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Static_Dynamic_TOrque

Thread Starter: Karnan   Started: 4/27/2012 12:13 PM   Replies: 8

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  4/27/2012, 12:13 PM
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Hai,

Anyone please give me simple discription to understand the difference between Static Torque and Dynamic Torque of a servo motor..

When these two torques are coming into picture???

Thanks&Regards,
Raj
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  4/27/2012, 5:19 PM
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Rnf:lblSmileyTooltip_ Hello Karnan,

Static Torque refers to the amount of torque a servo motor produces at zero speed of rotation. 
Dynamic Torque refers to the amount of torque a servo motor produces at some speed of rotation with load applied.

Typically with a servo motor it is rated by manufacturer with a continuous Static Torque rating, which means the motor is capable of supplying that Static Torque at zero speed of rotation continuously.

This Static Torque rating is quite often not extremely useful in the selection process of servo motors since usually the need of a servo motor is to move a load.

Therefore Dynamic Torque rating of a motor is more useful since this defines the continuous torque available from a motor at a certain speed (or range of speeds) to move a load. 

Typically a servo motor is rated by manufacturer in catalogs for a specific Rated Speed (for example 3000 rpm) where the servo motor would be capable of providing up to a corresponding Rated Torque value continuously.

The Static Torque capacity of a servo motor will be slightly higher than the Rated Torque capacity of a servo motor due to increased heat build up in motor due to rotation.  When viewing a torque vs. speed curve from zero speed to rated speed the line will be approximately linear.

Finally, in addition to the continuous rating for a servo motor all servo motors can achieve a higher torque for a short amount of time.  This is helpful for cycling loads. 
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  4/28/2012, 8:24 PM
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Thanks Buddylaughlaugh...

In motor nameplate showing  Mo =  XX Nm and MoMax = XX Nm what it defference ??
Is it maximum rated Torque of Motor at max rated Speed???


Thanks&Regards,
Raj
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  4/29/2012, 7:25 AM
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Generally Mo & Mn or Io & In are specified on servo motor nameplate. Significance is Mo = Motor torque at zero speed (static torque); Io= max motor current at zero speed. Mn = Max torque at rated motor speed & In = max current at rated motor speed.

Tell me and I will forget. Show me and I will remember. Involve me and I will understand. -Confucius
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  5/10/2012, 2:42 PM
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Hai Deputy,

Please help me to clear my Doubs,

About Torque at Zero speed -- The Torque required for motor to standstill ..... Is it right ???

I one of the document of a project,with constant speed application[in 6000mm/min linear movement,servomotor],considered the Static Torque of motor.They didnt mentioned about dynamic torque of a motor... 

After reading this i am just confused about Torque at zero speed [static torque]..Can you able to make options why they selected like this, as i didnt get proper answer from client... 

Thanks&Regards,
Raj
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  5/10/2012, 3:29 PM
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Hello Karnan,

Static torque for a servo motor in an actual application could best be described as the amount of torque necessary to maintain motor at zero rpm. 

This static torque would be for instance on a vertical lift application where a load is raised to position y and held (by the static torque) at that height for a short amount of time while a machine operation is being performed.  Then while load is moved to next position a dynamic torque would be necessary to be applied by the motor to move load.

In description of your application it appears that the motor wil be operating at a constant speed.  Therefore it would be technically most correct to select motor which could continuously supply the amount of torque (i.e. dynamic torque) at that speed.

A servo motor is designed and often documented for its torque capabilities in catalogs and some manufacturers do not supply all information.  Attached is an example catalog page and documentation for a servo motor 1FK7083-5AF71-1TA0 where its capability of supplying toque continuously is per below.

Static Torque 16 Nm (@ or near zero speed)
Rated Torque 10.5 Nm
Rated  Speed 3000 rpm
Rated Power 3.3 kW

Many manufacturers only display in catalogs the static torque rating of a motor, rated speed in rpm, and a power rating.

If rated torque is not listed in catalog this can be very misleading and can result in selection of a motor which is undersized since a servo motor will not be able supply as much torque continuously at rated speed as at zero speed.

When a manufacturer does not display rated torque in catalog pages for selection then the rated torque must be calculated or to be more exact refer to manufactuers torque / speed curves for that motor for the speed of operation range of application.

Calculation of Rated Torque would be performed as follows:
Power (kW) = Torque(rated Nm) * Rated Speed (rpm)  /  9550
Using above catalog example:
Power (kW) = 10.5 * 3000 / 9550 = 3.3 kW

 Deputy   

Attachment: Catalog Page 1FK7.pdf  (20 Downloads)

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  5/11/2012, 6:56 AM
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Hai Deputy,

Thanks for your Valuable information..

Please tell me how we defined the consatnt 9550 for the calculation ??/

Thank&Regards,
RAj
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  5/11/2012, 2:55 PM
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Hello Karnan,



Wikipedia is one source where the value 9550 is rounded slightly from what is documented on Wikipedia per below as 9,554.

The below copy and paste is better viewed from website, but if you take the following constants in the formula

60,000 / 2 * Pi

60,000 / 2 * 3.14

9,554

http://en.wikipedia.org/wiki/Torque

Conversion to other units

A conversion factor may be necessary when using different units of power, torque, or angular speed. For example, if rotational speed (revolutions per time) is used in place of angular speed (radians per time), we multiply by a factor of 2π radians per revolution.

\mbox{power} = \mbox{torque} \times 2 \pi \times \mbox{rotational speed}

Adding units:

 \mbox{power (W)} = \mbox{torque (N}\cdot\mbox{m)} \times 2 \pi \times \mbox{rotational speed (rps)}

Dividing on the left by 60 seconds per minute and by 1000 watts per kilowatt gives us the following.

 \mbox{power (kW)} = \frac{ \mbox{torque (N}\cdot\mbox{m)} \times 2 \pi \times \mbox{rotational speed (rpm)}} {60,000}

where rotational speed is in revolutions per minute (rpm).

Some people (e.g. American automotive engineers) use horsepower (imperial mechanical) for power, foot-pounds (lbf·ft) for torque and rpm for rotational speed. This results in the formula changing to:

 \mbox{power (hp)} = \frac{ \mbox{torque(lbf}\cdot\mbox{ft)} \times 2 \pi \times \mbox{rotational speed (rpm)} }{33,000}.

The constant below in, ft·lbf/min, changes with the definition of the horsepower; for example, using metric horsepower, it becomes ~32,550.

Use of other units (e.g. BTU/h for power) would require a different custom conversion factor.


Attachment: Kilowatt formula.pdf  (18 Downloads)

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  5/18/2012, 7:09 AM
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hi Deputy

   Nice explanation!!!

Regards

Mothilal

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