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.\\