Motor load inertia 3

[rockman7892]

I have been searching but have been unsucessful in finding out how a load inerita relates to a certain motor hp.

For instance if I have a 300hp 4160V motor is there a table of formula that relates this hp to the maxiumum load inertia that this motor can handle?

I have a motor that I am RVAT starting and am seeing high starting current which are actually much greater than the LRC at 80%V. I was wondering if a higher load inertia could drive these current high enough that the motor would still start but yeild a higher LRC at less voltage.

The motor is a 300hp motor and the load ineria from the load manufacturer is 27090 lb/ft^2

 

[jraef]

Load inertia cannot make the motor take higher than LRC, because LRC assumes the motor shaft is not turning AT ALL, which you could say is 100% inertia.

Load inertia affects the length of time it will take to accelerate, assuming it will accelerate with the torque produced by the motor. So if your crusher is not accelerating, the motor will stay in LRC until the overload protection trips it out, or hopefully before that if you have Stall Protection on it.

Generalized Motor Acceleration Formula for High Inertia Loads
t = WK2 x rpm
308 x T av.

WK² = inertia in lb.ft.²
t = accelerating time in sec.
T = Av. accelerating torque lb.ft..

T = WK² x rpm
308 x t



When you say the current is higher than LRC, where are you measuring? Ahead or behind the autotransformer?




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[rockman7892]

Jraef

The motor is indeed starting but like I said we are seeing a current of 320A on a motor with a LRC=250 using an 80% tap.

The current is being measured after the transformer

 

[electricpete]

Does not seem to compute.

Is it possible you are applying a higher voltage to the motor than you think you are?

Or somehow an error measuring current?

Are the currents balanced?

By the way, how long to start the motor? 3 minutes?

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[rockman7892]

Jraef

So what is it that determines weather a motor will turn a gien load. I know that I can compare the speed torque curves of the motor and loads to make sure the motor curve is above the load curve but from just looking at numbers what is looked at to determine this.

For instance does the locked rotor torque of a motor have to be greater than the locked rotor torque of the load? In other words if all you have is the hp and LRT of the motor and the load WK^2 can you make a judgement of weather a motor will start a load or do you need the load speed vs torque curve?

For instance obviously a 5hp motor cannot turn a mill because the motor will not be able to have enough LRT to be greater than the torque required to turn the load (mill)

 

[BobM3]

A motor will turn a load if the motor torque is greater than the torque produced by the load. If the motor torque is greater then the load will accelerate. If the motor torque is equal to the load torque then the motor will quit accelerating and run at constant speed.

Are there any other loads on the motor besides inertia? Like friction?

 

[electricpete]

For instance does the locked rotor torque of a motor have to be greater than the locked rotor torque of the load?

Motor torque speed curve exceeding load torque speed curve at all speeds is a necesssary but not sufficient condition for successful starting.

In other words if all you have is the hp and LRT of the motor and the load WK^2 can you make a judgement of weather a motor will start a load

Absolutely not. You need a lot more.

or do you need the load speed vs torque curve?

Yes, you need that and more. Particularly since you have a gynormous inertia.

Someone should have done a study of motor starting. With both motor and load inertia and torque speed curves (voltage curve adjusted for voltage), we can determine speed profile vs time. With current vs speed curve we, can convert that (speed vs time curve) into a current vs time curve. Current vs time curve should be compared to motor thermal limit curve. And the protection curve should be squeezed in between these two curves (between the actual current vs time and thermal limit current vs time).

If in doubt, consult the motor manufacturer.

Summary of needed info as input to analysis:
Motor torque speed curve
Motor voltage during start
Load torque speed curve
Motor and Load inertia
Motor current vs speed curve
Motor Thermal limit curve

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[electricpete]

Correction:
"(voltage curve adjusted for voltage)"
should have been
"(motor torque-speed curve adjusted for voltage)"

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[roydm]

Is this a new problem? A crusher that is not crushing should turn quite freely. Check things like oil level (it could be over full). Oil viscocity (is it too cold).
In my experience they will point to an electrical problem first until you can prove it's mechanical.
Regards
Roy

 

[rockman7892]

I am trying to run a transient motor starting analysis(TMS) in SKM to model this particular motor during starting.

Using this TMS I am creating a custom graphical motor representation and custom load representation based off the information I have recieved from the manufacturers on both.

I understand everything that I have for the motor in regards to speed vs torque and how to input them in the SKM model.

For the load however I am a little confused on how to input some of the parameters. Attached is the speed vs torque curve for the load (crusher) it is in european units so I will need to convert them.

The first input parameter in SKM asks for the load synchronous RPM. I am assuing that this value is the synchronous RMP of the motor reflected through the reducer. In this case we are using a 4.24:1 belt reduction so if the motor has a synchronous rpm of 1800rpm then I'm guessing that the load has a synchronous rpm of 424.5rpm. So my question for this input is weather I use the actual rpm of the motor or use the reduced rpm of the load?

The second parameter asks for an input for the rated torque of the load. This is the input that I am not sure what to put. The load curve does not specifically state a rated torque. To input this rated torque can you look at the torque curve and determine what the torque would be at the loads synchronous speed and use this value as the rated torque? In this particular case could I use the toruqe of 500NM at 424rpm as the rated torque for the load?

The last input asks for the load inertia. This value appears to be gien in kgm^2 at the top of the sheet. I will need to convert this unit to lbsft^2 to enter it into my model. When inputing this value however would I need to use the actual inertial seen at the motor which would be 4.24 times less than the listed inertia or do I just enter the full inertia and let the entered synchronous rpms of the motor and load take care of the reduction as seen at the motor?

I am new to this TMS and am tryin to fight through it as best I can and appreciate the help.

 

[electricpete]

I am trying to run a transient motor starting analysis(TMS) in SKM to model this particular motor during starting.

Now I'm not sure how to piece this together with your previous comments.

You measured (using a clamp-on or something similar) actual current above LRC and now you are running a simulation during troubleshooting of that real-world problem?

OR

The motor hasn't been run yet and you just had some calculated amps above locked rotor amps? (LOL - in that case you should have mentioned it).


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[rockman7892]

Yes we actually measured current above LRC. However today I have just recieved word back from the motor manufacturer and they have admitted that they provided the wrong LRC for this motor and corrected it. This new value is much higher and makes sense with what we are seeing in the field. The new LRC is 351A and we are seeing 320A at 80% tap which is still a little high, but makes a little more sense as to what we are seeing.

Now that I have all of this information corrected I am performing a TMS in order to try to determine what the best time is for switching the Autotransformer to across the line, and to use this particular problem as a learning exercise to learn how to perform this study in SKM.

 

[LionelHutz]

Do you enter a load torque vs speed curve into SKM? If so, what are the units of this curve? If you enter a load curve as a percentage then the program needs to know what 100% equals. To make life easy, you could enter 1000N-m = 100% which makes it easy to pick off the other curve values.

If you enter the load rpm and the load inertia I would believe SKM will reflect the inertia to the motor shaft.

Post up your results if you want.

 

[EddyWirbelstrom]

Mr Rockman7892. If you send :
- Motor Torque ( Nm ), Motor Effic, Motor pf, Motor Current versus rpm at rated voltage. ( in graph plots )
- Motor rotor inertia ( kgm² )
- Load torque vs speed curve ( Nm / rpm )
- Load inertia ( Nm / rmp also motor / driven load speed ratio)
- Fault level ( without motor contribution at the board the motor is supplied from )
- Size and length of motor cable
- Motor nameplate data

I will run a TMS and send you the results and PTW project files.
Murray Newman

 

[rockman7892]

mnewman

I have tried to run this TMS but as I mentioned earlier I am getting hung up on some of the motor and load inputs and coming up with results that are clearly wrong. My results show the motor accelerating in 2s and the motor and load torque curves look very weird. I am going to keep plugging away at this but would be curious to compare what I have vs your results.

Attached are the motor datasheet and load profile giving you the follwing information:


- Motor Torque ( Nm ), Motor Effic, Motor pf, Motor Current versus rpm at rated voltage. ( in graph plots )

Information on attached motor datahseet

- Motor rotor inertia ( kgm² )

149.3 lb-ft^2

- Load torque vs speed curve ( Nm / rpm )

Information on attached load speed vs torque curve

- Load inertia ( Nm / rmp also motor / driven load speed ratio)

Load inertia= 1200kgm^2
motor / driven load speed ratio = 4.24 : 1

- Fault level ( without motor contribution at the board the motor is supplied from )

Aproximately 8590A ( I have to look into this further)

- Size and length of motor cable

3/C #2 @ 80ft

- Motor nameplate data

Information on attached motor datasheet.

Thanks for the help.

 

[rockman7892]

The motor datasheet file did not attach.

 

[EddyWirbelstrom]

Rockman7892
The data you sent is sufficient, however can you please inform the impedance of 80ft of 3/C #2 cable. ( In Australia we use metric ).
I'm using PTW Ver6506b1, however I also have the earlier version PTW Ver6035b7. Please inform which version you are using because the versions are not backward compatable.
It may be easier to zip your PTW project file and send to me. ( note: to make the size of your PTW project file smaller so it can be sent as a 'stand-alone' project, go >Project > Backup and rename the backed up project )

 

[EddyWirbelstrom]

Rockman7892
No need for you to send a PTW project file, I've found the U.S. cables in the PTW default ( U.S.) library. Motor cable 3c 2 AWG impedance is Z = 0.202 + j 0.039 Ohms / 1000 Feet.
Question:
The motor / driven load rpm ratio is 4.24 / 1.
Therefore the driven load rpm at motor rated output would be 1785 / 4.24 = 421 rpm.
From the load torque / speed curves at a driven load speed of 421 rpm the load torque is 494 Nm and input power is 2*pi*(421/60)*494 = 21.8 kW ( 29 hp ).
The motor rated output power is 223.8 kW ( 300 hp ).
Can you please confirm the driven load torque / speed curve.

 

[EddyWirbelstrom]

Rockman7892
Attached is the PTW Ver6506b1 project folder and also the motor data spreadsheet which I have used to build graphical motor and load for the PTW TMS library.
Conversion factors I used were :
Inertia of 1kg.m² = 23.73 lb.ft²
1 Nm = 0.737 lb ft
Inertia seen by motor is load inertia x 1 / 4.24²
Torque seen by motor is load torque / 4.24

Load torque and inertia was referred to the motor to make the PTW TMS graphical load model.

Assuming a stiff supply the starting time was 9.4 sec.

 

[LionelHutz]

OK, I did a quick simulation and I came up with ~9 seconds starting full-voltage - this assumes no voltage drop at all. An 80% tap autotransformer start should increase this to a little over 15 seconds.

I didn't enter a lot of data points to create the curves so they are a little rough but the still follow the same basic shapes you provided. This will affect the results a little. I get 1585 lb-ft^2 reflected on the motor shaft.