Negative Terminal to Starter N
• Supply Negative Terminal to Motor Terminal AA

PROCEDURE:

1. Connections are made as per the circuit diagram.
2. After checking whether the motor is sufficiently loaded (at least 50%), DPST switch is closed and motor is started with help of starter. The starter resistance is gradually removed by pulling the handle of the starter towards the right-hand side and gets locked.
3. Apply load by tightening the belt.
4. For various loads, the Voltmeter, Ammeter readings, the spring balance readings, and speed are noted. Do not exceed the full load current. Do not exceed 1800 rpm speed.
5. The load is brought to the initial (50%) position and the supply is switched off while observing the necessary precautions.
6. Measure the radius of the drum.

OBSERVATION TABLE

 

Radius of the drum R = ---------------m

FORMULAE USED FOR CALCULATIONS:

MODEL GRAPH:

GRAPHS TO BE PLOTTED:

1. % Efficiency (η) vs Output Power
2. Torque vs Output Power
3. Speed vs Output Power

RESULT:

Thus the efficiency of the DC series motor is found and the torque and speed of the motor is plotted against the output power.

QUESTIONS:

1. Why can't a series motor be started at no load condition?
2. Why does the DC series motor have a high starting torque?
3. Why does a DC series motor have very high speed regulation?
4. How is speed control through field control achieved in a series motor? Write down the different ways of doing the same.
5. Write down the advantages and disadvantages of a DC series motor over a shunt motor.
6. Write down the differences between the shunt and series field windings.
7. State two applications of a DC series motor.