Control Techniques FXM5, User Manual

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FXM5 User Guide
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Issue Number: 5 www.controltechniques.com
1 Introduction
1.1 Purpose of the FXM5 field-current controller
The FXM5 field-current controller allows wound-field DC motors to be run at speeds
above base-speed. This is achieved by automatically reducing the field current which
reduces the magnetic flux. This, in turn, reduces the back-emf in the armature windings.
The result is an increase in armature current, and a consequent increase in speed.
When a motor is operated under these conditions above base-speed, power output from
the motor remains constant since the available torque reduces in inverse proportion to
increases in speed.
The motor is normally controlled in a closed-loop system by a variable speed drive
(such as a Control Techniques Mentor II Drive).
1.2 System control modes
The FXM5 controller can be used in the following system control modes:
1.2.1 Analog control
The motor speed is controlled by a variable speed drive which varies the armature
voltage. The FXM5 controller monitors the armature voltage. When this has reached a
preset level (usually the level for base-speed), the controller reduces the field current.
This gives automatic field weakening resulting in constant power above base-speed.
A suitable speed feedback device must be fitted to the motor shaft and used by the
Drive.
1.2.2 Digital control
The motor speed is controlled by a Control Techniques Mentor II Drive which varies the
armature voltage, and controls the field current using the FXM5 controller.
Basic PI loop gain adjustments can be made in the Mentor II Drive. Highly precise
control of motor speed above base-speed can be obtained.
The FXM5 controller can be controlled by the Mentor II Drive to give automatic field
weakening (see Analog control above), or to reduce the field current to a preset level
when a specified speed has been reached.
A suitable speed feedback device must be fitted to the motor shaft and used by the
Drive.
1.2.3 External control of the field current
An external source can be used for either of the following:
• Field weakening by applying a variable field-current reference signal (e.g. from a
system controller such as a PLC). A suitable speed feedback device must be fitted
to the motor shaft and used by the system controller.
• Fixed level of field current (e.g. using an external preset potentiometer).
1.2.4 Internal control of the field current
The field current is set at a fixed level using a preset potentiometer in the FXM5
controller. The motor can then be run with a reduced field current at fixed or varying
speed.
1.3 Main features
1.3.1 AC supply requirements
The FXM5 controller operates on a single-phase AC supply. The AC supply is rectified
and controlled by thyristors which can be used to give half-control or full-control as
required.