The paper 'Vector Control for Induction Motor' is a great example of a Business Case Study. Mukhtar (2010) argues that advances in solid-state power devices have resulted in the development of variable speed AC induction motors that are powered by switching power converters. The switching power converters control both the magnitude and frequency of the current and voltage applied to a motor. For much higher performance and efficiency, the motor drives should generate less noise. The speed of an induction motor which is given by N=120f/p (1-S) where f= frequency of supply, p=no of poles, s=slip; can be changed by either controlling the number of poles, slips or the frequency. The common principle applied is the use of constant V/Hz in which the magnitude and frequency of the voltage applied to the stator of a motor maintain a constant ratio.
In this manner, the magnitude of the magnetic field is kept at an approximately constant level throughout the operating range (Atmel 1994). Atmel (1994) says that another principle that can be applied is the use of vector control. This technique regulates both the amplitude and phase angle of the AC excitation voltage. Induction control methods V/F control for induction motor Description of V/F control for induction motor The induction motors can be controlled using the Volts/Hz control in which the frequency of supply f is varied (Mukhtar 2010).
It is possible to obtain a variable frequency supply with good quality output wave shape as shown in the diagram below. Fig 1. Voltage versus frequency curve under the constant V/F control derived from Trzynadlowski 1994. However, the frequency control also requires proportional control in applied voltage, because the stators flux, λ s =ν s/ω s (neglecting the resistance drop) remain constant.
Otherwise, if frequency alone is controlled, then the flux will change. Increasing the frequency results in a reduction in the flux and thus decreasing the torque by the motor. Decreasing frequency results in an increase in the flux and this lead to the saturation of the magnetic circuit. The drives in the PWM inverters are fed from a PWM nerve since their voltage and frequency can be controlled independently. In such a control scheme, the motor is supplied by three-phase supply through the filter, diode rectifier, and PWM inverters. Depending on the desired speed, the method uses frequency command which is applied to the inverter and thus directly generating the phase voltage command from the frequency command by a gain factor, and the input dc voltage of the inverter is controlled. The voltage command signal for the inverter is also generated using frequency command and V/F function generator. When the ratio V to f is made to be constant with a change in f, then the magnitude remains constant with the torque being independent of the frequency supply. Thus, the ratio between the frequency and magnitude of the stator voltage should be based on the rated values of these variables.
According to Tzou & Hsu (1997), both low frequency and low voltage leads to a drop across the stator resistance which cannot be neglected and thus must be compensated. Uses of V/F control for induction motor The method is used in applications such as blowers and fan drives where the speed response at a lower end is not serious (Rashid 2006). The open-loop Volts/ Hz control is a popular speed control method for induction motor drives that do not require high accuracy. Besides, this method is used in low-performance applications where precise speed control is not required. Advantages of V/F control for induction motor In this method, the stator flux remains constant despite the change in frequency supply and thus the torque developed depends on the slip speed only.
Regulation of the slip speed makes it easy to control the speed and torque of an AC induction motor by the use of the V/Hz principle (Mukhtar 2010). The use of the closed-loop speed control through the control of the slip speed of the motor results in inaccuracy in speed response and thus keeping the motor speed at its set value. It is only the rate, maximum or minimum frequency information is required to implement the method since the maximum voltage (rated voltage) is applied to the motor at a rated frequency. Disadvantages of V/F control for induction motor This method does not precisely control the speed of the motor since the frequency control only controls the synchronous speed. According to Mukhtar (2010), there is also a small variation in the speed of the motor under load conditions.
Such a variation is not much when the speed is high. When working at low speeds the frequency is low, and if the voltage is also reduced then the performance of the motor deteriorates due to the large value of static resistance drop. For low-speed operation, the relationship between the voltage and frequency is given by v=v0+kf where v0 =voltage drop in the stator resistance. For applications that do not have a concern for the accuracy in speed response such as heating air conditioning, fan and/ or blower applications, open-loop speed control is used whereby the supply frequency is determined by the assumption that the motor follows a synchronous speed and is based on the recommended speed.
In this case, the speed error as a result of the slip of the motor is regarded as acceptable, and yet it should not be the case. Another disadvantage is that for frequencies that are greater than the rated value, the principle applied in this method has to be violated in order to avoid the breakdown of insulation by ensuring that the stator voltage should not exceed its rated value (Rashid 2006). Vector control method Description of V/F control for induction motor The induction motors can also be controlled using the vector control method where the phase angle and the magnitude of the excitation current are controlled (Nagrath & Kothari 2004). In this method, the stator current is changed into a reference frame where the two parameters can be controlled independently, one for the electromagnetic torque and the other for the rotor flux. The basic principle of vector control is to separate the components of the stator current which is responsible for the production of flux and the torque.
The diagram below shows the vector control algorithm for induction motor
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