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Electric Machines
Published in Patrick Hossay, Automotive Innovation, 2019
The embedded control functions required to manage an induction motor are challenging, requiring complex mathematical models and high-performance control algorithms. At first glance, the task might look easy. After all, more current to the stator increases the magnetic field strength at each pole, which increases the resulting magnetic field of the rotor, in turn increasing the rotational force and thus the motor’s torque. However, unlike the DC motor, simple PWM or varying the voltage alone will not offer good speed control, since the frequency of the changing magnetic field is the principle determinant of rotational speed. At the same time, changing the frequency of the AC current alone is problematic as it combines with voltage to define the air gap flux, and shifting one without the other would have undesirable effects. So, we can adjust frequency and voltage together, maintaining a constant ratio between the two to alter the speed of the motor, a method called variable voltage variable frequency (VVVF). However, as discussed with DC machines, above the rated speed we can no longer usefully increase voltage. So, we increase frequency alone and experience reduced torque with decreased flux. Unfortunately, this method is imperfect, control is imprecise and response can be sluggish. So, it is not good for multi-motor EV configurations that require precise motor speed coordination, and it is similarly not good for hybrids that need to be accurately controlled to work with the ICE. So, in general VVVF is not suitable for advanced vehicle applications.
Induction Motor Basics
Published in Kwang Hee Nam, and Electric Vehicle Applications, 2017
The motor control method keeping the V/F constant is called variable voltage/variable frequency (VVVF) method. Specifically, consider an operation point marked by “x”. Note that it is impossible to operate the motor with 127V at 60Hz, since “x” is in unstable region. However with reduced voltage and frequency (25V, 12Hz), “x” is a stable operation point. Similarly, if both voltage and frequency are doubled at the same time (50V, 24Hz), another stable operation point is obtained “xx”. Hence, while keeping V/F constant, it is possible to drive crane like loads requiring high start torque.
Induction Motor Basics
Published in Richard E. Neapolitan, Kwang Hee Nam, AC Motor Control and Electrical Vehicle Applications, 2018
Richard E. Neapolitan, Kwang Hee Nam
With the use of inverters, it is possible to provide a variable voltage-variable frequency (VVVF) source to the motor. Changing the frequency means changing the synchronous speed. Note from (3.19) that the stator voltage is approximated as Vs=rsIs+jωe(Lls+Lm)Is≈jωeLsIs=jωeλs $ \mathbf V _s =r_s \mathbf I _s + j\omega _e (L_{ls} + L_m ) \mathbf I _s \approx j\omega _e L_s \mathbf I _s =j\omega _e \boldsymbol{\lambda }_s $ . Therefore, the magnitude of flux satisfies λs=Vsωe=Vs2πf. $$ \begin{aligned} \lambda _s= { \mathbf V _s \over \omega _e} = { \mathbf V _s \over 2\pi f}. \end{aligned} $$
Control of a converter system for an asymmetrical parameter type two-phase induction motor drive operating in motoring and generating modes
Published in Journal of the Chinese Institute of Engineers, 2018
Papol Sardyoung, Vijit Kinnares
A single-phase induction machine is always used as a motor rather than a generator for household appliances. For a braking operation, the machine will operate as a generator to convert mechanical energy into electrical energy. Because of low cost, high reliability, rugged construction low maintenance, and so on, presently the induction generator can be found in small-scale renewable energy conversion systems, Yildirim and Bilgic (2008). For fixed speed, single-phase induction motors (SPIMs) are widely used in low-power residential and industrial applications such as HVAC blowers and compressors Piyarat and Kinnares (2010). However, a wide range of speed and energy-saving operations cannot be achieved. For variable-speed applications, power electronic equipment such as a voltage controller using thyristor phase control and for an energy-saving concern an inverter is preferable. The inverter offering variable voltage and variable frequency (VVVF) is considered as a first choice because it has various advantages like high starting torque, and a wide adjustment range of speed. An SPIM can be connected in a two-phase induction machine configuration supplied with a two-phase source with 90° phase differential in voltages. Publications related to a three-leg VSI-fed asymmetrical parameter type two-phase induction motor modified from the existing SPIM are Sardyoung, Watcharin, and Kinnares (2014) and Chakrapong and Kinnares (2009). Moreover, a comparison between three-leg and four-leg VSIs was made in Kumsuwan, Premrudeepreechacharn, and Kinnares (2010) research. Although the performance of the three-leg VSI-fed two-phase induction motor drive using balanced and unbalanced two-phase outputs was fully evaluated, a machine acting as a generator and bi-directional power flow for the front-end converter has not yet been reported. More importantly, the comparison of the performance between balanced and unbalanced phase voltages of the two-phase machine in the generating mode has not yet been reported. The control of a three-phase induction motor for bi-directional flow, with reduced number of power electronic switches can be found in Dong-Choon and Young-Sin (2007). A full-bridge front-end converter for a two-phase three-leg VSI converter was reported on in Sardyoung, Watcharin, and Kinnares (2013) and Leeragreephol et al. (2013). However, this topology is costly.