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Step Motor Glossary

Common step motor terminology

Accuracy (step)
The correctness of the distance a step motor moves during each step. Does not include errors due to hysteresis.

Axial play
The axial shaft displacement due to a reversal of an axial force on the shaft. (End play).

Bifilar (winding)
Two windings wound (in parallel) on the same pole. This permits magnet polarity reversal with simple switching means.

Bi-level drive (dual voltage drive)
A drive where two levels of voltage are used to drive a step motor. A high (over drive) voltage is applied to the winding each time it is switched on. The high voltage stays on until the current reaches a predetermined level. The high voltage is turned off after a time period determined experimentally or by sensing winding current. The low voltage maintains the rated or desired current.

Bipolar drive
A drive which reverses the magnetic polarity of a pole by electronically switching the polarity of the current to the winding (+ or -). Bipolar drives can be used with 4, 6 or 8 lead motors. With 4 and 8 lead motors bipolar drives are usually more efficient than unipolar drives.

Chopper drive
A step motor drive that uses switching amplifiers to control motor current. Chopper drives are more efficient than L/R or voltage drives.

Controller (step motor)
A system consisting of a DC power supply and power switches plus associated circuits to control the switches in the proper sequence.

Detent torque
The maximum torque required to slowly rotate a step motor shaft with no power applied to the windings. This applies only to permanent magnet or hybrid motors. The leads are separated from each other.

Driver or drive
An electronic package to convert digital step and direction inputs to currents to drive a step motor.

Duty cycle
The percentage of ON time vs. OFF time. A device that is always on has a 100% duty cycle. Half on and half off is a 50% duty cycle.

End play
The axial shaft motion, due to the reversal of an axial force acting on a shaft with axial clearance or low axial preload.

Friction (coulomb)
A resistance to motion between non-lubricated surfaces. This force remains constant with velocity.

Friction (viscous)
A resistance to motion between lubricated surfaces. This force is proportional to the relative velocity between the surfaces.

Holding torque (static torque)
The maximum restoring torque that is developed by the energized motor when the shaft is slowly rotated by external means. The windings are on but not being switched.

Hybrid step motor (HY)
A type of step motor comprising a permanent magnet and variable reluctance stator and rotor structures. It uses a double salient pole construction.

Hysteresis (positional)
The difference between the step positions when moving CW and the step position when moving CCW. A step motor may stop slightly short of the true position thus producing a slight difference in position CW to CCW.

Indexer
An electronic control which converts motion commands from a computer terminal into pulse and direction signals for use by a step motor driver.

Inductance (mutual)
The property that exists between two current-carrying conductors or coils when magnetic lines of flux from one link with those of the other.

Inductance (self)
The constant by which the time rate of change of the coil current must be multiplied to give the self-induced counter emf.

Instantaneous start-stop rate
The maximum switching rate that an unloaded step motor will follow without missing steps when starting from rest.

L/R drive
A drive that uses external resistance to allow a higher voltage than that of a voltage drive. L/R drives have better performance than voltage drives, but have less performance and efficiency than a chopper drive.

Maximum reversing rate
The maximum switching rate at which an unloaded motor will reverse direction of rotation without missing steps.

Maximum slew rate
The maximum pulse rate at which a step motor with no load will run and remain in synchronism.

Microstepping
A technique in which motor steps are electronically divided by the drive into smaller steps. The most common microstep resolutions are 10, 25 and 50 steps per full step, but many resolutions, ranging from 2 to 256 microsteps per full step are available.

Oscillator
A device that is used to produce pulses for driving a step motor at a preset speed. Many Applied Motion drives are available with built in oscillators.

Overshoot
The amount the step motor shaft rotates beyond the commanded stopping position. Usually applies to a single step.

Permanent magnet step motor (PM)
A step motor having a permanent magnet rotor and wound stator.

Positional accuracy
The maximum error in one revolution of a full step in 360°. Expressed as a percentage of a full step.

Pull-in rate (response rate)
The maximum switching rate at which an unloaded motor can start without losing step positions.

Pull-in torque
The maximum torque load at which a step motor will start and run in synchronism with a fixed frequency pulse train without losing step positions.

Pull-out torque
The maximum torque load that can be applied to a motor running at a fixed stepping rate while maintaining synchronism. Any additional load torque will cause the motor to stall or miss steps.

Pulse rate
The rate at which successive steps are initiated or the windings switched.

Radial play
The side to side movement of the shaft due to clearances between the shaft and bearing, bearing to housing, and bearing internal clearance for ball and roller bearings. (Side play).

Response rate (pull-in rate)
The switching rate an unloaded motor can follow from a standing start without missing steps.

Settling time
The elapsed time starting the instant the rotor reaches the commanded step position and the oscillations settle to within a specified displacement band about the final position, usually ±3 to ±5 percent.

Stall torque (holding or static torque)
The maximum restoring torque that is developed by the energized motor when the shaft is slowly rotated by external means. The windings are not switched.

Step angle
The nominal angle through which the step motor shaft rotates between adjacent step positions.

Step rate (speed)
The number of steps a shaft rotates during a specified time interval.

Step-to-step accuracy
The maximum error that occurs between any adjacent step, expressed as a percentage of one full step.

Switching amplifier
A device that switches a high voltage on and off to control current. Some amplifiers (PWM types) switch at a constant frequency and adjust duty cycle to control current. Other types have a fixed off time and adjust the frequency.

Switching sequence (energizing sequence)
The sequence and polarity of voltages applied to the coils of a step motor that result in a specified direction of rotation.

Thermal resistance
The resistance to the flow of heat between two surfaces of the same body or different bodies. Thermal resistance as it pertains to step motors equals the rise in winding temperature above ambient divided by the Watts dissipated in the winding. (Winding temp rise above ambient) / (Watts dissipated in winding) = °C/Watt.

Thermal time constant
The time required for the motor winding to reach 63.2% of its final temperature.

Torque displacement curve
The holding (restoring) torque plotted as a function of rotor angular displacement with the motor energized.

Torque gradient (stiffness)
The ratio of the change in holding torque for a particular change in shaft position when the motor is energized.

Unipolar drive
The motor phase winding current is switched in one direction only. The polarity of the applied voltage to each winding is always the same. Unipolar drives require 6 or 8 lead motors.

Variable reluctance step motor (VR)
A step motor having a wound stator or stators with salient poles working with a soft iron rotor having salient poles on the periphery. There are no permanent magnets in a variable reluctance step motor.

Viscous damping
A damper which provides a drag or friction torque proportional to speed. At zero speed the drag torque is reduced to zero.

Viscous inertia damper
A damper with an inertia coupled to the motor shaft, through a film of viscous fluid, usually silicone oil to minimize viscosity variations due to temperature changes. This damper only responds when the velocity between the damper inertia and motor shaft changes. At steady state speed there is no effect from the damper.

Voltage drive
A drive operated at the minimum voltage required to safely limit motor current. Motors used with voltage drives produce less torque at higher speeds than when used with L/R or chopper drives.

Wave drive
Energizing the motor phases on at a time. Driving the motor one phase or winding on at a time.