DC Motor Applications 321

AC

Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz (Hz), in the US.

alternating current

AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz (Hz), in the US.

armature

The part of a motor in which a current is induced to create a magnetic field. The armature usually consists of a series of coils or groups of insulated conductors surrounding a core of iron.

armature windings

The coiled, insulated conductors wound through the armature that receive current in order to create a magnetic field. Armature windings come in three different configurations.

base speed

The speed at which a motor runs with maximum supplied voltage. Base speed requires maximum voltage be supplied to the armature and the field in compound and shunt motors but to only the armature in series and PMDC motors.

brush

A sliding electrical contact used to provide a connection between the armature and the external circuit of a DC motor. Brushes contact the commutator to maintain DC.

brush holders

A device that secures a brush in a DC motor. Brush holders work with springs to allow brushes to move slightly while maintaining proper position.

carbon

A common nonmetallic element that is very strong and has high resistance to heat. Carbon formations like graphite are often used to make brushes for DC motors.

chatter

Occasional unwanted vibration between components. Chatter decreases productivity and can cause wear.

commutator

The rotating switch that contacts the brushes of a DC motor to maintain constant polarity. The commutator maintains DC when the rotation of the armature switches the polarity of the conductor.

compound motor

A DC motor that has both series and shunt field windings. Compound motors have relatively high torque and good speed regulation.

constant-speed motors

A motor that provides good speed regulation. A constant-speed motor maintains a steady speed from no load to full load.

contact

A conductive device that connects to other conductive components to allow electricity to flow between them. Contacts connect points between conductors to create a circuit.

counter electromotive force

Counter EMF. Voltage induced in the armature of a DC motor that opposes the applied voltage and limits armature current. Counter electromotive force is also known as generator action.

counter EMF

Counter electromotive force. Voltage induced in the armature of a DC motor that opposes the applied voltage and limits armature current. Counter EMF is also known as generator action.

current

I. The flow of electricity. Current strength is called amperage and is measured in amperes (A).

DC

Direct current. Electricity that flows in one direction. DC does not reverse the direction of flow.

digital multimeter

DMM. A device that can measure voltage, current, or resistance of a circuit. A digital multimeter is the most versatile and common meter used for electrical maintenance and troubleshooting.

direct current

DC. Electricity that flows in one direction. Direct current does not reverse the direction of flow.

directly proportional

A relationship in which one number increases or decreases at the same rate or ratio as another number. Directly proportional is the opposite of inversely proportional.

distribution systems

A type of electrical circuit. Distribution systems provide power to specific end destinations.

electric motors

A machine that converts electricity into mechanical energy or motion. Electric motors operate on the principle of magnetic induction.

electromagnets

A magnet that gains an attractive force only when current passes through it. Electromagnets in motors are formed by pole pieces and field windings.

field windings

A coil of conductive wire connected to the armature that energizes the pole pieces. Field windings are connected in series or in parallel.

frogleg-wound armature

A series-parallel armature winding. Frogleg-wound armatures are more common than lap- and wave-wound armatures.

full-load torque

The torque a motor requires in order to produce its rated horsepower at full-load speed. A series motor can produce 500% full-load torque at startup.

graphite

A black, soft form of carbon that conducts electricity and is easily machined. Graphite is often used to make brushes for DC motors.

grounded circuit

A situation in which current strays from its intended path and flows through a grounded component, like the frame of a motor. Grounded circuits may occur when conductor insulation breaks down or is damaged.

horsepower

hp. A unit of power used to describe machine strength. One horsepower equals 33,000 foot-pounds (ft-lbs) (44,748 joules) of work per minute, or 746 watts.

housing

A protective cover designed to contain or support a component, such as an electric motor. The field of a DC motor forms in the housing.

inrush current

The initial surge of current into a device. Inrush current can be up to 10 times higher than the continuously needed operating current because there is low initial resistance.

inversely proportional

A relationship in which one number either increases as another decreases or decreases as another increases. Inversely proportional is the opposite of directly proportional.

lap-wound armature

A parallel armature winding. Lap-wound armatures are used for high-current, low-voltage loads.

load

A component in a circuit that converts electricity into light, heat, or mechanical motion. A load provides resistance to a motor.

lubricant

A fluid used to reduce the friction between components. A lubricant typically reduces resistance, heat, and wear, but DC motor brushes cannot be lubricated.

machine tool

A power-driven piece of equipment used to form and cut metals. Machine tools require motors with high startup torque.

magnetic field

The area in and around a magnet in which a magnetic force exists. Magnetic fields exhibit the powers of attraction and repulsion.

mica

A shiny mineral material with a layered structure. Mica is used to insulate the copper segments of a commutator.

open circuit

A circuit that has an incomplete path that prevents current flow. An open circuit will cause a motor to not function.

parallel

An electrical route that has multiple loads and multiple paths. Parallel windings consist of many turns of thin wires.

permanent magnet direct current

PMDC. A DC motor that uses permanent magnets instead of field windings to create a magnetic field. Permanent magnet direct current motors have a simple design and are used for low-horsepower applications.

permanent magnets

A magnet that retains its attractive force after it is removed from a magnetic field. Permanent magnets are used to create the stationary field in some DC motors.

PMDCs

Permanent magnet direct current. A DC motor that uses permanent magnets instead of field windings to create a magnetic field. PMDCs have a simple design and are used for low-horsepower applications.

polarity

Having two oppositely charged poles, one positive and one negative. Polarity determines the direction in which current flows.

pole pieces

A metallic object used to create a magnetic field inside a motor. Pole pieces are used with field windings to form electromagnets.

proportional

A constant ratio or relationship between two values. With proportional values, if one value changes, the other will change in a predictable manner to maintain the same ratio.

punch press

A machine that uses force to either cut or form a sheet metal workpiece. Punch presses require high starting torque and relatively constant speed.

resistors

A device that controls current flow by opposing the passage of current. Resistors cause some current to be lost in the form of heat.

rheostat

A continuously variable electrical resistor used to regulate current. Rheostats can be used to adjust counter EMF and current flow in the armature of a DC motor.

right-hand motor rule

A method used to determine the direction of conductor movement in a magnetic field. The right-hand motor rule uses the middle finger to show the direction of current, the index finger to show the direction of flux, and the thumb to show the direction of conductor movement.

robot

A mechanical device that can be programmed to perform a variety of complicated, repetitive tasks. Robots require motors with high startup torque.

self-excited shunt field

A magnetic field created by field windings that connect to the armature in parallel and share the same power supply with the armature. A self-excited shunt field still offers better speed control and regulation than a series field because the parallel windings are not directly connected to the armature.

separately excited shunt field

A magnetic field created by field windings that connect to the armature in parallel but are supplied with current from a source other than the armature. A separately excited shunt field helps maintain constant speed.

series

An electrical route that may have multiple loads but has only one path. Series windings consist of a few turns of thick wire.

series motor

A DC motor that has field windings connected in series with the armature. Series motors provide very high startup torque but must never be run without a load.

series-parallel

A compound connection that uses both series and parallel connections. Series-parallel connections are used in frogleg-wound armatures.

short circuit

A situation in which current takes a shorter, unintended path between two conductors. Short circuits often occur when the insulation of two conductors is worn or damaged.

shunt field

A winding designed to be connected in parallel with the armature of a DC motor or generator. Shunt fields consist of many turns of thin wire.

shunt motor

A DC motor that has field windings connected in parallel with the armature. Shunt motors are commonly used because of their excellent speed regulation.

speed

The rate at which an object travels in a given period of time. In motors, speed reflects rotational movement and is measured in revolutions per minute (rpm).

speed control

The external means of changing a motor's speed. Speed control is easier to accomplish on DC motors than AC motors.

speed regulation

The ability of a motor to maintain its speed when a load is applied. Speed regulation is typically measured by comparing a motor's no-load speed to its full-load speed.

torque

A force that produces rotation. Torque is measured in pound-feet (lb-ft) in the English system and newton-meters (N-m) in the metric system.

traction

The application of a sustained force to an object in order to create movement. Traction occurs in a motor when the motor produces torque.

voltage

E. The electrical pressure or potential that pushes electrons through a conductor. Voltage is measured in volts (V) and is also called electromotive force.

wave-wound armature

A series armature winding. Wave-wound armatures are used for high-voltage, low-current loads.

wear

The gradual removal of material on a surface caused by contact and friction. Wear occurs on DC motor brushes because they contact the hard commutator and may experience sparking.