User Guide Mentor ll (3part)

Origin: Part Number: 0410-0013-13 Issue Number: 13 Control Techniques Drives Ltd

3 Introduction

  Mentor II is the latest family of advanced, fully microprocessor-controlled DC variable speed industrial drives. The range of output current is from 25A to 1850A. All sizes share control, monitoring, protection and serial communications features.

  All units are available alternatively in either single-ended or four- quadrant configuration. Single-ended drives provide forward run operation only. Four-quadrant drives are fully-reversible. Both types offer comprehensive control of motor speed and/or torque, the four-quadrant drives providing full control in both directions of rotation. Operating parameters are selected and changed either at the keypad or through the serial communications link (interface). Access for writing or changing parameter values can be protected by the three-level security code system.

3.1 DC motor control

  The functions of a DC motor which must be controllable for practical use are the speed, the torque delivered, and the direction of rotation. Speed is proportional to armature back-emf and inversely proportional to field flux. Torque is proportional to armature current and field flux. Direction of rotation is simply a matter of the relative polarities of the armature and field voltages. It follows that it is necessary to control:

  1. The armature voltage; back-emf is a component of armature voltage. Thus, assuming the field to be constant, control of armature voltage provides complete control of speed up to the point where the voltage reaches the maximum value for which the armature is designed. Armature current is also a function of armature voltage, so that within the speed range up to maximum voltage, torque is controlled by voltage also. Provided that the field is fully-excited, the availability of maximum torque is normally maintained from zero speed up to armature voltage maximum (base speed).

  2. The field voltage; this determines the field current and, in consequence, field flux. If field voltage can be varied independently of the armature voltage, speed can be increased at full power (full armature voltage) beyond the point where the applied armature voltage and current are at maximum. Since torque is directly proportional to field flux, maximum torque is reduced if speed is increased by weakening the field.

  Basically, therefore, a variable speed DC drive is a means of controlling the voltage applied to the armature of the motor, and thus the current delivered to the motor. The drive may be equipped with means for control of the field if speeds higher than base speed are required. Separate control of the field within the operating range up to base speed can be exploited also, to obtain extended control of speed and torque for more-complex motor applications. If a suitable feedback is available, position control becomes possible.

3.2 Principles of the variable speed drive

  A single phase voltage applied to a fully-controlled thyristor (SCR) bridge and a resistive load produces an intermittent flow of current which is started by the firing of the thyristor (SCR), and stopped as a result of the supply voltage passing through zero at the end of each half cycle. Maximum voltage is delivered when the firing angle is fully advanced, that is, when f in Figure 3-1 becomes zero. Retarding the firing angle reduces the current output. When the load is inductive, such as a motor, or the firing angle is sufficiently advanced, current becomes continuous . The fundamental of the current characteristically lags behind the voltage due partly to the inductive nature of the load and partly due to firing angle delay.

3.3 Reversing

  Reversal of rotation is done in one of two ways, dependent on the type of drive bridge configuration. The simplest fully-controllable arrangement of thyristor (SCR) bridge configuration to operate from a 3-phase AC supply is a full-wave bridge but this is not capable of reversing the output polarity. This type, which is called single-quadrant or single-ended, requires a means of switching the motor terminals externally as shown in Figure 3-2 if reversing is required. For some applications this simple system is an adequate practical solution.

  If, however, the motor application is such that it demands complete control of motor operation in both directions, with the ability to reverse motor torque rapidly and frequently, two anti-parallel bridges must be used, Figure 3-3. This configuration provides full control of forward and reverse drive and forward and reverse braking without the need for reversing contactors, and is called four-quadrant, Figure 3-4.

  If braking is required with a single-ended drive, an external circuit has to be provided, Figure 3-5 (dynamic braking). In this case, deceleration is neither controlled nor linear.