Direct Current vs. Alternating Current Motors
Types of Power
All electrical power supplied by electric grid, transformer, generator or battery can be divided into two general types:
- Direct Current (DC)
- Alternating Current (AC)
Direct Current
Direct current power sources feed electricity directly to the line(hot) which the lights, power receptacles, motors, appliances, and other equipment are connected to. There are limitations to direct current power sources, though.
Most electrical equipment requiring power is designed for alternating current. Ordinary light fixtures, appliances and power receptacles are designed for AC so they will not work for DC. The two types of current are not interchangeable unless there is a converter, which is an expensive option. And, because direct current cannot operate AC equipment, it is not usable as standby power for emergency generator systems.
Typically, direct current power sources are normally used to supply special types of equipment including accessories, etc.
Alternating Current
Alternating current power sources feed electricity directly to the electrical network to which the lights, motors, appliances, and other equipment are connected. Generators are usually used between the power source and the line instead of batteries. However, in most cases, a battery is used for engine-starting purposes only. Generators and batteries with additional equipment can be utilized to produce the following voltage levels (600 volts or less):
- Single Phase
- 120 volts
- 120/240 volts 1ø, 3w
- 240 volts
- Three Phase
- 120/208 volts 3ø, 4w
- 480 volts
- 277/480 volts 3ø, 4w
- 600 volts
Direct Current Motors
A machine that converts electrical energy into mechanical energy is called a motor. The direct current motor is one of the first machines devised to convert electrical power into mechanical power. The DC motor consists of an electromagnet, an armature and a commutator with its brushes. Many DC motors (brush-type) have built in commutation, meaning that as the motor rotates, mechanical brushes automatically commutate coils on the rotor.
Figure 1-10 will serve to illustrate the operation of a direct current motor as well as a generator. The magnetic field, as indicated, will be the same for a motor because of current flowing in the field windings. Now, let the outside current at A have a voltage applied that causes a current to flow in the armature loop, as indicated by the arrows.
It must be remembered that any current flowing in a loop or coil of wire produces a magnetic field. This is exactly what happens in the armature of this motor. In addition, a second magnetic field is produced, with poles N and S perpendicular to the armature loop. The north pole of the main magnetic field attracts the south pole of the armature, and since the loop is free, it will revolve. At the instant the north and south poles become exactly opposite, however, the commutator reverses the current in the armature, making the poles of the field and the armature opposite, and the loop is then repelled and forced to revolve further. Again the armature current is reversed when unlike poles approach, and the armature is free to revolve. This continues as long as there is current in the armature and field windings.
Alternating Current Motors
When a coil of wire is rotated in a magnetic field, the current changes its direction every half turns. Thus, there are two alternations of current for each revolution of a bipolar machine. In an alternating current generator, also called an alternator, the current induced in the armature is led out through slip rings or collector rings.
A magnetic field is established between the north pole and the south pole by means of exciting current flowing in the winding. A loop of wire in this field is arranged so that it can be rotated on an axis and the ends of this loop are brought out to slip rings, on which brushes can slide.
This circuit, of which the rotating loop is a part, is completed through the slip rings at A. When the loop is rotating, voltage is produced in conductors F and G, which will cause a current to flow out to A where the circuit is completed.
The magnetic field – the coils of wire and iron core – are called simply the field of the generator. The rotating loop in which the voltage is induced is called the armature.
The rotating-armature type of generator is generally used only on small machines, whereas large machines almost without exception are built with rotating fields.
If the voltage completes 60 cycles in one second, the generator is termed a 60-hertz machine. The current that this voltage will cause to flow will be a 60-hertz current. The term hertz (Hz) indicates cycles per second or frequency. Click here for more about frequency.
|
