T7 - DC Machines

Stator and Rotor

DC machine components

Stator (Field)

Produces a stationary magnetic field via permanent magnets or electromagnets

Rotor (Armature)

Multiple loops of insulated wire formed around core (windings) connected to segmented copper commutator

Rotor turns, loop moves through stator’s magnetic field, emf induced in loop

Process of DC generator

Copper Commutator

Segmented copper cylinder that swaps connections to the loop every half-cycle to produce uni-polar output

Brushes

In contact with copper commutator to maintain connection

Uni-Polar Output

Output voltage of DC generator

Induces Peak Voltage

Loop is parallel to the field

Induces Zero Voltage

Loop is perpendicular to the field

Practical Machine

Many loops connected to many segments on commutator

Benefit of Practical Machine

One of the loops is always producing max voltage, output is approximately constant

DC Motor

Current passed into commutator through one of the loops, produces torque on shaft of the rotor

k, v/m/s

Back EMF symbol and units

Back EMF

Induced voltage caused by conductor motion in DC motor

Terminal Voltage

Ideally, back EMF is the same as this

Stall Current

When E (or EMF) = 0

No Load Voltage

When i (current) = 0

Stall Torque

When ω (rotational speed) = 0

No Load Speed

When 𝜏 (torque) = 0

Determining Resistance of Windings

Measure current, i, in stall conditional (ω=0)

Rated Speed

Some point close to the no load speed (high speed, low torque)

Load Line

Determines the speed at which the machine operates (dependent on torque-speed characteristic of load)

Operating Point

Intersection of DC machine characteristic and load line

Speed > No Load Speed

Condition for DC machine to act as a generator (i<0, 𝜏<0)

Speed < No Load Speed

Condition for DC machine to act as a motor (i>0, 𝜏>0)