In step 10, you reduced the output power of the motor until the power delivered

Question

In step 10, you reduced the output power of the motor until the power delivered by the alternator to the power line changed polarity. Comment on why the system continued to rotate at the same speed.

Alternator field, VA(FL) = 48 VDC

Current, IA(FL)                 = 0.46 Amps DC

DC motor, VM(FL)          = 120 VDC

Current, IM(FL)                = 3.4 Amps DC

Output, AC power output = -249 watts

Efficiency, -249/(3.4 x 120) = -61%.

From the results of step 10, discuss the difference between a three phase alternator and a three phase synchronous motor.

PROCEDURE: 1. Connect the equipment as shown in Figure 5-1. Set the toggle switch on the synchronous machine to SYN RUN and set the switch on the phasing lamp box to OPEN. Have the instructor check your connections. SYN-100 PHASING LAMPS YOKOGAWA DIGITAL MULTIMETER SM-100-3 SYNCHRONOUS MACHINE T3 FIELD 208 VAC 3 PHASE A FIELD 2 1 a DCA . 0-25 VDC FIELD RHEOSTAT POWER SUPPLY FIELD DCV ARM SHUNT FIELD FIELD 2 DM-100AL DC MACHINE •H DCA ) . 0 – 50 VDC POWER SUPPLY DCV FIELD Figure 5-1 2. Set the field rheostat on the DC machine for mid-range. Start the DC motor by switching on the 125 VDC power supply and slowly increasing the voltage to 125 volts. The direction of motor rotation is not important at this time.

Explanation / Answer

As seen that with the change in the value of reistance with rheostat, the amount of cuurent flowing through thr field winding changes but the direction of current flow still remains constant. Also,since there is no change in phase sequence of stator voltage, the direction of speed remains the same.

A three phase alternator is a three phase generator that generates 3 phase ac power whereas three phase synchronous motor uses 3 phase ac for rotation

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