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3. STARTING A PERMANENT SPLIT-CAPACITOR MOTOR 3.1 INTRODUCTION. DESCRIPTION OF THE EXPERIMENT This experiment refers to a permanent split capacitor induction motor having a cage rotor with two stator windings. The stator windings are named main windings and auxiliary windings. This type of motor has a capacitor (C) permanently connected in series with the auxiliary windings. This is the main reason why this motor is called single-phase with capacitor motor. This type of motor has not a starting switch and operates in the same way as a capacitor-start capacitor-run induction motor. It produces a uniform torque and it is less noisy while running. The split phase capacitor motor has a few main advantages: high efficiency, high power factor, no centrifugal switch. The main limitation is that its starting torque is quite low, between 30 to 90% of the rated load, so it cannot be used for applications which are hard to start. Permanent capacitor single-phase induction motors are commonly found in the drive systems of fans, refrigerators, compressors and pumps, etc. OBJECTIVES By running this experiment, the students will face some main objectives: ¾¾ To understand the schematic diagram corresponding to the permanent capacitor single-phase induction motors. ¾¾ To learn the split phase motor with capacitor wiring connections. ¾¾ To observe the motor starting and running operations. 3.2 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. B. HARDWARE ¾¾ DL 30130 Split phase motor ¾¾ DL 30135 Capacitor unit 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 10055NF Mechanical power digital measuring unit, frame type ¾¾ DL 30300 Electromagnetic brake ¾¾ DL 2006E Load Cell 150N 17 ¾¾ DL 1013A Universal base for electrical machines ¾¾ DL 2100-2M Two-level work frame 3.3 PROCEDURE OUTLINE SCHEMATIC DIAGRAM The permanent split capacitor motor has a cage rotor and the two windings named as main and auxiliary windings. It has only one capacitor connected in series with the starting winding. Figure 1. Circuit diagram of permanent capacitor single-phase induction motors. The capacitor CB is permanently connected in the circuit in both starting and running conditions. Since the capacitor remains in the circuit all the times, no centrifugal switch is required. CHARACTERISTICS CURVES The capacitor value is selected as per the requirement of the starting torque. The starting torque can be as high as 350 to 400 % of the full load torque. The torque-speed characteristic is as shown in figure 2. Figure 2. The torque/speed curve of a permanent capacitor motor. 18 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. Wiring diagram of a permanent split-capacitor motor. 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. 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 modules. In order to use the DL 30016 power supply, follow the next preparatory steps: 19 ¾¾ Turn the key clockwise. Be sure that the main switch three-phase output is switched ON and the overload protection is in 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 AC conditions, by using the supply module DL 30016 you are able to power the split-phase 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 AC voltage "L1L2L3 / 0÷240V" to the "on" position. Slowly rotate clockwise the knob corresponding to the AC voltage of the module DL 30016. Use the power supply until the split-motor will reach the rated parameters. Use the measuring units and check the current (about 3.2A), the voltage (about 220V) and the rated speed (about 2820 rpm) Fill the next table with the actual measured data based on the rated values presented above. Record in the data table the values of: voltage (U), current (I), power (P), power factor (cosϕ), and motor speed (n). 20 Table 1. Capacitor motor – rated parameters. When the experiment is complete, you are asked to turn off the power supplies of the split motor and of the electromagnetic brake. The easiest and more secure way to do this is to press the red “STOP” button from the power supply module. Switch all the selectors to “off”, the “a0b” to the zero position and completely return the knobs counterclockwise. 3.4 QUESTIONS Answer the following questions related to the experiment regarding the split phase motor with capacitor. 1. How are the split phase motors made self-starting? 2. What is a permanent capacitor motor? 3. What will happen if the capacitor of a single-phase motor is short-circuited? a) The motor will not start. b) The motor will run. c) The motor will run in the reverse direction. d) The motor will run in the same direction at reduced speed. 4. In capacitor start single-phase motors: a) Current in the starting winding leads the voltage. b) Current in the starting winding lags the voltage. c) Current in the starting winding is in phase with the voltage in the running winding. 3.5 CONCLUSIONS When running at no-load, the motor is always noisier than at full-load, because only under full load conditions it runs as a true two-phase machine. If the proper value of capacitance is cho21 sen, the currents through each of the two equal stator windings (under full-load) can be made such that the power factor is close to 100%. However, the starting torque is rather low and the capacitor run motor is not recommended for severe starting conditions. 22