Motor torque? 4

[zolka]

Hi!
Is it possible that the rated torque of a 3 phase AC motor is smaller than the starting torque?
In my case:
Tstart=13Nm
Trated=20Nm
Tpullout=33Nm

 

[zolka]

Sorry, rated torque is bigger than Tstart!!!
But it is still a question!

 

[desertfox]

Hi zolka

Usually the starting torque of a motor is much higher than the rated torque.
Have a look at this link:-

http://www.engineeringtoolbox.com/electrical-motors-torques-d_651.html

regards

desertfox

 

[electricpete]

Tstart would not be < Trated for any of the standard NEMA designs (A, B, C, D).

BUT is possible to adjust the parameters of a machine widely during design, depending on the objectives.

A relevant question in my mind is what spec was the machine purchased under and what did that spec say about starting torque.


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[7anoter4]

According to IEC-34-12 if the motor power is between 400-630 kw the starting torque may be 65% from rated.
Also for all motor power above 100 kW the starting torque may be less than rated.
But I think 33/20=165% pull-up torque is not possible even on this standard. Pull-up torque is the minimum torque then it must be less than starting. May be you meant breakdown torque instead of pull-up, did you?
See the attachment.
Regards

 

[zolka]

Helo Electricpete!
I dont know the machine's spec, all I know is that it is a 1KW motor, 1700rpm pole number=4, Rr=5.0815 Rs=5.62 Lm=0.4257 Ls=0.4631 Lr=0.4631, delta-star 220/380 60Hz, the torque values were computed by formulas!

Helo 7anoter4!
In the literature I used for the torque calculations, the term 'pull out' torque was used instead of 'breakdown torque', I thought both are correct, and have the same meaning!

Thanx for the feedback!

 

[desertfox]

Hi zolka

Your right pullout and breakdown torque are the same.
I think reading 7anoter4's post, last sentence of his paragraph "May be you meant breakdown torque instead of pull-up, did you?"
He as read pull-up torque instead of pullout torque.

regards

desertfox

 

[LionelHutz]

It's very possible. I often see large 5kV motors with the minimum torque during acceleration down to 60% of rated.

This motor could be specifically designed for operation with a VFD. When using a VFD all that matters is the torque curve above the breakdown or pullout speed.

Tstart really isn't a parameter I'm familiar with. Is that an IEC rating?

Typically, locked rotor torque and pull-up torque are the values I see. Locked rotor torque is the torque produced at zero speed. Pull-up torque is the minimum torque in the torque curve and typically occurs around 40-60% speed.

 

[zolka]

Helo LionelHutz!
I don't know if Tstart is an IEC rating!All I know is that Tstart can be computed by changing the slip value to 1 in the torque formula!

 

[LionelHutz]

OK, your definition of Tstart is the same as the locked rotor torque then.

 

[zolka]

Helo everybody!
Could anyone possibly check my calculation results, only the torque, I was wondering if I miscalculated something??!!
The motor parameters are given above!
Thanx in advance!

 

[7anoter4]

Hi Zolka
There are many problems with your data.
If the Trated is 20 Nm the rated power has to be 3.6 KW as T=60*1000/2/PI()*P[Kw]/rpm=60*1000*3.7/2/pi()/1700=20.2 Nm
The rotor data are stated for rated slip, I think. Usually, due to skin effect the rotor resistance may be more than 2-5 time and rotor reactance decrease in the same time at starting conditions.
Since at starting the supply voltage will be only 220 V the torque will decrease direct proportionally with (380/220)^2 =3 time.
I guess the resistance values are in ohm and inductances are in Henry and the rotor impedance is reported to the stator.
If you use the Steimetz model to calculate the torque you'll get the P=1 kW indeed, but the torque will be only 6 Nm. Both, rotor resistance and Lm have to be different a bit :R=3.7 ohm and Lm=0.62 H even for 1 kW.
Regards