Torque Motors 1

[buzzp]

I am trying to figure out what the NEC's definition is of a 'torque motor' per NEC(2005)430.6(C).
We have a motor (AC - 3phase) with a torque of 640 ft-lbs which operates at 173RPMs, 30 minute duty. I have never sized conductors for such a motor.
In 430.6(C) they say a torque motor needs to use the locked rotor current in sizing conductors according to 430.22. In 430.22 they don't talk about torque motors (dont expect them too as this just gives percentage of amps to use to size), but they do have a special case for intermittent duty motors(430.22(E)), which is what I have. This section says to use 150% of FLA (table 430.22(E) and short duty- raising/lowering).
So on one hand the torque motor requires 150% of LRA and the intermittent duty requires 150% of FLA for the purposes of sizing conductors. I don't have any other motor nameplate data as of yet so I can't offer more information. Any know which one I should be using? I would hate to have to use 150% of LRA. Thanks for any help.

 

[waross]

A torque motor is typically drawing LRA or close to it continuously. On the torque motors I have installed LRA and FLA were the same. (Full load on a torque motor is locked rotor.)
Torque motors are often used in place of springs in cable take-up reels. It is common for them to be rotated in reverse while energised when the cable is being drawn off the reel.
A torque motor (or the motor-gearbox package) may have a maximum RPM, but normally they rotate at a much slower speed.

 

[petronila]

Hello buzzp,

Torque is the rotating force produced by a motor.The units of torque may be expressed as Newton-meter (metric system ) or Pounds-foot,pound-inch(English system).

Try with this Useful Formula:

Horsepower = Torque (LB-FT)x RPM / 5250

LB-FT = 640 RPM = 173 HP = 21 (57 Amp at 230 V or 28.5 Amp at 460 V).

Now you can use the NEC.

Regards

Petronila

 

[waross]

Thanks for doing the math, petronila.
There is a problem, however: A torque motor rarely operates at listed RPM. Normal current is locked rotor torque or close to it, and normal speed is locked rotor or close to it.
One caveat, torque motors may be supplied as a package with an included gear reduction. The nameplate torque may be the final output torque and not the actual motor torque. Check this out, I was involved in the changes on a job where a designer was caught in this trap.
Your best action is to phone or e-mail the manufacturer for the name plate data.

 

[mc5w]

A torque motor has extra high resistance in the rotor so that it can operate at standstill or nearly standstill indefinitely. Cooling would need to be totally enclose nonventilated to totally enclosed blower cooled. In the case of the latter you need for power failure or phase failure or overload trip for the blower motor to shut off the torque motor.

 

[buzzp]

I just need to know if it qualifies as a torque motor per the NEC. I have the FLA of 34 and LRA of 135A. The literature I have also specifies a Kw of 8.31, pf 0.72 and an eff of 85%. There are no design letters, etc given to me yet. They are saying the time duty is 30 minutes with H class insulation. This is all from the vendor and I assumed (perhaps erroneously, that this is the nameplate data). So it appears the LRA and FLA are given seperately on this motor so does that mean it is NOT a torque motor?

This motor drives a gear box, which turns at 22RPM with an output torque of 4352 ft-lbs. I don't think this part will matter much in sizing motor conductors but wanted to get all the info on the table. The motor will be totally enclosed.

Anyone know how the NEC defines a torque motor, or the IEEE for that matter? Or which rating should I use, LRA or FLA in sizing the conductors? Thanks for all the replies.

 

[waross]

This would not be a torque motor as I know them.
Try looking in

http://nema.org/

yours

 

[petronila]

Hello Buzzp,

I think you could take a look of the motor´s lead size, the motor is 3,6,9 or 12 Leads? if it is 3 leads you know the motor wire gauge,and you will know the Full load amperage, if is 6,9 or 12 leads then connect the motor to rating voltage (star or delta) if is star the conductor size could be 1.25 the lead amperage, watch with the connection because if the motor is delta connected you have two motor leadsconnected together and the conductor size is 1.25 x (2 x one of the leads amperage) another thing made the corrections if the conductor is too long.

Regards

Petronila

 

[aolalde]

Per IEEE a torque motor: A motor designed primarily to exert torque through a limited travel or in a stalled position. Such a motor may be capable of being stalled continuously or only for a limited time .

So, if for the application the motor works stalled for several seconds or minutes, it is a Torque Motor. The wire should be capable of handle the LRC as per the NEC.

 

[buzzp]

The motor can work in a stall, although its not normal operation. We will have torque limits set to trip the motor before the stall torque is reached while the motor is operating as normal. So the "primary application" is not in stall mode so per the IEEE's definition, it appears this is NOT a torque motor.
There would be a rare case, in cold weather, where the motor torque switches would be set higher to allow unseating a stuck valve or something. However, they would be set back to below stall tork for normal operation.
I still have to size the conductors to 150% of FLA (not 125%) because it is intermittent duty (30 minute).
Thanks to all for your help. Have to give aolade a star for providing the definition I was looking for.

 

[petronila]

Dear Buzzp,

About your conductor sizing concept : "I still have to size the conductors to 150% of FLA (not 125%) because it is intermittent duty (30 minute)" I think 50% factor is high take acount The motor´s manufacturer must be calculate the motor´s leads to whistand the load condition so this motor could be operate with out lead damage independent if is intermitent duty or not, what happened if the motor operates 30 minutes under full load with leads with wrong ampacity????, the above 1.25 factor is for safety and taking acount the conductors could be travel by a duct and for voltage drops considerations.

Regards

Petronila

 

[buzzp]

Petronila,
I think I understand what your saying and its not true. Mfgs wiring, internal to their equipment, does not have to follow the NEC. So using their conductor sizes might be a bad idea (in my case since their are no motor leads only terminals). I could look internal to the terminal and see what size conductors but this won't help in sizing per the NEC.
Look at 430.22. The first paragraph is for 'continuous duty' which says to size for 125%. However, my application is intermittent duty as defined by 430.22(E), which requires the conductor sizes be sized for 150% of current rating for 30 & 60 minute rated motors operating valves, raising or lowering, etc per table 430.22(E).
So I know the 150% HAS to be used and not 125%. I was questioning whether this should be 150% of FLA or LRA (determined by being defined as a 'torque motor' then use LRA). Mine, I have reasoned, is not a torque motor so I will size to 150% of FLA.
Thanks

 

[waross]

I concur With buzzp and add that the internal leads are often rated at a higher temperature than allowed by NEC for building wiring systems. In addition, the leads are larger than the winding conductors which may operate at even higher temperatures than the lead conductors.

 

[UKpete]

Limited-motion rotary actuators are referred to as torque motors, e.g.

http://www.slmti.com/lat/default.asp

though I could understand the type of machine mentioned by waross (for cable take-up reels) might also qualify. This coincides with the IEEE definition posted above.

So I would say your motor ISN'T a torque motor, it's a motor running at low speed.

 

[buzzp]

UKPete, I believe the limited-motion rotary actuator is also called a quarter turn actuator. While this is an option for this installation, we are choosing to with a multi-turn actuator.
Thanks to everyone for your help.