he DC machines described in this chapter are motors or generators. The term machine is used when the test procedure is the same for both. A motor can be converted to a generator (and vice versa), so the terms motor and generator are used only when the explanation applies to one or the other.
DC generators produce very high-quality power, but (because of maintenance and other costs) AC powered DC drives are used for most motor applications.
DC power is used for some high-voltage power transmission. DC current flows through the whole area of a wire, making it more efficient than AC for long distance transmission. High-voltage AC current uses only the outer portion of the wire. Only two transmission lines are needed with DC, and in an emergency, the earth can be used as a second conductor. At the user end, DC is converted to three-phase power and distributed to substations.
DC generators also make the best power for arc welders. They provide steady voltage and a non-fluctuating current that flows in one direction.
The DC motor has excellent speed control with very good torque and horsepower characteristics. Because of its armature design and function, it has very smooth torque from 0 RPM to base speed. The DC motor also has full-rated horsepower above base speed.
Basic Electricity as It Applies to Motors
The properties of electricity are volts, resistance, and amperes. Voltage is the driving force, resistance is the work to be done, and amperes get the work done.
Volts
Voltage (electromotive force, or emf) is the driving force that causes the amperes to flow through the resistance of the load. Even if there is no circuit or path, voltage can be present. The volt can be compared to air or water pressure. When voltage is raised, more amperes will flow through the resistance (load). When voltage is lowered, fewer amperes will flow. When the voltage is varied, the number of amperes flowing through a given resistance (load) will go up or down with the voltage change.
Another comparison of the volt to air or water pressure is containment. Higher voltage requires stronger (thicker) insulation.
Resistance
Resistance controls the number of amperes that flow in a circuit. When a constant value of voltage is applied, as resistance goes up, amperes go down and as resistance goes down, amperes go up. As the resistance value varies, the number of amperes varies the opposite way. All loads have some form of resistance. The resistance of a device is measured in ohms. (Ohm’s Law is discussed later in the chapter.)
Resistance is opposition to ampere flow and measured in ohms. It is seldom measured, so is just called resistance. It is common to measure amperes.
Two factors furnish resistance to current flow in an electric motor. First is the resistance of the wire in the coils that form the poles. Each wire size has a resistance value per 1000 feet at a given temperature. Coils of wire used in the shunt field of a DC motor have enough feet of wire to limit the amperes to a safe level (and not overheat). The second factor is the interaction of the winding conductors and the magnetic circuit of the motor. This will be explained under “Counter emP’ in DC motors in the section “Counter-voltage” and in Chapter 3 under “Inductive Reactance” in AC motors.
Amperes
The ampere is a measurement of the number of electrons flowing in a wire. The number of amperes flowing in a circuit is controlled by two factors: the voltage applied and the resistance of the load. The voltage and/or the resistance are varied to control the amperes. The formula called Ohm’s Law (described later in the chapter) calculates the number of volts and/or the amount of resistance needed to predict the number of amperes in a circuit.
Most electrical breakdowns involve ampere flow. When insulation breaks down, heat created by ampere flow destroys it. Excessive amperes flowing in a wire cause a wire to become hot.
The number of amperes flowing through a coil controls the coil’s magnetic strength. As the number of amperes changes, the coil’s magnetic strength will vary with the change.
The direction of ampere (current) flow determines the polarity of the coil.
Figure 1.1 shows the left-hand rule for determining the polarity of a DC coil.
The right wire size is a very important part of motor design. The wire size is determined according to its cross-sectional circular mil area. The number of amperes that flow in a motor’s circuits and the motor’s cooling ability determine the wire size.
The coils used in the shunt field of a large DC motor have much larger wire size (circular mils per amp) than the coils used in single- or three-phase induction motors. This is because the coils have a large mass and do not cool easily. It’s common to find 1000 (or more) circular mils per amp in the shunt field coils of a large DC motor. It is also common for single- and three-phase motors to have 300 to 350 circular mils per amp. Table 1.1 shows the wire size converted to circular mils plus other data.