Transfer Function of DC Motor

[curious_mind]

I was going through this link https://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed&section=SystemModeling for understanding DC motor transfer function. But for real motor, how do I get values such as motor viscous friction constant ,electromotive force constant etc. Can we get it from data sheet?

 

[KlausST]

Hi,

Can we get it from data sheet?

Generally: The datasheet should always be the first place for any devices specifications.

Klaus

 

[curious_mind]

I do not see parameters like motor viscous friction constant for example

 

[FvM]

Depends on the data sheet completeness. Most important parameters are emf and torque constants. They can be derived from a good datasheet. Also friction constant from the no-load current. Rotor inertia and inductance are rarely specified and must be measured or estimated.

 

[curious_mind]

I have attached the image of DC motor datasheet. Could you please guide, how to calculate motor viscous friction constant ,electromotive force constant

 

[KlausST]

Hi,

It's not a motor, it's a generator.

Klaus

 

[FvM]

No it's a motor (see item 20). Item 7 specifies the power supply.

The datasheet clarifies that the motor has among other features variable excitation and a compound winding, thus the simple DC motor model derived in the link in post #1 doesn't apply. Refer to an electrical machines text book for a detailed analysis.

 

[KlausST]

Hi,

Indeed you are right. I misinterpreted the "generator" and "excitation" ...

Klaus

 

[curious_mind]

Yes, It is separately excited dc motor. Would viscous friction and emf depend on type of motor(simple or compound).

 

[FvM]

Sketch a complete equivalent circuit of the motor with all windings. You'll realize that there more parameters involved than with the simple PMDC motor analyzed in the post #1 link.

There are no parameters in the above datasheet that allow estimation of friction constant.

 

[curious_mind]

ok. Now one last question. I wanted to know the dimension normalization in a typical DC motor control system. For example inertia is usuallyt given in kg-m2 and speeds are usually rad/s. Can we subsititute these values as it is or any conversion is required?