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Electric Motor Terminology

MOONS' provides Electric Motor Terminology for reference. The Electric Motor Terminology is a list of common motor parameters. Electric Motor Terminology Absolute Position: Position referenced to a fixed zero or "home" position. Absolute Programming: A positioning coordinate reference wherein: all positions are specified relative to some reference or "home" position; this is different from incremental programming where distances are specified relative to the current position. Ambient Temperature: The temperature of the medium immediately surrounding a device. Amplifier: Electronic device that converts command signals (analog or digital) to high power voltages and currents for the operation of the motor. ASCII: American Standard Code for Information Interchange; this code assigns a number to each numeral and letter of the alphabet allowing information to be transmitted between machines as a series of binary numbers. Axial Play (End play): The axial shaft displacement due to a reversal of an axial force on the shaft. Baud Rate: The number of binary bits transmitted per second for serial communications such as RS-232. Bi-level Drive (Dual Voltage Drive): A driver where two levels of voltage are used to drive a stepper 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 desired current. Bipolar Drive: A drive that 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 and generally produce more torque. Brushless Servo Drive: A servo drive used to control a permanent magnet synchronous AC motor. Chopper Drive: A step motor drive that uses switching amplifiers to control motor current. Class B Insulation: Specifies motor insulation that is rated for operation up to 130°C. Class H Insulation: Specifies motor insulation that is rated for operation up to 180°C. Closed Loop: A system that uses some form of feedback device to monitor the system output; the signal from the device is used to correct any errors between actual and demanded output. Cogging: Term used to describe uneven velocity in motors usually at low speeds. Commutation: Refers to the action of steering currents or voltage to the proper motor phases to produce optimum motor torque. In brush type motors, commutation is done electromechanically via the brushes and commutator. In brushless motors, commutation is done by the switching electronics using rotor position information typically obtained from hall sensors, tachometers, resolvers or encoders. Controller (Stepper Motor): A system consisting of a DC power supply and power switches plus associated circuits to control the switches in the proper sequence. Damping: An indication of the rate of decay of a signal to its steady state value; related to settling time. Dead Band: A range of input signals for which there is no system response. 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. Drive (PWM): A motor drive utilizing Pulse-Width Modulation techniques to control current to the motor; typically a high efficiency drive that can be used for high response applications. Drive (Servo): A motor drive that utilizes motor position feedback with a control loop for accurate control of motor position and/or velocity. Drive (Stepper): 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. Dynamic Braking: A passive technique for stopping a permanent magnet brush or brushless motor; the motor windings are shorted together through a resistor, which results in a motor braking with an exponential decrease in speed. Encoder: A device used to translate motion into electrical signals used to provide position information; often used as a position/motion feedback device in closed loop systems. Encoder Marker Pulse: A once-per-revolution signal that is provided by some incremental encoders to specify a reference point within that revolution. End Play: The axial shaft motion due to the reversal of an axial force acting on a shaft with axial clearance or low axial pre-load. Following Error: The positional error during motion between a load's actual position and the commanded position. 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. Hall Sensors: A feedback device built into a motor used by a servo amplifier to electronically commutate the motor. Holding Torque (Static Torque): The maximum restoring torque that is developed by the energized motor when the shaft is slowly rotated by external means. Hybrid Stepper Motor: 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. I/O (Inputs/Outputs): The reception and transmission of information between control devices; I/O has two distinct forms: Digital - switches, relays, etc. which are either in an On or Off state; Analog – a continuous signal such as speed, temperature, low, etc. Idle Current Reduction: Reduction of phase current to a step motor when no motion is required. Indexer: An electronic control device that sends 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 rate of change of the coil current must be multiplied to give the self-induced counter EMF. Inertia: Measure of resistance of an object to changes in velocity; the larger the inertia, the more torque required to accelerate and decelerate the load. Inertial Match: Ratio of reflected load inertia to motor inertia. Instantaneous START/STOP Rate: The maximum switching rate that an unloaded step motor will follow without missing steps when starting from rest or stopping from moving. 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. Loop, PID: A high performance control loop that uses Proportional, Integral and Derivative type control parameters. Loop, Position: A feedback control loop in which the controlled parameter is motor position. Loop, Velocity: A feedback control loop in which the controlled parameter is velocity. Maximum Reversing Rate: The maximum stepping rate at which an unloaded motor will reverse direction of rotation without missing steps. Maximum Slew Rate: The maximum stepping 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. Open Frame Drive: Refers to amplifiers where a separate DC power source must be provided to the unit. Open-Loop: A system with no feedback; most step motor systems are run in this mode. Oscillator: A device that is used to produce pulses for driving a step motor at a preset speed. Overshoot: The amount a motor shaft rotates beyond the commanded stopping position. Packaged Drive: Refers to amplifiers where the power supply is included in the enclosure and 110/220VAC is used to power the unit. Permanent Magnet Step Motor: A step motor having a permanent magnet rotor and wound stator. 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 stepping rate 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; the pulse rate divided by the resolution of the motor/drive combination (in steps per revolution) equals the rotational speed of the motor in revolutions per second. PWM (Pulse Width Modulation): A method of controlling motor voltage and current used in servo and step motor drivers. Radial Play (Side 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. Ramping: The acceleration and deceleration of a motor; may also refer to the change in frequency of the step pulse train. Rated Torque: The torque producing capability of a motor at a given speed; this is the maximum continuous torque the motor can deliver to a load. Regeneration: The action during deceleration, in which the motor acts as a generator and takes kinetic energy from the load, converts it to electrical energy, and returns it to the amplifier. Repeatability: The degree to which the positioning accuracy for a given move performed repetitively can be duplicated. Resolution: The smallest positioning increment that can be achieved; frequently defined as the number of steps or feedback units required for a motor's shaft to rotate one complete revolution. Resonance: The effect of a periodic driving force that causes a large amplitude increase at a particular frequency Response Rate (Pull-In Rate): The stepping rate an unloaded motor can follow from a standing start without missing steps. Ringing: Oscillation of a system following a sudden change in state. RS-232, RS-422/485: Serial communication hardware definitions. Serial Port: A digital data communications port that uses a serial bit stream for data transfer. Servo Amplifier/Servo Drive: An electronic device that converts a control signal into a current that is fed into the motor windings to produce torque in the motor. Servo System: A feedback control system for mechanical motion in which the controlled output is position or velocity; servo systems are closed loop systems. 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 around the final position. Si: MOONS' Simple Indexer operating environment; sequences for machine operation are programmed by the use of point and click instructions. Slew: The portion of a move made at a constant nonzero velocity. Stall Torque (holding or static): The torque available from a motor at stall or zero rpm. Step Angle: The nominal angle through which the step motor shaft rotates between adjacent step positions. Step or Stepping Rate (Speed): The number of steps a shaft rotates during a specified time interval. Step-to-step Accuracy (relative 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, others have a fixed off time and adjust the frequency. Switching Sequence (Energizing Sequence): The sequence and polarity of voltages applied to coils of a step motor that result in a specified direction of rotation. Thermal Time Constant: The time required for the motor winding to reach 63.2% of its final temperature. Thermal Resistance: The resistance to the flow of energy between two surfaces of the same body or different bodies; thermal resistance = degrees C/watt in the winding. Torque: The rotary equivalent of 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: A number representing the relationship between motor input current and motor output torque, usually expressed in units of torque/amp. 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 to a particular change in shaft position when the motor is energized. Torque Ripple: The cyclical variation of generated torque given by the product of motor angular velocity and number of commutator segments. Torque-to-inertia Ratio: Ratio of a motor's torque divided by the motor's rotor inertia; the higher the ratio, the higher the acceleration may be. 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 (V/R): A step motor having a wound stator or stators with salient poles working with a soft iron rotor having salient poles on the periphery. Velocity: The change in position as a function of time; velocity has both magnitude and direction. Viscous Damping: A damper that provides a drag or friction torque proportional to acceleration; a quality used to damp unwanted oscillations of a step motor. 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 phases one at a time; driving the motor one phase or winding at a time. Inertia Conversion Tables: To convert from A to B multiply by entry in table. Electric Motor Terminology Example: Convert a rotor inertia of 90 g-cm2 to oz-in-s2 The multiplier from the table above is 1.416 x 10-5 The new inertia is 90 x 1.416 x 10-5 = 1.27 x 10-3oz-in-sec2 Torque Conversion Tables: To convert from A to B multiply by entry in table. Electric Motor Terminology Example: Convert a torque of 53 oz-in to kg-cm The multiplier from the table above is 7.201 × 10-2 The new torque value is 53 x 7.201 x 10-2 = 3.816kg-cm

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