Previous in Forum: HT Poles   Next in Forum: Sensitive Microphone Array
Close
Close
Close
22 comments
Active Contributor

Join Date: May 2012
Posts: 11

Motor Torque Measurement.

05/17/2012 10:13 AM

Is there an easy way to measure the torque being applied by a (small <1hp) motor to start the rotation of a shaft?

The motor is often failing to initiate rotation and this measurement would aid in diagnosis/rectification of the problem.

Thanks

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30332
Good Answers: 818
#1

Re: Motor Torque Measurement.

05/17/2012 10:32 AM

What does the torque/speed characteristic curve of the shaft load it it trying to turn look like?

Where is the intersection point with the motor's curve?

What sort of motor?

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
3
Guru
United States - Member - New Member

Join Date: Jun 2008
Location: Alabama
Posts: 1610
Good Answers: 61
#2

Re: Motor Torque Measurement.

05/17/2012 11:13 AM

You can measure the required torque with a torque wrench. Using the wrench apply a force on the shaft until it begins to turn. This will give you the approximate torque required.

Register to Reply Good Answer (Score 3)
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16499
Good Answers: 662
#3
In reply to #2

Re: Motor Torque Measurement.

05/17/2012 11:35 AM

Or similar to what Wareagle says, (but you don't have a torque wrench) clamp a lever to the shaft and add weights until it starts to turn, length of lever x weight gives the torque.
Similarly to measure motor toque atach alever to the output shaft and attach it to a spring balance. (Apply common sense safety precautions... e.g get the new guy to do it)
Del

__________________
health warning: These posts may contain traces of nut.
Register to Reply
Guru

Join Date: Oct 2008
Posts: 42294
Good Answers: 1662
#4

Re: Motor Torque Measurement.

05/17/2012 11:58 AM

If the motor has a nameplate, it should list the LRA. Locked rotor amps. If the amp draw exceeds this value, and the "rotor won't go", get a bigger motor.

Register to Reply
Power-User

Join Date: Nov 2011
Posts: 131
Good Answers: 5
#9
In reply to #4

Re: Motor Torque Measurement.

05/18/2012 1:18 AM

Are you sure that the amp drawn by the motor will exceed the LRA? I thought the locked rotor current indicated by the manufacturer in the data sheet or name plate is the maximum current which can be drawn by the motor in the stand still condition.

If the motor doesn't rotate, the locked rotor current will continue till the motor trips on overload protection (either by a bimetallic type thermal overload relay or an electronic over current relay)

Register to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30332
Good Answers: 818
#11
In reply to #9

Re: Motor Torque Measurement.

05/18/2012 7:39 AM

...or it burns out.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30332
Good Answers: 818
#5

Re: Motor Torque Measurement.

05/17/2012 12:14 PM

...and if it needs a bigger motor, don't forget to look at the motor overload trip device, the motor power cable, the circuit protective device(s) and the upstream power supply to make sure the whole shebang is up to the job.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Guru

Join Date: Dec 2008
Location: Cd. Juarez, Chihuahua, Mexico.
Posts: 1023
Good Answers: 69
#6

Re: Motor Torque Measurement.

05/17/2012 1:11 PM

Well, the principle of operation is pretty easy, but NOT the construction of the testing station, the load is a stationary fixture connected to the motor under test by a magnetically coupled variable torque collar.

The more duty cycle on the variable torque coil, the more load is applied to the motor, therefore more "twisting" effect over the stationary fixture.

The stationary fixture is actually afixed to the chassis plate by one piezo-electric load cell (without it, it would spin freely), so the more torque applied to the load, the higher amplitude from the cell signal, process it as necesary to attain the units of your preference.

Regards

__________________
No hay conocimiento ni herramienta que sustituya al sentido comun.
Register to Reply
Guru

Join Date: Jun 2010
Posts: 1296
Good Answers: 102
#7

Re: Motor Torque Measurement.

05/17/2012 3:30 PM

How about a torque measuring coupling or this?

Easy, yes. Cheap, no. (You could probably install a hydrostatic drive and use the hydraulic drive motor pressure to measure torque more cheaply. Actually, depending on your load characteristics, a hydrostatic drive might not be a bad final solution.)

Register to Reply Score 1 for Good Answer
Active Contributor

Join Date: May 2012
Posts: 11
#15
In reply to #7

Re: Motor Torque Measurement.

05/18/2012 10:53 AM

I have to appologize to all for not being clear as to what I was asking. Bigg actually gave me what I needed in the way of a couple of links to suppliers of torque measuring devices. My sincerest thanks to all -- especially Bigg.

Register to Reply
Active Contributor

Join Date: Jan 2012
Posts: 22
#8

Re: Motor Torque Measurement.

05/18/2012 12:33 AM

As there is no 'easy' way to measure the torque, start by measuring the terminal voltage on the motor at startup and run. This is easy and may show the problem is not in the motor but in the supply. You may have to increase the size of the supply cable.

Register to Reply
2
Power-User
Belgium - Member - New Member

Join Date: May 2007
Location: Flanders (Belgium)
Posts: 344
Good Answers: 21
#10

Re: Motor Torque Measurement.

05/18/2012 6:08 AM

Method 1

Measuring the motor speed will give a good idea of the required torque.

  • At no load, the motor speed will be approximately the synchronous motor speed. (suppose : P = 0 kW and n = 1500 rpm)
  • At full load (as indicated on the motor name plate) the motor speed will be equal to the speed indicated on the motor name plate. (suppose : P= 0.75 kW and n = 1350 rpm)
  • The motor speed will lower when the applied motor torque increases. Measure the actual motor speed, when the motor is driving the load (suppose : n = 1400 rpm)
  • The relation between output power and motor speed is almost linear.

So, if you know the actual motor speed when driving the motor load, you can calculate the output motor power.

  • P = 0.75 x (1500-1400) / (1500-1350)
  • P = 0.50 kW (= 500 W)

The relation between the output motor power and shaft torque equals

  • output motor power = shaft torque x motor speed x Pi x 2 / 60
  • [kW] = [Nm] x [rounds per minute] / [seconds per minute]
  • 500 W = motor torque x 1400 x 3.1415 x 2 / 60
  • motor torque = 3.41 Nm

Method 2

An other way, which is less accurate, depends on measuring the motor currents.

  • Measure the motor current when the motor is running without any load. (no mechanical connection between motor and load). (P = 0.00 kW and I = 1.1 Ampères)
  • Connect the motor to the mechanical load and start the motor. When the motor is driving the load, measure the motor current (I = 1.5 Ampères)
  • Look at the motor name plate, at full load the motor current is indicated (P = 0.75 kW and I = 1.9 Ampères)

Estimate a linear relation between power and current.

  • 0.00 kW at 1.1 A and
  • 0.75 kW at 1.9 A
  • P = 0.75 x (1.5 - 1.1) / (1.9 - 1.1)
  • P = 0.375
  • this tells me that the power at 1.5 A equals to 0.375 kW.

Estimate the shaft speed when the motor is driving the load to be equal to the average between synchronous shaft speed and full load shaft speed.

  • (1500 + 1350) / 2 = 1425 rpm

To determ the actual shaft torque, use the same way as in Method 1 :

  • output motor power = shaft torque x motor speed x Pi x 2 / 60
  • [kW] = [Nm] x [rounds per minute] / [seconds per minute]
  • 375 W = motor torque x 1425 x 3.1415 x 2 / 60
  • motor torque = 2.51 Nm

Method 3

  • Measure the motor speed with a tachometer.
  • Measure the motor input power with a power meter.
  • Estimate the mechanical output power to be 95% of the electrical input power (mechanical power equal efficiency times electrical power)

To determ the actual shaft torque, use the same way as in Method 1

Register to Reply Good Answer (Score 2)
Active Contributor

Join Date: Jan 2012
Posts: 22
#12
In reply to #10

Re: Motor Torque Measurement.

05/18/2012 8:55 AM

Which method gives the starting torque? That was the original question, or am I mistaken?

Register to Reply
Power-User
Belgium - Member - New Member

Join Date: May 2007
Location: Flanders (Belgium)
Posts: 344
Good Answers: 21
#13
In reply to #12

Re: Motor Torque Measurement.

05/18/2012 9:10 AM

None of them gives the starting torque.

Starting torque has to be seen in relation with acceleration time.

The faster the application is on speed, the higher the starting torque.

Register to Reply
Power-User

Join Date: Oct 2007
Location: S. California, USA
Posts: 279
Good Answers: 12
#17
In reply to #12

Re: Motor Torque Measurement.

05/18/2012 11:55 AM

The starting torque, also called the locked rotor torque, is typically 3x the motor's rated torque. Rated motor torque = 5250 x hp / speed.

Register to Reply
Active Contributor

Join Date: May 2012
Posts: 11
#18
In reply to #10

Re: Motor Torque Measurement.

05/18/2012 2:03 PM

RUDY:Thanks very much for this; I have almost made resolve to simply buy torque measurement devices as were suggested by "bigg", however we will have a closer look at these formulas and how they will appeal. Thank you.

Register to Reply
Power-User
Belgium - Member - New Member

Join Date: May 2007
Location: Flanders (Belgium)
Posts: 344
Good Answers: 21
#19
In reply to #10

Re: Motor Torque Measurement.

05/20/2012 6:00 AM

There is a (small ?) mistake in the formula

  • [kW] = [Nm] x [rounds per minute] / [seconds per minute]

has to be changed in

  • [W] = [Nm] x [rounds per minute] / [seconds per minute]

Additional information:

the calculation is based on a motor with the following data on the motor name plate

  • output power P = 0.75 kW / 1 Hp
  • 4 pole motor @ 50 Hz synchronous speed 1500 rpm
  • full load speed n = 1350 rpm
  • full load current I = 1.9 A

The no load current is not mentioned on the name plate and has to be measured.

Register to Reply
Associate

Join Date: Mar 2012
Posts: 31
Good Answers: 1
#14

Re: Motor Torque Measurement.

05/18/2012 9:52 AM

If this motor has functioned properly in the past, and previously accomplished it's designed task. Check to see if something is impeding the mechanism; something as simple as proper lubrication could be the cause (a problem I have experienced with my Garage Door Opener). If this is a single phase motor with capacitors, (run and/or start) check for a faulty one. A larger motor with greater Hp. may be required when all else has failed.

Register to Reply Score 1 for Good Answer
Active Contributor

Join Date: May 2012
Posts: 11
#16

Re: Motor Torque Measurement.

05/18/2012 10:57 AM

I have to appologize to all for not being clear as to what I was asking. Bigg actually gave me what I needed in the way of a couple of links to suppliers of torque measuring devices. My sincerest thanks to all -- especially Bigg.

Register to Reply Score 1 for Good Answer
Active Contributor

Join Date: Jan 2012
Posts: 22
#20

Re: Motor Torque Measurement.

05/20/2012 7:20 AM

Check the voltage and current on the motor when it does start. You can then work out the starting VA and hence the torque. Your question is answered. Cost nil. Then check what has altered when it does start. Please let me know what you find. Thank you.

Register to Reply
Power-User
Belgium - Member - New Member

Join Date: May 2007
Location: Flanders (Belgium)
Posts: 344
Good Answers: 21
#21
In reply to #20

Re: Motor Torque Measurement.

05/20/2012 1:54 PM

@ Rainmain, Please explain the relation between the starting VA and the torque.

Register to Reply
Active Contributor

Join Date: Jan 2012
Posts: 22
#22
In reply to #21

Re: Motor Torque Measurement.

05/20/2012 2:28 PM

Please see the comment by Snave

Register to Reply
Register to Reply 22 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

bigg (1); Darcy Eleniak (3); debata07 (1); Del the cat (1); lyn (1); PWSlack (3); Rainmain (4); rudy.leurs (4); Snave (1); Thomas J. Becker (1); wareagle (1); Yahlasit (1)

Previous in Forum: HT Poles   Next in Forum: Sensitive Microphone Array

Advertisement