AC drives - a practical application of power semiconductors01 July 2008The speed control of ac motors has always been of importance to industry. Since the invention of the ac motor, various means of speed control have been used. Rob Easthope describes one of the more common methods in use todayThe speed of an ac motor is calculated using the following formula:
n = [(f x 60)/p] – ns, where n is the speed in revolutions per minute, f is the frequency of the supply voltage, p is the number of pairs of poles of the motor and ns is the slip.
The number of pairs of poles of a motor is determined by its physical construction. Therefore, variable speed control of the motor can only be effectively achieved by varying the frequency of the supply to the motor. An additional complication is that the torque produced by an ac motor is proportional to the motor’s current. The inductance of the motor causes the reactance of the motor to change as the supply frequency varies. This variation in reactance causes the current, and hence the torque, to vary. To keep the torque constant the voltage also needs to be variable and therefore an ac drive needs to produce a variable frequency, variable voltage output.
Both the rectifier and the inverter sections of an ac drive use power semiconductors. In the basic ac drive the rectifier consists of a three phase uncontrolled diode bridge. This diode bridge provides the dc supply to the dc link.
The inverter section is where the dc voltage from the dc link is converted or inverted back to an ac, supply to the motor. The inverter section consists of six switching devices. Various semiconductor devices can be used for these switches, such as transistors, thyristors, MOSFETs (metal oxide semiconductor field effect transistors) and IGBTs (insulated gate bi-polar transistors). By opening and closing the switches in a specific sequence, a voltage is developed that alternates between positive and negative. The firing, or turning on, of these switches is regulated by a microprocessor in the controller section. By varying the firing of the semiconductor devices a variable frequency ac output can be produced.
If the speed of a motor is suddenly reduced, negative torque is developed in the motor. The causes the motor to act like a generator, mechanical power from the shaft is converted into electrical power, which is returned to the ac drive. This is known as regeneration, and helps slow the motor. This electrical energy returned by the motor can cause the voltage on the dc link to become excessively high. To avoid damage to drive components this excess voltage has to be dissipated. One method is to fit a braking resistor, this resistor is connected across the dc link and when the voltage rises above a safe level, a thyristor or similar semiconductor device is used to turn the device on and off. This is known as pulsed resistor braking.
Another solution to this problem is to allow the excess energy to be returned to the ac supply in the form of ac current. This requires the diodes in the rectifier section to be replaced with thyristors; the bridge changes from being uncontrolled to controlled. In addition, a series of thyristors are added in an inverse parallel configuration, creating a regeneration circuit that returns the excess energy to the supply.
Another option available to control regenerative voltage is the ‘Active Front End’. In this configuration, the diodes in the bridge rectifier are replaced with another type of semiconductor, the IGBT. The IGBT modules operate in both the motoring and the regenerating modes and are controlled by the microprocessor in the controller section.
- Rob Easthope is training officer, School of Computing and Engineering, University of Wales, Newport (www.newport.ac.uk). He can be contacted on 01633 432474 or email: rob.easthope@newport.ac.ukRelated Articles...Most Viewed Articles...<< Back to company search results
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