Technical information

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Glossary
Speed
Used to describe the linear or rotational velocity of a motor or other object in motion. (see Velocity.)
Speed Regulation
For a speed control system, speed regulation is the variation in actual speed expressed as a percentage of set
speed.
Stall Torque
The torque available from a motor at stall or zero rpm.
Static Torque
The maximum torque available at zero speed.
Stator
The non-rotating part of a magnetic structure. In a DC motor the stator usually contains the mounting
surface, bearings and non-rotating windings or permanent magnets.
Stiffness
Ratio of an applied force or torque to a change in position for a mechanical system.
Synchronization
A motor rotating at a speed correctly corresponding to the applied master frequency is said to be in
synchronization.
Tachometer
An electromagnetic feedback transducer which produces an analog voltage signal proportional to rotational
velocity. Tachometers can be either brush type or brushless.
Torque
The rotary equivalent to force. Equal to the product of the force perpendicular to the radius of motion and
distance from the center of rotation to the point where the force is applied.
Torque Constant (Kt)
A number representing the relationship between motor input current and motor output torque. Typically
expressed in units of torque/amp.
Torque Margin
Whenever possible, a motor/drive that can provide more torque than the minimum necessary should be
specified. This torque margin accommodates mechanical wear, lubricant hardening and other unexpected
friction. Selecting a motor/drive that provides at least 20 to 50 percent margin above the minimum needed
torque is good practice.
Torque Ripple
The cyclical variation of generated torque with constant current applied to the motor. In a brush type motor,
the ripple is caused by the non-linearity of torque that occurs when the commutator switches from one
winding to the next. In a brushless motor, the ripple is caused when the applied current wave form does not
precisely match the motors characteristic wave shape.
Torque-to-Inertia Ratio
Defined as a motor’s torque divided by the inertia of its rotor; the higher the ratio, the higher the
acceleration can be for a given torque value.