Power Supplies

The five basic types of power sources for DC motors are: 1) batteries, 2) generators, 3) six-step SCR's, 4) three-step SCR's, and 5) single phase SCR's. These supplies fall into four categories, or codes, based on the type of output power used.

Power supply type is an important consideration in the application of DC motors. The most common way to provide DC voltage to a motor from an AC line is through an electronic drive. An electronic drive is a device that provides a pulse wave similar to the perfect voltage supplied by a battery. These pulses are characterized by a form factor, defined by NEMA power supply code.

NEMA POWER CODE A: Defines a pure DC power supply such as a battery or a generator. All NEMA DC motors may operate with this type of power supply.

NEMA POWER CODE C: Defines a nearly pure supply, consisting of six silicon controlled rectifiers (SCR's), connected in a three phase, full-wave bridge configuration. All NEMA DC motors may operate off this type of power supply with the aid of an external reactor. A reactor is a device, other than a pure resistor, that opposes the flow of current.

NEMA POWER CODE D: Connotes a slightly more distorted pulse than code C. It consists of six silicon controlled rectifiers (SCR's) and three diode style rectifiers connected in a three phase, full-wave bridge configuration. DC motors rated 250 HP or less may operate on this type of supply without external reactors. DC motors rated 300 HP or greater may require an external reactor with this type of power supply.

NEMA POWER CODE E: Indicates a pulse of average quality. The mechanism comprises three silicon controlled rectifiers connected in a three phase, half-wave bridge configuration. Most DC motors require either de-rating or an external reactor when using this type of power supply.

NEMA POWER CODE K: Is for use with a very limited number of applications. It is made up of two silicon controlled rectifiers and two diode style rectifiers connected in a single phase full-wave bridge configuration. This type of power supply is normally used with motors rated up to 7-1/2 horsepower.