hi,
we want to use an ac motor in wide speed range
20-1450 rpm. the motor is 380v ac 15 Kw.

we will decide to use siemens micromaster vector 440 driver.(is it right choose? )

is there any problem in low speeds (it will work in 20 rpm continiously 2-3 hours).
our load is water pump for labrotory use.
we dont want to use reductor.
how torque changes?
and is motor damaged.
Can anyone please clarify it for me.

 

This type of drive application is done all the time across sundry industries. Your drive will provide a constant torque output so there is no problem there. I see two potential problems,
1) heat: depending on load, your motor internal fan may not be able to deliver enough air flow at low speeds to cool the motor. If this is a problem consider using a motor with a secondary ventilation fan motor.
2) pump performance: If your using a centrifugal pump, the pump performance falls off dramatically and non-linearly with decreasing speed, making loop tuning difficult.
What are you tring to accomplish? Your motor is small so the pump must not be large. If your trying to save energy the savings may not recoup drive costs. If your trying to derive a specific pressure at variable flow rates, you can trim your pump impeller to provide a specific pressure over a broad flow range. Work with your pump vendor, I've seen lots of drives applied to small pumps where a bit of engineering on the pump would have saved a bunch on drive and setup costs.

 

It's OK, but with separate driven motor cooling fan.

 

At very low speeds probably you will have problems.
The micromaster vector is rigth choose. Use of vector control (you can choose it in one of the parameters) is very dificult to adjust but you can control exactly the torque.

At low speeds it is necesary to have seperated ventilation for the motor because the fan is doing nothing.

I think you can contact your local Siemens dealer for more accurate information.

 

The Micromaster Vector has parameters which allow you to give extra torque at very low speeds. The only problem I see will be the heat disipated in the motor, because the internal fan also turns low speed. You may have to attach an external cooling fan to the motor to provide extra cooling.

 

You will need a separate cooling fan for those low RPM. Even for a 1200 RPM motor 20 RPM is really slow.
Motors and drives applications can be quite complicated. Maybe your Seimens drive vendor
can help you with the torque curve and other
quetions.
Have a look at this thread you may find it helpfull.
"http://www.plctalk.net/qanda/search...hid=8599&sortby=lastpost&sortorder=descending":http://www.plctalk.net/qanda/search...hid=8599&sortby=lastpost&sortorder=descending

Good Luck

Jcblanch

 

You should have no trouble doing this. Here is some information and suggestions:

Check the specs on the motor or contact the manufacturer to be sure that the motor won't overheat at low speeds. Fan-cooled motors don't get enough ventilation at low speeds and sometimes require additional cooling. VFDs generally have derating provisions to limit current when operating below a certain frequency, to protect the motor from low-speed overheating. You might consider an "inverter-duty" motor which is designed to better tolerate these conditions.

Learn about the torque charateristics of the pump. Fans and centrifugal pumps are generally variable-torque (VT)loads that require less torque at low speeds. This would work to your benefit, requiring less work (and less heat buildup) from the motor at low speeds.

If you can verify that the pump does represent a VT load, give this information to your rep and size the drive accordingly. A given VFD can be rated for a higher HP in a VT application than for a constant-torque(CT)application. Most manufacturers rate VFD horsepower according to VT requirements. In fact, its usually best to ignore the HP rating on the drive and base your VFD size selection on it's current capacity. Pay particular attention to current maximums for starting, since VFDs usually provide much less starting current (150%-300% FLA)than standard across-the-line starters (6x FLA). Since VT loads are easier to start, this shouldn't be a problem for you.

Knowing the torque characteristics of the pump will also permit you and your rep to select the optimum speed/torque control method, either flux-vector (sensorless vector) or V/f. The first is easy to set up and works well in most applications, the latter allows customization of the V/f curve when needed. If you can't get sufficient information prior to purchase,make sure that the drive has provisions for selecting the type of control applied.

Regards,
DG

 

If the motor is tefc, then the fan for cooling the motor is driven by the motor shaft. This means that at speeds below 50% of the rated speed, the motor will have effectively no cooling. This will pose a serious problem for your thermal rise, if the thermal protection schem used assumes the motor is fan-cooled. You would be ok if you provide externally propelled cooling, and derate the thermal overload protection proportionally.

Hope this helps.
- s5_guy

 

The Only problem I see is Cooling of the AC motor, as these motors are designed to cool itself perfectly when Run at full speed, ( 1450RPM ) by their own fan mounted on shaft, as you are gona run it at 20 or so rpm the cooling will be practically absent !!.. So just add a Small Fan mounted piggy back on the non driving end of motor. (which can be any single phase axial fan ) You might have to modify the existing Fan gaurd to accomodate this additional cooling Fan.

All the Best

 

Hi

In low speed, overheating of motor can be a problem... But extra cooler could help in this problem. Torgue is not very high in low speed, because in low frequency, voltage of motor is low - so torque is too low...

Hope this helps

Yours, Tommi

 

I have not used the drive in question, but one thing to watch out for in very low speed continuous motion is granularity of the position feedback. If you have problems with unstable or jittery velocity control you may need to increase the resolution of your feedback.

Davis Gentry
Delta Tau Data Systems

 

HI All

In Vector mode the motor should be able to generate 100% torque at 0RPM, or close to it. But if it is not a force cooled motor it will likely get a mite on the hot side while it is doing this.

Bye
Donald Pittendrigh

 

We have found that if you use a TENV motor, you can run full torque at stall 24 hours a day without heating problems. Dyne tests with TEFC motors show that typically you can expect between 60 and 80 percent of full torque at stall indefinitely without exceeding normal operating temps. Much depends on the specific motor design. Oddly enough, many of the European Vector
design motors have less ability to dissipate heat and require an external fan regardless of slow or high speed operation. A plain vanilla Reliance XT
TEFC motor can run at full torque / current at stall for hours on end and never get warmer than 60 deg C. All of these tests were done with a
standard Control Techniques UniDrive running in closed loop vector mode.

Ken Brown

 

Also investigate the effects of power restoration following a short-duration interruption!

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Sorry, but I take exception to the comment that an external fan will adequately cool a TEFC motor. The internal fan precludes extreme air-gap
temperature. Remember, the rotor generates most of the heat. In addition, internal air circulation improves heat transfer to the
carcass!

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Two options are available for your application.
1. TEBC (totally enclosed blower cooled) motor
2. Variable Ratio Gearbox

Since variable ratio gearboxes are few and far between, not to mention the ungodly costs; you might have only option 1. available to you.

We use TEBC motors exclusively when using 1000:1 speed ratios on VFDs. These motors are designed to run at low speeds using the blower to dissipate heat. We have never run into any problems but you do have to interlock the VFD with the blower contactor. The costs for this type of motor maybe a 10% adder compared to standard TEFC motors. Highly recommended.

 

Many motors are more prone too stall out at a low RPM (You can see this from the torque-speed curve. At low RPM's, the motor generates low torque - so it's easy to stop in its tracks.)
One way is to run the motor at a higher RPM,
then "downshift it" by connecting a small pulley to the motor shaft and a larger pulley to the load.

 

Motors can not produce constant torque below %10 of the nominal speed. Especially with speed controllers. As a result, it is wrong to use the motor in 20 rpms. My question is, why don't you want to use a reductor? In case of need, you can contact us. We can provide you a solution.

Emre Sayman M.Sc.
MAY Sistem Mühendislik
[email protected]