POWER SUPPLIES FOR DC MOTORS BASIC INFORMATION AND TUTORIALS

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.