Direct-drive mechanism


A direct drive mechanism is one that uses the torque coming from a motor without any reductions.
Direct drive mechanisms can be categorized in groups
Torque motor mechanisms. Output shaft is driven either by brush torque motor or brushless torque motor. The brushless torque motor requires commutation. Battery driven devices such as cordless hand drills are generally use brush torque motors.
AC drives. Generally driven by 60 cycle voltage. Examples- turntables low speed, fans medium speed, hard drives high speed.
The major alternative to direct drive mechanism is geared stepper motor mechanism driven by current pulses.

Advantages

The main disadvantage of the system is that it needs a special motor. Usually motors are built to achieve maximum torque at high rotational speeds, usually 1500 or 3000 rpm. While this is useful for many applications, other mechanisms need a relatively high torque at very low speeds, such as a phonograph turntable, which needs a constant 33 rpm or 45 rpm.
The slow motor also needs to be physically larger than its faster counterpart. For example, in a belt-coupled turntable, the motor diameter is about. On a direct-drive turntable, the motor is about.
Also, direct-drive mechanisms need a more precise control mechanism. High speed motors with speed reduction have relatively high inertia, which helps smooth the output motion. Most motors exhibit positional torque ripple known as cogging torque. In high speed motors, this effect is usually negligible, as the frequency at which it occurs is too high to significantly affect system performance; direct drive units will suffer more from this phenomenon, unless additional inertia is added or the system uses feedback to actively counter the effect.

Applications

Direct drive mechanisms are present in several products:

High-speed