Electromechanical

Calculating Winding Temperature of Servo Motors

Calcuulating Winding Temperature of Servo MOtors - Parker Hannifin Electromechanical Division North America

So, you want to know how to calculate the actual winding temperature of a Parker permanent magnet (PM) servo motor? In the course of applying rotary motors in applications, it sometimes becomes desirable to figure out the actual winding temperature given certain data.  

Setting the context

First off, we will define some terms: 

 

Calulating Widing Temperature of Servo Motors - Terms, Definitions, Units and Vallue (BE232D) - Rth w-c    Thermal resistance from winding to case    oC/w    .56 (Parker Supplied)   Rth c-a    Thermal resistance from case to ambient    oC/w    1.02 (Parker Supplied)   R25C    Winding resistance at 25oC    Ohm    7.72 (Parker Supplied)   w    Shaft speed    Rad/s    Assumed or measured   FT    Total friction from all sources    Nm    .0141 (Parker Supplied)   B    Total damping from all sources    Nm/rad/s    3.227E-5 (Parker Supplied)   Wc    Case losses (speed related losses)    Watts    Calculated   Wr    Resistive losses (power-related losses)    Watts    Calculated   Rhot    Winding Resistance at Winding Temp    Ohms    Calculated   Tw    Temperature of the winding    oC    Calculated   Tc    Temperature of the case    oC    Calculated or measured   Irms    RMS current output from drive to motor    Arms    Assumed or measured

Calculating the winding temperature

First, calculate the resistance of the winding at the measured or assumed RMS current from the drive to the motor.  The equation for this is: 

 

Rhot = R25C *(0.90175 + 0.00393Tw)                                  

Equation 1 

This will give the value for Rhot in terms of the unknown value, Tw - our desired term. 

Next, the losses due to shaft rotation must be calculated.  These losses are the case losses, Wc, and are due to the motor friction and motor damping.  Fail safe brake and gearbox losses can also add to the case losses - but will be omitted from this example. 

 

Wc = (FT * w) + (B * w2)                                                                    

Equation 2 

 

The friction and damping values are supplied by Parker. 

Once the case losses are calculated, the resistive losses should be calculated.  This equation is: 

 

Wr = 1.5(Irms)2 * Rhot                                                        

Equation 3 

 

Wr will be given in terms of Tw once we plug in the equation for Rhot

 

Wr = 1.5(Irms)2 * (R25C *(0.90175 + 0.00393Tw)           

Equation 4 

 

Insert all known values and simplify. Equation 4 will be reduced to a simple form with Tw still as an unknown. The final equation for the winding temperature is as follows: 

 

Tw =[(Rth c-a) *(Wc + Wr) + (Rth w-c) * Wr] +25               

Equation 5 

 

Plug in the simplified Equation 4 for Wr and the thermal resistance values supplied by Parker. Simplify. Now you can easily solve for the winding temperature, Tw

If you prefer, you can put everything in terms of Tw and plug in the values at the end to solve, but the equation gets long and complicated. 

Also, if you are able to reliably measure the case temperature, you can increase the accuracy of the winding temperature calculation and simplify the Equation 5 to the following: 

 

Tw = Tc + (Rth c-a) *(Wr) + 25                                             

Equation 6 

 

At that point, the case temperature would include all losses and cooling due to convection and conduction. 

 

 Example calculation 

 

Find the winding temperature (Tw) for the BE232D rotating at 5000RPM with RMS current of 1.8Arms 

Plugging in the 7.72 for R25C and simplifying, Equation 1 becomes: 

 

Rhot = R25C *(0.90175 + 0.00393Tw)                             

Rhot = 6.962 + 0.03034Tw                                                                              

 

Converting 5000 rpm to radians/second gives us 523.6rad/s.

Plugging this and the friction and damping values into Equation 2 yields:

 

Wc = (FT * w) + (B * w2)                                                

 

Wc = (.014123 * 523.6) + (.00003278*523.62) 

Wc = 16.377 W 

 

Find the resistive losses for the RMS current of 1.8A from Equation 3 and 4: 

Wr = 1.5(Irms)2 * Rhot                                                                        

Wr = 4.86 * (6.962 + 0.03034Tw)                                  

Wr = 33.84 + 0.147T                  

                                                                                         

Finally, use Equation 5 to put it all together and solve for Tw

Tw =[(Rth c-a) *(Wc + Wr )+ (Rth w-c) * Wr] +25                           

Tw =[(1.02*(16.377 + 33.84 + .147Tw)) + (.56 * (33.84 + .147Tw))] +25 

 

This simplifies to: 

Tw = 95.17 + .2323Tw 

Tw = 124o

The speed and the current happen to be the rated speed and current for the BE232D motor. 

If you would like to calculate the winding temperature for your motor, you will need to contact your local Parker distributor for some of the unpublished data required to perform the calculations.  Let them know to look on the Parker Distributor Extranet Product Pages for this unpublished data.

 

Calculating Winding Temperature of Servo Motors - Jay Schultz, Industry Market Manager - Hybrid Electric Vehicle ProductsArticle contributed by Jay Schultz, Product Manager-Motors, Parker Electromechanical Automation North America. Originally published on ParkerMotion Blog June 26, 2013.

 

 

 

 

Other articles by the Electromechanical Team:

What You Should Know About Frameless Motors

GVM Motor Gives Traction to Brammo's Winning Motorcycles

What You Should Know About Gearhead Sealing

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