As we start into ESA theories and application, we will have to spend a little time on electric motor theory. As we will start out with a basic system, AC motors are the simplest in terms of operation and components, we will start out with AC induction motors. Using this base knowledge, we can later expand it to cover wound rotor motors, synchronous motors, machine tool motors, servos, traction machines, generators, DC motors, transformers, etc.
In fact, in order to open this topic, we will have to briefly discuss some transformer theory. Keep in mind, an AC induction motor is just a transformer with a rotating secondary. As a transformer transforms one level of voltage and current to a second level of voltage and current, an AC induction motor converts electrical energy to mechanical torque.
Trivia: One of the purposes of using high voltages for transmission is to reduce losses in the transmission wires. Voltage does not produce losses, only resistance and current as electrical losses through a conductor are Watts = I2R. Therefore, if you were moving 480 Volts and 1,000 Amps across a conductor with a total resistance of 10 Ohms, you would have 10 million Watts or 10,000 kW (10MW) of losses. However, if you increase the voltage to 13,200 Volts, the current will be 36.4 Amps which would produce losses of only 13,250 Watts or 13kW, a reduction of 99.9%! Therefore, the purpose of a transformer is to increase T&D (Transmission and Distribution) voltages then to reduce them to a useable level. This reduces the overall T&D system losses. Transformers also work to isolate systems from each other.
The primary purpose of a transformer is to increase or reduce voltage and to have the inverse effect on current. This concept took the brilliance of a Croatian immigrant electrical engineer by the name of Nikola Tesla (we will discuss Tesla in more depth in a following part of this lecture series, including the technology battle between Tesla and Thomas Edison, which I briefly covered in an earlier Blog). The concept is simple and has to do with using alternating current, magnetic fields and the use of a transformer ratio. For the purpose of this first part, we will be working with the concepts with an ‘ideal transformer’ (ie: no losses, no connections, theoretical). We will represent transformer ratio as Na and will use a subscript 1 for the primary (high voltage, low current side) and a subscript 2 for the secondary (low voltage, high current side).
The transformer ratio can be determined by comparing the number of conductors (turns – T) on the primary side to the secondary side such that Na = T1/T2. The effect is due to the mutual inductance between the primary and secondary circuits as described in the “Motor Diagnostics and Quantum Mechanics Part 7” lecture. Therefore, if a transformer has 100 turns in the primary (T1 = 100) and 10 turns in the secondary (T2 = 10), then the transformer ratio would be described as a ratio 10:1.
Now, if you have 480 Volts and 100 Amps required at the secondary, and 13,200 Volts available at the primary, you would use the formula: V1I1 = V2I2 in order to calculate the current at the primary. Therefore: I1 = (480V * 100A)/13,200V = 3.6 Amps on the primary. The transformer ratio can be determined as 13,200V/480V = 27.5. For example: The transformer may have 275 turns in the primary and 10 turns in the secondary.
Now, the question is, how does this impact our understanding of a three phase induction motor? Simple: The stator windings are the primary and the rotor bars in the motor are the secondary of a transformer.
How is the voltage and current induced into the transformer?
This is where we fall back onto a basic understanding of physics. If I pass a magnet over a conductor, it causes electrons (classical physics) to move in the inductor, creating a current (electron flow). Now, if I pass a current through a conductor, I will generate a magnetic field. If I create a coil, the magnetic fields add, and the magnetic field increases. If I then place a medium (such as a piece of iron) within the coil, I begin to direct the magnetic field such that the medium has a North and South pole. This is the action in a DC field.
Now, in an AC field, as the voltage and resulting current increase in a coil (ie: the primary), a magnetic field increases. If you have a coil in close proximity, the increasing field will effectively ‘cut through’ the conductors in the second coil, generating a voltage and current in the second coil. Because you will also have a magnetic field in the second coil (secondary), you will generate a torque between the fields. This torque is referred to as Electro-Motive Force (EMF). The currents in both coils will depend upon, not only the impedance of the transformer, but the impedance of any loads attached to the secondary of the circuit. The frequency will also be maintained in both the primary and secondary.
In the next part of the series, we can begin describing the operation of an AC induction motor, using the basic principles of this Blog. However, as we will discover, there are a few complex principles required as we move forward (such as the interaction of fields in a three phase system).
Additional Definitions:
In this lecture series, we will be discussing Electrical Signature Analysis (ESA), which is a method for evaluating electrical machinery while energized. The topic will be quite broad and is to include an analysis of supply power through the driven load.
While we will rely upon some of our previous discussions to provide information and definitions for some of our new information, we will start this series by providing some definitions unique to ESA:
- Voltage: Electrical pressure, is also termed as electromotive force. Voltage is generated.
- Current: Defined in classical physics as electron flow. Current is demanded in order to produce work and is a result of the load.
- Upstream/Downstream: Upstream refers to the electrical system in the direction of generation or distribution from the point of test. Downstream is towards the motor and load from the point of test.
- FFT: Fast Fourier Transform (FFT) is a mathematical method of separating the frequencies of a ‘sine wave’ and presenting them as frequencies and amplitude.
- Spectra: Is the graph of frequencies and amplitudes resulting from an FFT.
- Voltage and Current FFT: Spectra of voltage and current.
- Motor Current Signature Analysis (MCSA): A method of viewing demodulated current and current FFT’s to evaluate the condition of machinery downstream of the point being tested.
- Voltage Signature Analysis (VSA): A method of viewing voltage FFT’s to evaluate the condition of machinery upstream of the point being tested.
- Torsional Analysis (TA): A method of viewing the current resulting from the load and its torsional effect (pulsating loads, etc.).
- Inrush Analysis: A method of viewing the inrush effects on voltage and current when electrical machinery is started.
- Power Quality: The industry has defined this as reviewing voltage and current. Voltage unbalance, over/under voltage, voltage and current harmonics and current unbalance.
- Power Analysis: This is defined as viewing power quality as well as surges, swells, transients, interruption, etc. and requires datalogging capabilities.
- Electrical Signature Analysis (ESA): A method of evaluating the motor system, which includes supply, control, motor, coupling, load and process, utilizing MCSA, VSA, TA, Inrush Analysis and Power Analysis.
The purpose of ESA is to obtain enough information, concerning the circuit being tested, to evaluate the health of the electrical system from supply through load.
ESA has been successfully applied in these applications:
- AC induction motors
- Variable Frequency Drives (VFD’s)
- Wound Rotor Motors
- Synchronous Machines
- DC Motors
- Alternators and Generators
- Machine Tool Motors and Servos, including robotics
- Driven equipment including Belted, Direct Drive and Geared
- Transformers
- Traction Equipment
- And numerous other applications
What it comes down to is the ability to evaluate the information provided by ESA. That is the purpose of this lecture series.