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Section 6 — Motion Control Tutorial 6 - 2 7
Disadvantages
Some of the main disadvantages of the stepper motors are:
• could loose steps (synchronization) in open loop operation
• requires current (dissipates energy) even at stop
• Generates higher heat levels than other types of motors
• moves from one step to another are made with sudden motions
• large velocity ripples, especially at low speeds, causing noise and
possible resonances
• load torque must be significantly lower than the motor holding torque
to prevent stalling and missing steps
• limited high speed
6.6.2 DC Motors
A DC motor is similar to a permanent magnet stepper motor with an added
internal phase commutator (Figure
6.6-13).
N
S
A B C D E
Figure 6.6-13 — DC Motor
Applying current to phase B pulls in the rotor pole. If, as soon as the pole
gets there, the current is switched to the next phase (C), the rotor will not
stop but continue moving to the next target. Repeating the current switch-
ing process will keep the motor moving continuously. The only way to stop
a DC motor is not to apply any current to its windings. Due to the perma-
nent magnets, reversing the current polarity will cause the motor to move
in the opposite direction.
Of course, there is a lot more to the DC motor theory but this description
gives you a general idea of how they work. A few other characteristics to
keep in mind are:
• for a constant load, the velocity is approximately proportional to the
voltage applied to the motor
• for accurate positioning, DC motors need a position feed-back device
• constant current generates approximately constant torque
• if DC motors are turned externally (manually, etc.) they act as generators
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