Page 158 - Electric Drives and Electromechanical Systems

Chapter 5 Brushed direct-current motors 151

The armature or load current’s form factor is a function of the motor’s armature
inductance, L a , the amplifier switching frequency, f s , and the supply voltage, V s .Since
the form factor is also a function of the current I, it is convenient to specify a minimum
load inductance for a specific drive at its maximum average current. A form factor of
1.01 is considered typical for a PWM drive.

nnn
Example 5.1
Consider a drive that requires a PWM amplifier to drive a printed circuit motor with an
armature inductance of 40 mH. If the drive switches at 20 kHz, calculate the addition
inductance required, to maintain a form factor of 1.01, at an average current of 10 A. The
drive operates from a 50 V dc. supply.
Rearranging Eq. (5.21) allows the minimum armature inductance to be calculated.
V s
L min ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 0:256 mH
6:9 f s I a 1:01 1
2
Given that the motor inductance is 40 mH, an external inductance of 0.216 mH is required to
maintain the armature current at the required form factor.
If the additional inductance was not added, it can be shown by using Eq. (5.21),thata
form factor of 1.38 results; if this is substituted into Eq. (5.20), the peak-to-peak current is
31.2 A, compared to 4.9 A with the additional inductance.
nnn

The analysis of the current waveform given above has shown that the quality of the
waveform is significantly determined by the inductance of the motor; a low inductance
will give a high-current ripple leading to excessive motor heating which requires a larger
frame size and perhaps forced cooling. This is of particular concern with the application of
very-low inductance printed circuit and ironless-rotor motors. The form factor can be
improved by using very high-frequency switching (greater than 40 kHz), or, in certain
cases, additional inductance has to be added to the armature circuit this will improve the
form factor, but it may degrade the dynamic performance of the system.
5.3.4 PWM amplifiers

A block diagram of an analogue PWM amplifier is shown in Fig. 5.12. The design consists
of three main elements: an analogue servo loop, digital control logic, and the main power
bridge. The major feature of the operation of a PWM servo amplifier is the generation of
the switching waveform. This can be achieved in a number of ways, of which two
predominate:

Current-controlled hysteresis
Sub-harmonic modulation.

In a current-controlled-hysteresis system, the power devices are controlled by the
load current (see Fig. 5.13). When the load current exceeds a predetermined value, the