AC Synchronous Machine Characterization

Synchronous motors are ideal in applications requiring constant rotor speed independent of varying shaft load, and are almost ubiquitous in power plants for regulating frequency and voltage. Synchronous machines consist of an inner rotating core called the rotor and an outer stationary ring called the stator. The rotor magnetic field is fixed and is generated using either permanent magnets or a DC power source. In three phase synchronous motors, current flows to the machine’s stator with each phase connected to its own separate set of stator coils. This produces a separate rotating magnetic field that corresponds to oscillations in the supply line current. The stator and rotor magnetic fields are coupled or locked, causing the rotor to spin at a speed exactly the same as the stator magnetic field’s rotation rate. The overall goal of this video is to introduce three phase synchronous machines, demonstrate protocols for starting and locking the rotor and stator magnetic fields, and illustrate a protocol for finding the effect of motor loads on torque angle.

To overcome initial inertia before the stator and rotor fields are aligned, a three phase synchronous machine is initially run as an induction motor. In this procedure, the stator’s rotating magnetic field induces current in the squirrel cage rotor, subsequently creating a magnetic field around the rotor and inducing rotation. Once the machine speed approaches synchronous speed, a DC voltage is applied across the field winding. From this point on, electromagnetic excitation controls the rotor magnetic field. With the rotor magnetic field fixed, the rotor and stator magnetic fields become locked to achieve rotor-stator synchronism. Consequently, the synchronous motor’s speed is controlled by the stator magnetic field rotation speed, and is independent of load. While rotor load does not affect the rotor’s speed in a synchronous motor, it does cause the rotor pulls to fall slightly behind the stator pulls. While the motor continues to run at synchronous speed, the angular displacement is called the torque angle, which is smaller at lower loads and larger at higher loads. As the mechanical load increases, torque angle increases until the angle is so high that the rotor is pulled out of synchronism. This high mechanical load is therefore above the limit to which the motor can handle, and is called the breakdown torque. Now that synchronous motors have been introduced, we will demonstrate procedures for startup, synchronization, and characterization.

Before starting the synchronous motor, test the DC power supply used to lock the rotor and stator magnetic fields. First, short-circuit the low-powered DC power supply and turn it on. Reduce the current on the supply to one point eight amps, and then turn off the supply and disconnect the short-circuit. The DC test measures the stator winding resistance. First, connect the DC supply terminals across ports one and four of the synchronous motor, and turn on the supply. Then, record the DC voltage and current across these ports. Vary the voltage as needed to reach a current limit of one point eight amps. Record the voltage and then turn off the supply. Repeat the voltage and current measurements as described across ports two and five, and then for ports three and six. Finally, disconnect the DC supply to complete the DC test.

In the next step of the protocol, the synchronous machine is started in induction motor mode, and then the rotor and stator magnetic fields are locked. First, check that the three phase disconnect switch, synchronous motor switch, and DC motor switch are all off. Then, check that the variac is set to zero percent output voltage. With the equipment shut off, wire the variac to the three phase outlet, and connect the setup as shown. Then, attach a small piece of tape to the AC synchronous machine rotor shaft. Finally, set the five to 100 A-scaling of the digital power meter current probe. Now, start the motor by powering the equipment on. First, check that the ‘Start-Run’ switch is in the ‘Start’ position. Second, turn on the three phase disconnect switch. Third, quickly increase the variac output until the digital power meter reads around 115 volts. These measurements correspond to phase A, the line to neutral phase voltage and current so that the power factor measurement correctly reflects the per phase power factor. Then, measure the motor torque in induction mode. Finally, measure motor speed using the strobe light technique. Please refer to the Science Education video, “DC Motors” for more information on this technique. With the machine started, and initial parameters measured, it is ready for synchronization. First, turn on the 125 volt DC power supply. Then, flip the ‘Start-Run’ switch to the ‘Run’ position. Pay attention to how the machine sound changes. As the rotor magnetic field locks to the stator rotating magnetic field, the machine sound becomes smoother. With rotor and stator magnetic fields locked, or synchronized, measure the armature current and voltage, power, and power factor. Then, measure the field voltage and current from the DC power supply display. Next, measure the mechanical characteristics, torque, and speed. Finally, turn off the equipment starting with the DC power supply. Then, flip the ‘Start-Run’ switch to the ‘Start’ position, and set the variac back to zero percent output. Last, turn off the three phase disconnect switch.

When a DC motor is mechanically coupled to the synchronous machine to provide a mechanical load, the torque angle in the synchronous motor can be modified by the shunt field current in the DC motor. This protocol examines the relationship between motor field load and torque angle. With the equipment shut off, connect the set up as shown, and set the shunt load resistor to two kilo ohms. Now, power up the equipment as described previously. Record the electrical and mechanical parameters as before. Next, record torque angle with the shunt field loaded. To do so, use the strobe to visually freeze the shaft of the synchronous motor. Adjust the strobe frequency using the ‘course’ nob to approximately match 1800 RPM, the synchronous speed of four pull 60 hertz machine. Then, aim the strobe light at the motor shaft’s edge, and adjust the ‘fine’ nob until the shaft appears stationary Initially, measure the torque angle with RL set to 200 ohm, and switches S1 and S2 off. Then, repeat the angle measurements with the shunt field loaded as follows. Flip S1 on and measure angle delta one, then turn S2 on and measure angle delta two. Last, turn off S2, change RL to 300 ohm, and turn S2 back on. Finally, turn off the equipment as described previously.

The DC phase resistance was estimated from the DC test as the ratio of DC voltage to DC current when applied between a phase terminal and the neutral. Phase resistance contributes to losses in the machine, and causes voltage drop across the armature. The field resistance was measured in a similar manner by applying DC voltage to the field winding and measuring the field current. Field resistance controls field current. Field voltage can be varied with a fixed field resistance to vary field current. Finally, the torque angle became larger with increased mechanical load modified by varying the shunt field current in the DC motor. The real power of the machine is then related to the torque angle as shown. This tells us that the output power is highest when the torque angle is zero.

Synchronous machines are common in applications requiring constant speed on the motor’s shaft with very tight speed regulations. Three phase wound rotor synchronous generators are the main source of electrical power worldwide. In order to connect the generator at one plant to the electrical grid, three factors in the generator output voltages must match those of the grid, magnitude, frequency, and phase sequence. While automatic synchronizers are usually utilized in large power plants, a simple method is demonstrated for manual synchronization in the Science Education video, “AC Synchronous Machine Synchronization.” Synchronous motors are often used for simple devices such as ball mills. A ball mill is a device that blends and grinds materials by rotating a cylinder containing small metal balls. The impact of the balls grinds the materials placed within the cylinder. These grinders are used frequently to blend materials such as paints, or to pulverize materials such as plant grain.

You’ve just watched Jove’s introduction to AC synchronous machine characterization. You should now understand how AC synchronous machines work, how to start and synchronize the machine, and recognize the effect of motor loads on torque angle. Thanks for watching.