Motors Terminology

Many electronics equipment, devices, and appliances contain motors as part of their design to achieve their functionality. We have compiled the following glossary of terminology related to motors that we thought could be useful. If we missed anything that you think should be included, feel free to comment or contact us, and we will add it…

Breakdown Torque – The maximum torque that the motor develops at rated voltage and frequency, without an abrupt drop in speed.

Brushless motor – A class of motors which operates using electronic commutation of phase currents rather than electromechanical (brushes) commutation. Commutation is a function of rotor position. 

Cogging – A term used to describe non-uniform angular torque. Cogging appears as jerkiness especially at low speeds.

Commutation – A term which refers to the action of steering currents or voltages to the proper motor phases so as 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 obtained by Hall sensors or resolver.

Controller – A term describing a functional block containing an amplifier, power supplies, and possibly position-control electronics for operating a motor.

Demag current – The current level at which the motor magnets will be demagnetized. This is an irreversible effect which will alter the motor characteristics and degrade performance.

Duty cycle – A measure of a motor’s “on” time or operating time relative to “off” time or rest time. Typically expressed as a percentage of “on” time to total cycle time.

Dynamic braking – A technique for stopping a permanent magnet brush or brushless motor. The motor windings are shorted together through a resistor which results in motor braking with an exponential decrease in speed.

Dynamometer – A system used to measure the power output by a motor or other source. Can be found for many different applications sizes range from fractional horsepower to over 5,000hp.

Inertia – The property of an object to resist changes in velocity unless acted upon by an outside force. Higher inertia objects require larger torque to accelerate and decelerate. Inertia is dependent upon the mass and shape of the object.

Inrush current – The current surge generated when a DC motor is initially connected to a power source. This surge is due to the lack of armature inductance at zero speed and is limited by the motor resistance.

Locked Rotor Current – The steady state current from the line at rated voltage and frequency with the rotor locked.

Locked Rotor Torque – The minimum torque that the motor develops at rest for all angular positions of the rotor at rated voltage and frequency.

Maximum Theoretical Acceleration – The acceleration developed by the motor alone, from standstill, at the moment when maximum voltage is applied is the maximum theoretical acceleration in radians per second.

Moment of Inertia – The moment of inertia of the armature is measured about the motor’s axis of rotation. The ratio of the motor moment of inertia to the damping factor with a zero-impedance power source gives the mechanical time constant of the motor. In direct drive systems, load inertia and damping factor have to be added to the motor inertia and damping factor to determine the mechanical time constant.

Motor Constant – This is the ability of a motor to convert electric power input to torque. This is a figure of merit that can be used to compare motors in their ability to produce torque per unit of power input. It is the ratio of torque to the square-root of the power input.

No Load Speed – This is the maximum speed of the motor at no load when the voltage that is required to produce peak torque is applied.

Peak Torque – This is the maximum useful torque that can be obtained at maximum recommended current input.

Permeability – The relative ease of establishing a magnetic field in a material. A high permeability material is extremely easy to establish magnetic fields in.

Power – The rate at which work is done. Mechanically, this is the torque multiplied by the speed. Electrically, this is the voltage multiplied by the current. The units are horsepower or watt.

Power Rate – The ratio of peak torque squared to inertia which is useful in applications where the acceleration of a load through a gear train is the prime consideration. An initial motor selection is made which has a power rate of at least 4 times the product of the load inertia and the load acceleration required. A gear ratio is then chosen which will match the motor and load inertia.

Pull up Torque – The minimum torque developed during the period of acceleration from rest to the speed that breakdown torque occurs. 

PWM – Pulse Width Modulation. An acronym which describes a switch-mode control technique used in amplifiers and drivers to control motor voltage and current. This control technique is in contrast to linear control and offers the advantages of greatly improved efficiency.

Regeneration – The action during motor braking, 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.

Residual Induction – The remnant flux density that remains in a permanent magnet after it has been saturated by a magnetic field and then the field has been removed.

Ripple Torque – A small change in torque with armature position is caused by the switching action of the commutator. The armature rotates through a small angle before its field is returned to its original position through commutation. This variation is known as ripple torque and is usually expressed in percent of torque level.

RPM – Rotations per minute. Term used to indicate rotational speeds in system.

Stall torque – The torque available from a motor at stall or 0 rpm.

Tachometer – An electromagnetic feedback transducer which produces an analog voltage signal proportional to rotational velocity. Tachometers can be either brush or brushless. 

Torque – A measure of angular force which produces rotational motion. This force is defined by linear force multiplied by a radius e.g. lb.-in or oz-in.