6
MOTOR TORQUE
The torque a motor develops to turn a certain load
depends on the amount of current drawn from the
battery. The greater the load the more torque required
and the greater the current flow will be. The lighter the
load, the less torque required and the smaller the current
will be.
COUNTER ELECTROMOTIVE FORCE
When the armature rotates the coil windings “cut” the
magnetic field, inducing a voltage or electromotive
force in these coils. Since this induced voltage opposes
the applied terminal voltage, it is called the “counter
electromotive force,” or “counter–emf.” This
counter–emf depends on the speed, direction of rotation
and the field strength. The stronger the field and the
faster the rotating speed, the larger will be the
counter–emf. However, the counter–emf will always be
less than the applied voltage because of the internal
voltage drop due to the resistance of the armature coils.
The internal resistance of the armature is very low, (less
than one ohm). If the armature coil resistance was all
that limited the armature current, the current would be
very high. For example, if the armature resistance is
0.01 ohm and the applied voltage is 50
volts, the
resulting current, according to Ohm’s law, would be
5000 amps. This excessive current would completely
burn out the motor. However, the counter–emf is in
opposition to the applied voltage and limits the value of
armature current that can flow. If the counter–emf is 48
volts, then the effective voltage acting on the armature is
the difference between the terminal voltage and the
counter–emf (2 volts). The armature current is then only
200 amps.
TYPICAL MOTOR
Typical motors basically include a housing, armature,
brush assemblies and field windings. The housing is the
main structure of the motor which provides protection
for motor components and a magnetic path for the field.
The armature is the rotating part which has coils of wire
wrapped around an iron core connected to a
commutator. The brushes ride against the commutator
which allows battery current to reach the armature
windings. The field windings mounted on iron pole
pieces within the housing form electromagnets which
provide the magnetic field necessary for motor
operation.
1
2
3
4
5
1. BRUSH ASSEMBLIES
2. MOTOR HOUSING
3. COMMUTATOR
4. ARMATURE
5. FIELD WINDINGS
FIGURE 8. TYPICAL MOTOR
6927
TYPES OF MOTORS
There are four basic types of DC motors:
a. Permanent–Magnet
b. Series Wound
c. Parallel or Shunt Wound
d. Compound Wound
Each has characteristics that are advantageous under
given load conditions. The permanent–magnet, series
wound, and compound wound motors are the most
commonly used in lift truck systems.
Permanent–Magnet
Permanent–magnet motors have no field windings
because the field is provided by permanent magnets. All
the current flows through the armature windings.
Permanent–magnet motors are normally physically
smaller and less expensive than comparable field
wound motors. They are sometimes used in the lift truck
power steering system.
NOTE: The field on Series, Parallel, and Compound
motors is provided by an electromagnet (See Page 2).
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