Motor constants: Difference between revisions
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== Motor velocity constant, back EMF constant == |
== Motor velocity constant, back EMF constant == |
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''K<sub>v</sub>'' is the motor velocity constant, measured in [[Revolutions per minute|RPM]] per volt (not to be confused with ''kV'', the abbreviation for ''kilovolt'').<ref> |
''K<sub>v</sub>'' is the motor velocity constant, measured in [[Revolutions per minute|RPM]] per volt (not to be confused with ''kV'', the abbreviation for ''kilovolt'').<ref>http://learningrc.com/motor-kv/</ref> The K<sub>v</sub> rating of a [[brushless motor]] is the ratio of the motor's unloaded RPM to the peak (not RMS) voltage on the wires connected to the coils (the ''[[Counter-electromotive force|back EMF]]''). For example, an unloaded motor of K<sub>v</sub>, 5,700 rpm/V, supplied with 11.1 V, will run at a nominal 63,270 rpm (5,700 rpm/V × 11.1 V). |
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The terms ''K<sub>e</sub>'',<ref name="kk">{{citation| url = http://hades.mech.northwestern.edu/images/6/61/Asst7.pdf| title =Mystery Motor Data Sheet| work = hades.mech.northwest.edu}}</ref> ''K<sub>b</sub>'' are also used,<ref>{{citation| url =http://www.smma.org/pdf/SMMA_motor_glossary.pdf|title = GENERAL MOTOR TERMINOLOGY| work = www.smma.org}}</ref> as are the terms ''back EMF constant''.<ref>{{citation| url = http://www.mathworks.co.uk/help/toolbox/physmod/elec/ref/dcmotor.html|title =DC motor model with electrical and torque characteristics - Simulink| work =www.mathworks.co.uk}}</ref><ref>{{citation| url =http://www.micro-drives.com/motor-calculations.aspx| title = Technical Library > DC Motors Tutorials > Motor Calculations| work = www.micro-drives.com}}</ref> or the generic ''electrical constant''.<ref name="kk"/> In contrast to ''K<sub>V</sub>'' the value ''K<sub>e</sub>'' is often expressed in SI units <math>\frac{V\cdot s}{rad}</math>, thus it is an inverse measure of ''K<sub>V</sub>''.<ref>http://www.precisionmicrodrives.com/tech-blog/2014/02/02/reading-the-motor-constants-from-typical-performance-characteristics</ref> Sometimes it is expressed in non SI units krpm/V.<ref>http://www.smma.org/pdf/SMMA_motor_glossary.pdf</ref> |
The terms ''K<sub>e</sub>'',<ref name="kk">{{citation| url = http://hades.mech.northwestern.edu/images/6/61/Asst7.pdf| title =Mystery Motor Data Sheet| work = hades.mech.northwest.edu}}</ref> ''K<sub>b</sub>'' are also used,<ref>{{citation| url =http://www.smma.org/pdf/SMMA_motor_glossary.pdf|title = GENERAL MOTOR TERMINOLOGY| work = www.smma.org}}</ref> as are the terms ''back EMF constant''.<ref>{{citation| url = http://www.mathworks.co.uk/help/toolbox/physmod/elec/ref/dcmotor.html|title =DC motor model with electrical and torque characteristics - Simulink| work =www.mathworks.co.uk}}</ref><ref>{{citation| url =http://www.micro-drives.com/motor-calculations.aspx| title = Technical Library > DC Motors Tutorials > Motor Calculations| work = www.micro-drives.com}}</ref> or the generic ''electrical constant''.<ref name="kk"/> In contrast to ''K<sub>V</sub>'' the value ''K<sub>e</sub>'' is often expressed in SI units <math>\frac{V\cdot s}{rad}</math>, thus it is an inverse measure of ''K<sub>V</sub>''.<ref>http://www.precisionmicrodrives.com/tech-blog/2014/02/02/reading-the-motor-constants-from-typical-performance-characteristics</ref> Sometimes it is expressed in non SI units krpm/V.<ref>http://www.smma.org/pdf/SMMA_motor_glossary.pdf</ref> |
Revision as of 00:16, 11 May 2016
The constants KM (motor size constant) and Kv (motor velocity constant, or the back EMF constant) are values used to describe characteristics of electrical motors.
Motor constant
KM is the motor constant[1] (sometimes, motor size constant). In SI units, the motor constant is expressed in (N⋅m/sqrt(W)):
where
- is the motor torque (SI units, N·m)
- is the resistive power loss (SI units, W)
The motor constant is winding independent (as long as the same conductive material used for wires); e.g., winding a motor with 6 turns with 2 parallel wires instead of 12 turns single wire will double the velocity constant, Kv, but KM remains unchanged. KM can be used for selecting the size of a motor to use in an application. Kv can be used for selecting the winding to use in the motor.
Motor velocity constant, back EMF constant
Kv is the motor velocity constant, measured in RPM per volt (not to be confused with kV, the abbreviation for kilovolt).[2] The Kv rating of a brushless motor is the ratio of the motor's unloaded RPM to the peak (not RMS) voltage on the wires connected to the coils (the back EMF). For example, an unloaded motor of Kv, 5,700 rpm/V, supplied with 11.1 V, will run at a nominal 63,270 rpm (5,700 rpm/V × 11.1 V).
The terms Ke,[3] Kb are also used,[4] as are the terms back EMF constant.[5][6] or the generic electrical constant.[3] In contrast to KV the value Ke is often expressed in SI units , thus it is an inverse measure of KV.[7] Sometimes it is expressed in non SI units krpm/V.[8]
The field flux may also be integrated into the formula:[9]
- where is back EMF, is the constant, is the flux, and is the angular speed
An inverse measure is also sometimes used, which may be referred to as the speed constant.[3]
By Lenz's law, a running motor generates a back-EMF proportional to the RPM. Once the motor's rotational velocity is such that the back-EMF is equal to the battery voltage (also called DC line voltage), the motor reaches its limit speed.
Motor Torque constant
KT is the torque produced per unit armature current.[10] It can be calculated from the motor velocity constant Kv.
where is the armature current of the machine (SI units A ). KT is primarily used to calculate the armature current for a given torque demand:
References
- ^ http://www.motioncomp.com/pdfs/Motor_Constant_Great_Equalizer.pdf
- ^ http://learningrc.com/motor-kv/
- ^ a b c "Mystery Motor Data Sheet" (PDF), hades.mech.northwest.edu
- ^ "GENERAL MOTOR TERMINOLOGY" (PDF), www.smma.org
- ^ "DC motor model with electrical and torque characteristics - Simulink", www.mathworks.co.uk
- ^ "Technical Library > DC Motors Tutorials > Motor Calculations", www.micro-drives.com
- ^ http://www.precisionmicrodrives.com/tech-blog/2014/02/02/reading-the-motor-constants-from-typical-performance-characteristics
- ^ http://www.smma.org/pdf/SMMA_motor_glossary.pdf
- ^ "DC motor starting and braking", iitd.vlab.co.in
- ^ Understanding motor constants Kt and Kemf for comparing brushless DC motors
External links
- DC Motors, Electric motor reference center
- "Development of Electromotive Force" (PDF), biosystems.okstate.edu