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Thermal Time constant of motor | 05 Mar 2020 | |

Thermal Time constant of motor


I  am looking some information about Thermal Time constant of Kollmorgen motors.

I find the white paper "Kollmorgen Managing Servo Motor Overloads WhitePaper.pdf" which is very interesting..

But when I look some figures in detail, I am slightly at a loss...

Take for example a AKM41E :

On the Motor database of the S300/S700 (mdb46) , we see a Thermal Time constant of 20 sec

On the Workbench, we see a value for the Coil Thermal time constant of 5,436 mHz..

Even with the formulae Coil Termal Constant in mHz = 1000 / (2*PI*(Thermal Time constant in sec)/30.0)) , I don't see any coincidence between these 2 figures...

In fact, for an application, I need 22 Nm rms during 2 minutes, then 18 Nm during 1 minutes, and the motor is disabled during 15 minutes. 

I try to see if a Cartdridge C052D with 16.9 Nm in nominal torque could sustain such a cycle...

Thanks for your help.

Best regards.


Comments & Answers

hurley.gill | Thu, 03/05/2020 - 15:04

22_Nn for 120sec
18_Nm for 60sec
0_Nm for 900sec
Trms (all inclusive motion-profile) = ~4.84_Nm (~3.2_Arms); excluding dwell at no-load Trms = 20.75_Nm (~13.4_Arms) for 180sec.
C052D (kt=1.55_Nm/Arms) similar to AKM63L; Tc(motor) =16_Nm with Ic(motor) =10.5_Arms
TCT_motor = ~25_minutes, estimated based on AKM63L; TCT_coil (Workbench) = 49.7_sec.

Question: What drive are you using?

hurley.gill | Thu, 03/05/2020 - 16:24

1) Standard drive selection will be initial limiting factor; overcome by going to a larger continuous current drive.
2) Assuming Nrms > 0rpm; it is likely Trms(required)/Tr(Nrms) is greater than 130%.
You should go to the next size large motor; perhaps reviewing a C053B with AKDx02406!
With the data i have, i can not say for sure it would not work; but if it did and you had some undesirable event at the wrong time during motion-profile, i would expect potential thermal damage.

defty | Thu, 03/05/2020 - 17:39

The problem is that I need to use a low voltage drive (48 Vdc)...
I think to use a AKD 01206-0069 (in 48 Vdc) because I need no more than 35 rpm, but perhaps 12 Amp is not enough ...
I have a Trms (excluding dwell at no load) equal to 22 Nm...
Just to be sure : the Thermal Time constant given in the S700 database in sec is the Coil Termal constant isn'it ?
And you consider that there is a factor of 30 between the Coil CTC and the Motor CTC ?
Best regards.

hurley.gill | Fri, 03/06/2020 - 16:59

Based on the Motion profile information sent yesterday, i can not recommend the C052D w/12_Arms continuous drive; and suggested in its stay, a C053B & AKDx02406, due to overload requirements.
Even with normal 3-phase power source and C052D, you would need to utilize a larger continuous capability drive (e.g. AKDx024). The AKDs and S700 series drives can provide published Ipk for 4-5_sec; based on the provide motion-profile, you need to operate in the drive's intermittent (above continuous) duty zone for ~180_seconds, which requires about twice the intermittent I2t capability of a 12_Arms drive.

However, the low RPM and limited 48VDC source operation changes everything; both being very important to know!

Low RPM (<35rpm);
22_Nn for 120sec;
18_Nm for 60sec;
0_Nm for 900sec.

Utilizing Trms = 20.75_Nm [~13.4_Arms based on Kt(C052D)] for 180sec we get an I2t requirement of 6401; but only have (max.) 3780 [min. available: 3024].

What this means is we basically need to be able to provide the Tpk required (22_Nm) off the continuous capability of the drive or less, and still meet the [up to] 35rpm max. off a 48VDC source.
This results in a required Kt(AKDx012) = 22_Nm/12_Arms = ~1.83Nm/Arms; see suggested solution below:
If the following don't work for you we can look for a winding using a 24_Arms continuous drive.

Assuming from previous statements you wish to stay with low-voltage version of the AKDx012 (30_Arms peak):

Motor: C053S (S = special 53tpc winding (for new winding ref: FW65007]), with AKDx01206-xxx-0069 drive, and cables.

Closest PC (performance Curve) i can provide is basically equivalent: AKM64(53tpc); which is the base model for C053S.

Hope this is of some help.

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