Power supplies to dc motors may be
batteries, a dc generator, or rectifiers. The permanent-magnet and
miniature motors use battery power supplies. Large
integral-horsepower dc motors such as rolling-mill motors use dc
generators as the power supply. Most fractional-horsepower and
integral-horsepower dc motors operate with rectifier power supplies.
Some of the types of rectifier power supplies are as follows:
1. Single-phase, half-wave
2. Single-phase, half-wave, back
rectifier
3. Single-phase, half-wave,
alternating-current voltage controlled
4. Single-phase, full-wave, firing
angle controlled
5. Single-phase, full-wave, firing
angle controlled, back rectifier
6. Three-phase, half-wave, voltage
controlled
7. Three-phase, half-wave, firing angle
controlled
The NEMA standard letter designations
of dc motor test power supplies are as follows:
Power supply A—dc generator
Power supply C—3-phase 6-pulse
controlled rectifier (230 V L-L, 60 Hz)
Power supply D—3-phase 6-pulse
controlled rectifier (with three thyristors and three diodes)
with free-wheeling diode (230/460 V
L-L, 60 Hz)
Power supply E—3-phase 3-pulse
controlled rectifier (460 V L-L, 60 Hz)
Power supply K—1-phase full-wave
controlled rectifier with free-wheeling diode (230/115 V, 60 Hz)
When a direct-current
integral-horsepower motor is operated from a rectified
alternating-current supply, its performance may differ materially
from that of the same motor when operated from a low ripple
direct-current source of supply, such as a generator or a battery.
The pulsating voltage and current waveforms may increase temperature
rise and noise and adversely affect commutation and efficiency.
Because of these effects,
direct-current motors must be designed or specially selected to
operate on the particular type of rectified supply to be used.
Armature-current form factor and ripple are two important parameters
to be specified for motors which are required to operate with
rectifier power supplies.
The form factor is defined as the ratio
of the rms value to the average value of the armature currents.
Recommended rated form factors vary from 2.0 for 1-phase half-wave
rectifier supplies to 1.1 for 3 phase full-wave rectifier supplies
(see NEMA MG1-14.60).
Because the letters used to identify
the power supplies in common use have been chosen in alphabetical
order of increasing magnitude of ripple current, a motor rated on the
basis of one of these power supplies may be used on any power supply
designed by a lower letter of the alphabet. For example, a motor
rated on the basis of an E power supply may be used on a C or D power
supply.
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