A search of the internet provides articles and videos showing the general operation of a synchronous motor. I thought it would be interesting to make some actual measurements and display some real-world voltage and current waveforms. I was fortunate to find a synchronous motor that had external wiring to its two coils, allowing independent characterization of each coil. The motor is small, but the characterization would scale to a larger motor.
Synchronous Motor Basics
An AC signal powers a synchronous motor. It uses the positive/negative power cycle to create a changing magnetic field. Our motor uses two coils that are 90 degrees out of phase. The fields change such that the magnetic rotor turns to keep aligned with the fields. Synchronous 3-phase motors are popular due to the simplicity of using each phase to create a rotating field. You can view many good animations of how a synchronous motor works on the internet.
Figure 1. A drawing of a synchronous motor with two coils. The colors represent the colors of the motor leads.
The AC source causes the motor to rotate at a rate proportional to its frequency (typically measured in Hz). Our example motor is powered by a 60 Hz AC signal and rotates at 3600 RPMs. The synchronous motor can turn in either direction. It starts turning toward the attracting magnetic force, which could be either of the coils depending on its configuration. A minimum voltage is required to create a magnetic field strong enough to cause the rotor to spin. Once spinning, the AC source voltage does not affect the speed of rotation.
Electrical Connections to the Motor
I wanted to characterize the voltage and current through each coil. I used our PA2201A power analyzer to characterize each coil. Each channel of the power analyzer has an input for voltage and an input for current. It can make measurements of each input or measurements that combine two inputs. While not shown in Figure 2, I added a switch to connect the AC source to coil A or coil B.
The AC6801B AC source supplies the 110 VAC input power. The AC source will characterize all the power parameters for the motor. We can also vary the frequency of the input voltage.
We will start with the AC source connected to the coil A as well as channel 1 of the power analyzer, shown in Figure 3. We expect to see the 110 V on channel 1 (top yellow waveform) of the power analyzer. Notice the coil B voltage shown on channel 2 (top green waveform) lags channel 1 almost perfectly by 90 degrees.
Figure 4. Screen capture from the PA2201A. The display is split into three parts: the top is voltage, the middle is current, and the bottom is power. Yellow waveforms represent channel 1, coil A, and green waveforms represent channel 2, coil B.
I do not work with inductors and capacitors every day and was surprised that coil B receives much more current and power. If you look at the equations for reactance, it makes sense. The capacitor reduces the reactance X in the branch with coil B, X = XL - XC. More current flows through this branch, which creates more voltage across coil B.
The AC source is measuring the input to the motor, which is consuming 0.05 ARMS, 5.1 W, and 5.4 VA. Coil B uses most of the energy. I was also surprised to learn the coil's reactance is nearly 50% resistive, 50% inductance. A phase angle ɸ of 45% or power factor PF .707 would be precisely a 50% mix, and the measurements were only slightly more inductive. You can see the phase angle (the delay between voltage and current) in the Figure 6. Lastly, I had expected the current waveforms to be more sinusoidal. In reality, the motor acts as a transformer and sends energy between the coils. If I remove the capacitor, you can see 50 V across coil B; No current flows, as I left the branch open.
Figure 7. With the capacitor removed, the motor can turn in either direction. Energy in coil A creates a voltage on coil B. On the left, the motor is turning in a counterclockwise direction (normal), and on the right it is turning clockwise. Notice in the first case, the coil B voltage (shown in green) lags, and in the second it leads.
Reversing the Motor Direction
Applying the source voltage to coil B instead of coil A causes the motor to run in the opposite direction.
The coils are not perfectly symmetrical, as the power through each branch is a bit lower than in the counterclockwise setup. Using the AC source, the overall power measurements are also lower, reduced to 4.8 and 5.1 VA. You will notice the voltage in coil B, green waveform, leads coil A. You can compare Figure 9 and Figure 6 to see the differences.
Changing the Source Frequency
The AC source can vary its frequency from 40 Hz to 500 Hz. Changing the frequency of the source affects the speed of the motor. It also changes the impedance of the circuit, as the reactance of the capacitor and inductors are a function of frequency.
I was impressed with the motor’s overall power factor. The AC source measured the PF to be 0.94. Earlier, we measured the current through each branch of the circuit. The total current consumed by the motor is a point-by-point summation of the two current waveforms (i.e., the sum of the current through each of the coils). A PF of 0.94 indicates that the summation has nearly identical phase to the input voltage. The power analyzer can sum the two current waveforms and display them overtop the input voltage.
As you can see, the system has a PF near 1.0, as the input current and voltage have nearly identical phase.
The capacitor used with our motor is considered a run or permanent split capacitor, as it maintains the 90-degree shift in the coil voltages. People sometimes generically refer to a capacitor used with a motor as a start capacitor. A start capacitor is disconnected by a switch once the motor starts spinning.
To overcome the difficulties in changing the speed of our single-phase motor, the manufacturer added a gearbox so it spins at 33.3 RPMs.
The AC6801B AC source made it easy to supply the required AC power (110 VAC at 60 Hz) while making power measurements. The measurements include current, real power, apparent power, reactive power, and power factor. The PA2001A was used to characterize the voltage and current through each coil.