2. Measurement of the Windings Resistance for Universal Motors

2. MEASUREMENT OF THE WINDINGS RESISTANCE FOR UNIVERSAL MOTORS

2.1 INTRODUCTION. DESCRIPTION OF THE EXPERIMENT Motors that can be used with a single-phase AC source as well as a DC source of supply volta- ges are called universal motors. Universal motors differ in design from the induction motors. They have a series wound rotor circuitry similar to that of DC motors. The rotor of a universal series motor is made of a laminated iron core with coils around it. The ends of the wire coils directly connect to the commutator.

When a universal motor is fed with a DC supply, it works as a DC series motor. When it is fed with AC supply, it still produces a unidirectional torque. The armature winding and the field winding are connected in series, so they are in phase. Since the polarity of AC changes perio- dically, the direction of the magnetic field and the direction of the armature current reverse in such a way that the direction of the force experienced by the armature conductors remains the same. Thus, regardless of AC or DC supply, universal motors work on the same principle that a DC series motor works. This experiment aims to measure the windings resistance of the armature winding and of the field winding of the universal motor.

OBJECTIVES Students are expected to learn about the measurement of the windings resistance for universal motors. By running this experiment, the students will face some main objectives: ¾¾ To understand the schematic diagram corresponding to the measurement of the windings resistance for universal motors. ¾¾ To learn the universal motor wiring connections, in order to measure the windings resistance. ¾¾ To obtain the characteristic curves related to the measurement of the windings resistance for the armature winding and the field winding of the universal motor:

2.2 COMPONENTS LIST According to the formulated objectives, for running this experiment the following modules are needed: A. SOFTWARE

Optionally, a dedicated software can be ordered that allows the characteristic curves to be displayed. Software (DL 10065NSW) for data acquisition from DL 10065NF. Software (DL 10055NSW) for data acquisition from DL 10055NF.

11 B. HARDWARE ¾¾ DL 30150 Universal motor ¾¾ DL 30016 AC/DC Power supply ¾¾ DL 2109D10 AC/DC Voltmeter ¾¾ DL 10065NF Electric power digital measuring module, frame type ¾¾ DL 1013A Universal base for electrical machines ¾¾ DL 2100-2M Two-level work frame

2.3 PROCEDURE OUTLINE

SCHEMATIC DIAGRAM The circuit diagram for measuring the electrical resistance of the universal motor internal windings is presented in figure 1. In the figure, both schematic diagrams are shown, according to the internal resistance configuration for armature winding and field winding.

Figure 1. Circuit diagram for universal motor windings resistance measurement.

CHARACTERISTICS CURVES The internal resistance of the split phase motor characteristics refers to the following curves presented in figure 2. The same method is used as in the previous experiment that applies a power source to the motor windings and measure the voltage and current flowing through them.

12 Figure 2. The characteristic curves for measuring the windings resistance of a universal motor.

SETUP AND CONNECTION DIAGRAM Figure 3 shows the wiring recommendations for communication and power cables connection. Arrange the De Lorenzo modules in a convenient way for accessing and wiring. Identify the relevant ports for communication, cables and connectors.

Follow the diagram from the next figure and connect the power cables accordingly:

Figure 3. Power and communication cables are connected by following the general powering rules (colors, connectors): Before starting any wiring activity, check all the power connections: all switches must be OFF.

Do not miss the ground connecting terminal! As you can see from the diagram, with specific symbols, all the equipment are connected to the protective network through specific connector and cable.

EXPERIMENTAL PROCEDURE AND LEARNING PLAN At the beginning of this experiment, connect all the modules to a main power supply by using the supply cables.

13 Perform the circuit configuration that is shown in the wiring diagram presented in figure 3. Complete the first configuration in order to obtain the main windings (3.a) and then follow the same procedure for the second configuration (3.b). Operate the network power switch (I/O) that can be found on the front panel of the measuring DL 10065NF module. In order to use the DL 30016 power supply, follow the next prepara- tory steps:

¾¾ Turn the key clockwise.

Be sure that the main switch three-phase output is switched ON and the overload protection is on the START position (both are found at the back of the DL 30016 module).

¾¾ Press the green “start” button from the power supply module.

Make sure that the “temp” terminals from the DL 30016 module are connected in series with the “temp” terminals from the other modules used in the experiment (see the brown cables from figure 3).

¾¾ Switch the central selector "a0b" to position "a". Since the measurement is performed in DC conditions, by using the supply module DL 30016 you are able to power the universal motor.

At this point, make sure that the left knob of the power supply is turned counterclockwise, at “0” position.

Switch the selector corresponding to the variable DC voltage "L+L- / 0÷240V" to the "on" position. Gradually increase the voltage while measuring the current through the armature winding. The table presents the correlation between I and U.

14 Table 1. Measurement of the armature winding resistance. The same procedure must be done with the field winding. See the wiring diagram 3.b.

Table 2. Measurement of the field winding resistance. The resistance mean value is:

2.4 QUESTIONS Answer the following questions related to the experiment regarding the windings resistance measurement.

1. What is the name of the universal motor windings?

2. Using an ohmmeter, measure and record the resistance of the:

Armature windings = ______Ω

Field windings = ______Ω

4. Calculate the resistance mean value of the (D1-D2) windings and of the (H1-H2) windings:

15 Rmean_(D1-D2) = ______Ω

Rmean_(H1-H2) = ______Ω

2.5 CONCLUSIONS The test was carried out in direct current with the voltammetric method and it aims to obtain the winding resistance of the universal motor.

Students will compare the calculated resistance winding values with the values measured by using an ohmmeter.