inverter normal duty e heavy duty

[meccanicamg]

Good evening, everyone. I think it is a great question that goes on, in my opinion a lot of people or in the yard then things do not go as planned.
I would like to have clarifications regarding the use of inverters for industrial application on alternating current engines 400v standard iec series (to mean engines 5.5/7.5/11 kw to cage of classical squirrels).
Generalthere are different types of applications and each has its own problem. in substance two cases may occur:
1) Engine sized to work in class s1 with speed adjustment in v/hz from 10 to 120hz
2) Engine sized to work in class sx with need to use in addition to v/hz regulation also torque control management and perhaps use of braking resistance (also in open ring)
problem(a) using a inverter normal duty in case (1) you do not have enough to tick to the ignition under load.
b) using a normal duty inverter in case (2) you can not manage the load, especially in braking and inertia wins the drive pair (in positive or braking due to the too high current not supported by the fetecchia inverter)
note(i) it is appropriate in case (1) mount double or oversized motor type 1.5 times or inverter heavy duty?ii) agrees in case (2) mount double or oversized motor type 1.5 times so that the brake current is lower than the rated engine current or use a inverter heavy duty with correctly calculated motor (not oversized)?
requests- how to correctly choose a motor + inverter knowing the characteristics of the load and therefore of the kinematics/dynamics of the mechanics?
- as never with servoventilated motors you can not express all the couple if adjusted in v/hz (perhaps because they : normal duty Are they very squeezed if purchased exactly for engine size? )
- with a normal or heavy duty inverter, as long as enabled in the internal logic to manage open ring, you can do torque control in both cases and then have c = cnom * x without use of encoder or do I have to by force to make closed ring logic with inverter?
- often it is said that the engines connected directly to the network express more torque, while under inverters they sit fearfully. is it true or is it legend? What is the mistake you make commonly? that I know the engines can absorb up to 6 times the nominal current in direct ignition, which is not possible to do with the inverter purchased the for the right motor size. if motor sucks nominal 13a and take a normal duty inverter from 14th I would say that there is nothing to do... will be very limited (both in c/cn and in time) in expressing torque over the nominal, right?

this dark way for the mechanics must be opened by the electrics because really there are too many theories for air and too many times the applications make wag not for mechanical problems but for electrical problems of inverter not correct for the application.

is there someone who wants to synthesize and give key points on the use and choice inverter in relation to the application answering the above questions?

I thank in advance those who want to help shed light on the subject.

 

[Er Presidente]

the motor in case of heavy work, if you do not exceed the limits of plate, suffers by temperature, just put it in correct thermal conditions and also the motor more to the limit survives. different the problem of the inverter that instead must be sized also for the load factors

 

[meccanicamg]

the motor in case of heavy work, if you do not exceed the limits of plate, suffers by temperature, just put it in correct thermal conditions and also the motor more to the limit survives.

and this is perfect and respected, both for the use of servoventilated engines, and for the respect of current/time limits. never success of cremating an engine for overload fortunately

 

[Er Presidente]

inverters are more rognosis for two problems. the first is that manufacturers tend to overestimate presumptions for commercial, weight and size etc. In addition to varying frequencies the performance varies and a lot with the result that the thermal cycle is stressful and technological evolution has led to reduce its size and the ability to dissipate heat. during heavy cycles of localized heat bubbles lead to aging by cycle and also because the temperature in concentrated areas can exceed the thresholds. Moral inverters should be oversized to support the worst hypothesized cycle. I noticed that the inverters with liquid cooling are much more stable and compact. cool with air leads to such a number of variables that often accounts do not return.

 

[paulpaul]

hello mechanics

I have clashed myself several times with the argument, without ever getting a unique theory: I press to be a pure mechanic who no longer wants to “open the box” of the vfd for several reasons, and then I do work a lot of suppliers, giving him the information they strictly need to mate vfd/motor correctly. I carry cmq my experience, relative to the drive of alternative compressors up to a few hundred kw (maybe you refer to inverter for automation, field that I do not know at all).

I always start from the machine conducted, distinguishing the two conditions: start and adjustment.

start-up
with a xls sheet, I quickly calculate the resistant torque (without inertia) starting according to the speed, once set the various parameters from which it depends (races, ales, number of stages and cylinders, molecular weight gas, pressure of stop, etc.).

adjustment
my calculation (with thermodynamic simulation program) resistant pairs for all compressor work points, depending on pressure and suction temperatures, pressure pressure, speed required to obtain the required flow rates from the customer.

engine preselection
I carry the points above on a chart xls torque vs. speed, which overlaps with the torque curve of the motor under inverter reported on the catalogs (previously digitized on xls) to vary the power frequency. preselect with drop-down menu an engine (power, poles) present drive couple above all points considered, depending on the margin that I mortgage (or that is given to me by the customer)

then communicate to the engine and vfd supplier (often coincides, e.g. abb, weg):

- torque curve to start
- map of the operating points
- inertias for calculation start time (in the case of start-up star/triangle I calculate independently, in the case of vfd no: I know that the delivery of couple depends on the programming of the inverter, but I don’t master the details and I can’t reconstruct the drive couple)
- type of engine selected
- local network frequency and voltage
- Atex zone of installation if applicable and max surface temperature
- installation fee
- number of startups/hours (not more than 10 in my case)

and I have to verify in writing the suitability of the coupling, with the return of the d/s motor and the operating curves with superimposed the operating points of the compressor (also because very often the customer asks me during the project phase). if the engine and inverter supplier does not coincide, once the engine is pre-selected I send all the features of this to the vfd manufacturer by analog check.
practically, at the same power and for a typical 500-1300 rpm adjustment range (for a 6-pole engine) I saw that engines (and vfd) of a size higher than the suitable engine for star/triangle start (e.g. 250 kw vs. what would have been a 200 kw)
with regard to the torque, on some catalogs (e.g. abb) to 50 hz the motor under inverter provides about 90-95% of the torque that would if powered by network. I have never heard the fact that the “motor under inverter sits”, unless the voltage supplied by the vfd does not fall to protect the motor (e.g. high loads at low speeds for motor self-ventilated), resulting in a decrease in speed.
I have had bad experiences starting with the soft starters, which instead customers prefer compared to the triangle star (no apparent advantages in my case).

frankly I do not have time to study deeply electrotechnical (and above all these topics are often dealt with from the point of view of the inverter designer more than the one who has to select it): doing as I said above I never had big problems. Sure is that if some electrical deepened existing relationships between torque, current, frequency and power voltage would not be bad!
Thank you!:wink:

 

[meccanicamg]

thanks to you for the surgery and your experience that you brought us.

your mode is already the one used by me for engines from 60 to 900 kw which are the main ones of power and so far there have never been any problems except for some application a little so particular in the past. In fact with excel sheet, curves, motor, supplier etc. I can brush motor and inverter as well as turn reducer.

the problem arises with the motors of small power, say up to 15kw where we have an electric company that unfortunately has no experience to be able to support us and often buy them the inverters without questioning us mechanics and therefore without knowing if it is necessary more or less current of the nominal. This is the hippo.

it could be good thing to convey to them the curves also of small applications and everything indicated by us mechanics, but we need more specific training to correctly determine everything.

 

[Er Presidente]

often for particular applications for motors not too big, it is advisable to send a motor to the supplier of the inverter that at the counter can make the checks and calibrations of the soft-start

 

[meccanicamg]

our technician told me that for the next applications I have to take a double power engine, so in service s1 I manage both the normal and the extra that would be the overload. so you use a normal duty inverter. boh does not seem to me a sensible thing especially because also the epicycloidal gearbox with ratio 1300 if I put 15 or 30kw is not exactly the same as engine. then energyly nothing happens even if the overload of the reducer is 200% because it is used 30 seconds every 2 months, so it is ok.



but it is so difficult that if I have a 15kw to use in overload and braking resistance with a heavy duty does not make sense to take a 30kw of motor and normal duty inverter? It is true that as amper we are but makes no sense.