Frequent Speed Changes 1

[joe818]

I have a one horsepower, three phase, 480 volt motor on AB 160 drive. It runs a conveyor that moves cases of jars. This machine runs 24 hours a day, five days a week, 52 weeks a year. I would like to change the speed of the motor from 40 to 60 and back to 40 Hz every two seconds. I would like a half second accel and a one and a half second deccel. Will this overstress the motor or drive?

 

[jbartos]

Suggestion: It depends. If the motor experiences abrupt changes in its operation, there will be current inrushes that tend to elevate motor temperature excessively. To avoid this obstacle, try to avoid abrupt motor changes with associated inrush currents. The motor design and size will also affect the motor temperature development in such applications. A heavier duty motor that withstands plugging may be the better solution.

 

[hacksaw]

some motors, not necessarily ac, may be able to accomplish that, but a half second is only 16 cycles and that does not really give you much chance to control acceleration. Make sure your conveyor can take the acceleration too.

 

[jOmega]

JB,

One of the features of VFDs or drives, is their inherent ability to limit or control the current and/or rorque.

The inrush current you talk about is more appropriate for DOL operation of a motor.

Consider that on a drive, if you set the current limit to 100%... you can accelerate, run, decelerate, run... all day, all year, without overstressing the motor. This of course is predicated upon the motor being appropriate for dissipating the heat properly within the specified operating speed range.

Generally speaking, operation from 40 to 60 Hz should not be a problem for most motors...with the current limited to 100% or just above that. A 56 frame motor would have less thermal capability than would a 143 frame unit. 1 HP motors could be in either frame size.... depending upon manufacturer and model.

Now, all of this is predicated upon the motor being able to achieve the duty cycle times required with the current limited to a nominal 100%..

An RMS duty cycle calculation would have to be performed, as well as a determiniation of the motor's capability to develop the required torque for the accel/decel portions of the required duty cycle with current limited to 100%.

Another approach would be to perform the RMS Duty Cycle calculation and then see if the motor has sufficient torque within it's 100% rating ... to achieve the duty cycle times. .. If not.... then a larger motor would be required for the application.

A plugging duty motor would not be required as you are not going to be subjecting the motor to the same rigors as DOL plugging (for which such motors are designed).

Remember, this is a drive application, and as such, you have much control over the sresses that the motor is subjected to.

Joe818 provides us with no information about the load characteristics.... maximum continuous load torque, inertia, gear-in information, etc... etc.... so it is impossible to determine if he can implement his required duty cycle stress free.

This type of application is not unusual for a drive/motor combination, properly sized....

On a global basis, JB, there are thousands of such applications working successfully. The key is analyzing the speed-torque-duty cycle requirements, and selecting components (motor, drive, etc.) that can do the application within their design limits.

 

[jbartos]

Suggestion to Omega (Electrical) Sep 28, 2003 marked ///\\JB,

One of the features of VFDs or drives, is their inherent ability to limit or control the current and/or rorque.
///Yes, trivial.\\The inrush current you talk about is more appropriate for DOL operation of a motor.
///Essentially, yes. However, the original posting refers to very short time, i.e. 2 seconds. To achieve the required operation within such short time, the squirrel-cage induction motor will experience inrushes or high motor starting currents on the average.\\Consider that on a drive, if you set the current limit to 100%... you can accelerate, run, decelerate, run... all day, all year, without overstressing the motor.
///This sounds like commercial rhetorics since the short time of 2 seconds posted in the original posting poses a significant constraint; especially for motors that normally have large locked-rotor currents. Obviously, had there been a plenty of time for a smooth motor acceleration and deceleration, there would not be any problem. Then, your statements are o.k.\\ This of course is predicated upon the motor being appropriate for dissipating the heat properly within the specified operating speed range.
///Now, this sounds a little better in view of the posted 2 seconds in the original posting.\\Generally speaking, operation from 40 to 60 Hz should not be a problem for most motors...with the current limited to 100% or just above that. A 56 frame motor would have less thermal capability than would a 143 frame unit. 1 HP motors could be in either frame size.... depending upon manufacturer and model.
///Again, this sounds as an extract from a manufacturer commercial overlooking that tight 2 second time frame for the motor abrupt changes in speed.\\Now, all of this is predicated upon the motor being able to achieve the duty cycle times required with the current limited to a nominal 100%..
///Agree. A very good general statement with 2 seconds posted in the original posting being overlooked.\\An RMS duty cycle calculation would have to be performed, as well as a determiniation of the motor's capability to develop the required torque for the accel/decel portions of the required duty cycle with current limited to 100%.
///Certainly.\\Another approach would be to perform the RMS Duty Cycle calculation and then see if the motor has sufficient torque within it's 100% rating ... to achieve the duty cycle times. .. If not.... then a larger motor would be required for the application.
///Agree.\\A plugging duty motor would not be required as you are not going to be subjecting the motor to the same rigors as DOL plugging (for which such motors are designed).
///Certainly, since there is no change in the motor direction required in the original posting.\\Remember, this is a drive application, and as such, you have much control over the sresses that the motor is subjected to.
///Of course, I noticed that too.\\Joe818 provides us with no information about the load characteristics.... maximum continuous load torque, inertia, gear-in information, etc... etc.... so it is impossible to determine if he can implement his required duty cycle stress free.
///This is a very good point that lends itself to mention the motor inrush current potential problems.\\This type of application is not unusual for a drive/motor combination, properly sized....
///Certainly.\\On a global basis, JB, there are thousands of such applications working successfully.
///Agreed, may be millions.\\ The key is analyzing the speed-torque-duty cycle requirements, and selecting components (motor, drive, etc.) that can do the application within their design limits.
///Yes certainly. It is good to be reminded that.\\

 

[rodar]

The primary consideration is the weight of item on
the belt. Or total system inertia.
I would set the accec and decel rates into the drive
and monitor motor current during the interval with true RMS
type of current monitor. The worst case will be on the
accel. If the accel motor current more than 200% of rated
I would see if the gearbox can take that.
If it can try running at this rate and watch the motor
carefully.
It all depends on inertia of the load.

 

[jOmega]

JBartos:

With regard to the "inrush current" .. you said,

"........the original posting refers to very short time, i.e. 2 seconds. To achieve the required operation within such short time, the squirrel-cage induction motor will experience inrushes or high motor starting currents on the average."

JB... The original posting also said that the acceleration was to be from 40 to 60 Hz.. in 0.5 seconds

Do you think that a 1-HP motor (as stated in original post)... could accelerate 20 Hz in less than 0.5 seconds at 100% current ????

Let's see... 1 HP motor.... 56 Frame .... 0.1 lb-ft^2 inertia ... 3 ft-lbs rated torque .. 40 Hz = 1166.66 & 60 Hz = 1750 rpm
Change in speed = 583.33 rpm ..

Now, assuming no load... the motor would accelerate from from 40 to 60 Hz @100% current(torque).. in 0.0631 seconds.

Now then, JB, if the motor was 50% loaded so that it only had half its rated torque available to accelerate the motor.. (1.5 ft-lbs).... it would do so in 0.126 seconds.

And if the load consumed 75% of the motor rated torque so that the motor only had 1/4 of its rated torque available to accelerate with... (0.75 ft-lbs)...it would do so in 0.253 seconds.

And if the load consumed 87.4% of the motor rated torque so that the motor only had 12.6% of its rated torque available with which to accelerate ...(0.379 ft-lbs) ...it would do so in 0.5 seconds ....

So you see, JB.... the VFD set at 100% torque limit or 100% current limit.... could very well accelerate the load from 40 to 60 Hz in the alloted 0.5 seconds... no strain....no sweat.! and it could do it 24/7..

If the motor is a 143 frame... with a nominal inertia of 0.11 lb-ft^2 .... it would have some impact upon the numbers stated above.... you can do the math to verify.. that for a 0.5 second accel from 40 to 60 Hz, a load torque not exceeding 86.1% rated could be accelerated with a torque of 13.9% rated

Of course, this is all conjecture but shows it to be very possible.... the actual mechanical parameters, which are missing in Joe818's original post.... would need to be known for surety...

 

[jbartos]

Suggestion to the previous posting: The posting appears to be missing a thermal accumulation in the motor due to frequent starts - stops. I have addressed this issue in my first posting.
The original posting states:
""I would like to change the speed of the motor from 40 to 60 and back to 40 Hz every two seconds. I would like a half second accel and a one and a half second deccel. Will this overstress the motor or drive?""

It did not state:

"The original posting also said that the acceleration was to be from 40 to 60 Hz.. in 0.5 seconds"

Now, the statement from the previous posting:
"""Now, assuming no load... the motor would accelerate from from 40 to 60 Hz @100% current(torque).. in 0.0631 seconds."""
The 6.31 msec for the motor acceleration is somewhat questionable since it is just about quarter of the 60Hz cycle.

 

[jbartos]

Correction/clarification (I beg your pardon):
"The 6.31 msec for the motor acceleration is somewhat questionable since it is just about quarter of the 60Hz cycle."
should read:
The 63.1 msec for the motor acceleration is somewhat questionable since it is just about 3.8 cycle of the 60Hz."
The motor acceleration curve should be obtained from the manufacturer or constructed from the motor torque-speed curve.

 

[GGOSS]

Hello all,

One of the 'facts' that is often over-looked is that when a motor is started/accelerated via a variable frequency drive, the motor is in-fact in 'run mode', not 'start mode'. In the main, the over-current conditions normally experienced with traditional full voltage and reduced voltage starting techniques do not in VSD (VFD) applications.

Regards,
GGOSS

 

[jOmega]

JB:

According to Joe818's original post, the motor is NOT subjected to START & STOPS.... rather, it is going from 40 Hz to 60 Hz and back to 40 Hz.

Here are Joe's exact words:

"I would like to change the speed of the motor from 40 to 60 and back to 40 Hz every two seconds. I would like a half second accel and a one and a half second deccel."

Nothing there about starting and stopping, JB.

What Joe is stating, is that the conveyor is running at 40 Hz... and he wants to accelerate to 60 Hz in 1/2 second and decelerate back to 40 Hz in 1.5 seconds... and to do this every 2 seconds.. Notice, JB, that Joe doesn't state there will be any dwell time[/bb] at 60 Hz or at 40 Hz.... His stated intent is to operate continuously between 40 & 60 Hz .. in a constant state of change as stated.....The duty cycle profile would look like a sawtooth waveform.

Are we agreed on this, JB ?
(If I've misinterpreted Joe818's statements, I'm sure he'll correct accordingly).

Next let's address your expressed doubt of being able to accelerate he motor from 40 to 60 Hz in 63.1 milliseconds.

The equation for average time to accelerate/decelerate is as follows:

time(sec)= (WK² x N) / (308 x T)

where: WK² = Inertia in Lb-ft²
N = change in RPM
T = Torque in ft-lbs

You will recall that I stated the inertia for a 56 frame 1-hp 4-pole motor was a nominal 0.1 Lb-Ft² and that a 1-HP 1750 RPM motor has a rated torque of 3-ft.lbs.
The change in RPM when going from 40 to 60 Hz for a 1750 RPM base speed motor would be ... 583.33 rpm

So you say that the motor, under no load conditions...i.e., no mechanical load....CANNOT accelerate from 1166.66 RPM (40 Hz) to 1750 RPM (60 Hz) ... a change of 583.33 RPM ... in 0.0631 seconds when a constant force of 3 ft.lbs is applied to the rotor of the motor. {neglecting friction & windage of course}.

Suggestion: plug the numbers into the equation ... and see for yourself, JB.

Unless my calculator is broken, I get a value of 0.063130952380952380952380952380952 seconds which, rounds off to 0.0631 seconds

And neglecting friction & windage, I truly believe that the motor can make a 583.33 RPM speed change in that time when subjected to a constant torque force of 3 ft.lbs and no mechanical load torque applied to the motor shaft.

Ref:

http://www.sdp-si.com/D220/HTML/D220T015.htm

Finaly, JB... I leave it to you to do the RMS load calculation for the duty cycle that Joe818 has stated.
Joe818 hasn't supplied sufficient data to be able to just plug numbers in... but you can set up an example... and pick the load torque that would allow sufficient accelerating & decelerating torque to achieve the accel & decel times that Joe818 specified.

I think you will find that if the motor torque (current) is limited to 100% that the motor can perform the duty cycle on a continuous basis, 24/7/52 without any thermal buildup exceeding motor design capability.

 

[GGOSS]

Hello Jbartos & JOmega,

JB; I'm not sure if you bring out the best or worst in jOmega, but as this thread is proving more informative than many others in recent times, I would say......to both of you.....keep up the good work

Regards,
GGOSS

 

[jOmega]

GGOSS

A conundrum perhaps ?

jO