Main Body




Cogging torque (Tcog)
Cyclic torque in an un-energized motor resulting from the tendency of the rotor and stator to align themselves in a position of minimum magnetic reluctance

Continuous stall torque (Tcs
Maximum continuous output torque that the motor can develop under a stall condition without the motor exceeding its rated temperature

Peak torque (Tp)
Maximum torque developed by a motor under specified conditions when the maximum allowable peak current is applied.

Rated torque (Tr)
Continuous torque
Full load torque
Output torque
Torque developed at rated voltage and rated speed without exceeding the temperature rating.

Torque ripple
Variation of torque within one shaft revolution under specified test conditions, expressed as the ratio of peak-to-peak torque amplitude to average torque (not including cogging torque)

Viscous torque
Dynamic friction torque
The resistance to motion that is velocity dependent

Torque constant (KT)
The ratio of change in torque to change in current developed by a motor.
Torque constant is temperature dependent and can be mathematically derived from the back EMF constant.

(KE). For SI (metric) units, KT = KE. For English units, the following conversions apply:
a) KT = KE /11.834 for brush commutated or trapezoidal EMF brushless motors;
b) KT = KE /13.017 for sinusoidal EMF brushless motors with square wave current;
c) KT = KE /13.662 for sinusoidal EMF brushless motors with sinusoidal current.
The available shaft output torque is the developed torque reduced by frictional and rotational losses.


NOTE In industry, velocity is commonly referred to as speed and is expressed in rpm.

No load speed (?nl)
Actual motor speed with no external load and specified terminal voltage

Rated speed (?r)
Continuous speed
Maximum motor speed that can be achieved while maintaining a specified rated torque.

Maximum mechanical speed (?max)
Highest speed at which the shaft can be rotated without mechanical damage


No-load current (Inl)
The current drawn by an unloaded motor at the rated voltage

Continuous current (Ic.)
Current required to develop rated torque without exceeding the temperature rating.

Continuous stall current (Ics.)
Maximum current that can be continuously applied to a stalled motor without exceeding the temperature rating of the motor.

Peak current (Ip.)
acceleration current
Maximum intermittent current that does not cause motor damage or irreversible degradation of motor performance.

Rated current (Ir.)
Current developed at rated voltage and rated speed without exceeding the temperature rating.

Demagnetization current (Idm.)
The winding current that creates a magnetic flux opposing the main field flux which irreversibly decreases the field strength of the motor magnets.


Ambient temperature (Tamb)
Temperature of the cooling medium, usually air, immediately surrounding the device.

Case temperature (Tcase)
Temperature of the surface of a device.

Maximum allowable winding temperature (Tmax)
Maximum temperature of the winding permitted by the temperature class of the insulation system used.

Temperature rise (?T)
Increase in temperature (in oC) of a device above ambient temperature at designated conditions.


Electrical time constant (?e)
Time required for the current to reach 63.2% of its steady state value when a step input voltage is applied.

Mechanical time constant (?m)
Time required for a device to reach 63.2% of its steady state velocity after a zero source impedance step voltage input is applied.

Thermal time constant (?th)
Time required for a device to reach 63.2% of steady state temperature rise with constant power dissipation.


Back EMF
Counter EMF (Eg)
Internally generated voltage produced by the relative movement between the magnetic field and the armature winding when measured on an open circuit.

Back EMF constant (Ke)
Amplitude of the counter EMF waveform generated at the motor terminals as it is moved through the magnetic field at a constant velocity divided by that velocity:
Motor terminal inductance (La)
Inductance of the armature as measured across the terminals of the device.

Motor terminal resistance (Rm)
Resistance of the armature winding and, where applicable, of the connections measured at the brush contact points.


Rated power (Pr)
The ability of a motor to output its full power for a sustained period of time
NOTE Power is calculated by the formula
Pr = Tr x ?r
?r is the rated speed, Tr is the rated torque

Duty cycle
Relation between the on time and the off time of a device.

Efficiency (h)
Ratio of power output to power input of a machine, expressed as a percentage.

Safe operating area
Area defined by speed and torque conditions at which the motor will operate within its thermal limitations.


Moment of inertia (Jm)
Property of matter that causes the mass to resist any change in its motion.

Breakaway torque (Tbr)
Starting torque
Static friction torque
Mechanical resistance that a device must overcome before motion can occur.

Friction torque (Tf)
Resistance to motion that is independent of velocity.

Torsional resonance
Rotational oscillation that occurs in any rotating system when it is being excited at or near its natural frequency.

Locked rotor
Stalled motor
Condition where the rotor is held stationary while power is applied to the motor terminals.

Manufacturer of Highly Engineered Motors and Motion Control Products
SL Montevideo
2002 Black Oak Ave., Montevideo, MN 56265 USA

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