Motor Draws more amps at certain HZ

[Thomas Sullens]

Hi DickDv
Thank you
I always wondered how all that worked.So does this mean when using a constant torque drive. The torque stays the same from say 0 to 60 hz?
Does this mean we can`t use Hp X 5252 / rpm to figure torque accross
this range? Please exsplain.
Thanks Thomas

Reading your post again I think I understand

 

[atatari]

Thank you all for spending time to answer this question.

The issue that I don't understand is:

we ended up increasing the torque boost parameter from 6% to maximum which is 25% and then there is another parameter that automatically sets the maximum boost based on the torque required.

this drive used to trip for O.L. because didn't have enough voltage to produce the torque needed.
by increasing the boost ,I assume that in lower frequencies drive produce more voltage so obviously more current will required ,so again more current ...so why drive doesn't trip?

 

[DickDV]

Thomas, whether you use a variable torque or constant torque drive, the only thing that matters is that the drive regulates voltage and frequency so that the ratio is constant from 60 down to 0 hz. Remember, the terms variable and constant torque with respect to drives refer only to short-term overload capacity, not torque output of the motor. And the formula works as it should. If rpm drops at the same ratio as hp, then the torque must remain constant. And that's just what happens.

And atatari, the torque boost parameter can be dangerous. It is intended to add a little extra voltage at the bottom end of the V/Hz curve to compensate for IR drop in the copper conductors at very low Hz. Normally, 2 or 3% is enough for this compensation. If you add too much compensation voltage, you saturate the motor and the torque falls off rapidly. In fact, if you have this parameter set too high, this may account for why the motor needs 12hz of slip to start the load moving.

 

[Bitmore]

Well that's allot of good advice!
You might stop to think that perhaps the drive in question might have S-curve edit. Some drives come with a pre-packaged set of parameters to deal with manipulating torque/speed curves.
Just a thought from an old clown

Bitmore

 

[DickDV]

You are correct, bitmore. Many drives have parameters which allow you to customize the V/Hz curve which in turn modifies the available torque curve of the motor.

A common use of this is on motors direct or belt coupled to centrifugal pumps or fans. Since the pump or fan needs torque only as a function of the square of the speed, the drive can be set up to provide a V/Hz curve that also corresponds to the squared function.

There is a small amount of energy to be saved in the middle of the speed range when this is done. It also starves the motor severely for voltage at the low end of the speed range so operation there is not possible. Of course, pumps or fans don't work well down at those low speeds either so it all works out ok.

You mentioned S curves. Normally these refer to accel and decel rates, not torque curves, but I suspect you already know that.

 

[atatari]

but this extra voltage at lower frequency helped the motor to produce enough torque to run the auger .
what I don't understand is :

by doing this does current remains the same or increases?
drive doesn't trip for O.L. anymore ,so I assume current doesn't change that much,...so how this extra voltage affects the motor?

???

 

[Bitmore]

Well somebody is listening.. perhaps I used the wrong terminolgy. There are drives capable of manipulating the power curve based on frequency. I originally thought from the original complaint that the failure was caused at a certain frequency. Manipulation of the power curve (if available) would float the bill.

Bitmore

 

[DickDV]

atatari, as you raise the low speed voltage the current will increase. This assumes the motor doesn't start. When the motor starts to saturate, the current will increase even tho the torque is dropping off. The energy is simply going into heat which will damage the motor soon enough. Hopefully, the drive will trip on OL before the motor fails.

Now, if the extra voltage results in more torque which results in the load starting to move, then the current will not rise nearly as much. This would be the best situation possible, under the circumstances.

 

[Thomas Sullens]

atatari I think starting torque might vary directly with the square of the voltage you put into your winding and by you raising the low end voltage, you increased the torque. Since your motor starts now I guess it`s posible you're seeing a higher current, but as your rotor starts to turn it is cutting more lines of flux which should help your starting torgue ( not sure about this ). Your higher current would fall off almost instaneously so the drive does not trip. Since DickDv has cleared up "constant torque & variable torque" and Hp If you have to run the auger at this lower end I would put a bigger motor on it. I think some drives, like danfoss, you can go 1 motor size over or under and still be ok - not sure about jt woods. If I had known what DickDv explained, as the hz went down so did the hp, it would have saved me 2 weekends of work. I guess that`s why they call them constant torque? Now all I can think is well DUH!
Have a good day

 

[Lancie1]

DickDV,
I guess we are talking about apples and oranges. I was referrring to the times that I have tried to increase the speed of an operating motor in the field, whereas you are apparently talking about the design stage, where you can work it all out on paper and make it work as you said, adjusting the motor size and the gear reducer size and the speed and so on. But what about when you are up to your neck in aligators, the plant won't run, and there is no time to change out the motor or any other equipment? All I know is that when I have tried to increase the speed under actual real world conditions on existing equipment, I usually have got a high current overload trip on the VFD. So my question is: if the higher speed reduces horsepower requirements, why does the drive trip out?

 

[DickDV]

Ahh! Lancie, I did misunderstand. You are talking about pushing the motor into overspeed on an existing machine simply to get more speed.

The controlling factor here is not motor hp but load hp. Since the motor enters a constant available hp mode above its design frequency and speed, the machine speed can only be increased to the point where load hp equals available motor hp.

Now, if the original system has been designed with lots of extra motor/drive capacity, then you can often get quite abit of extra speed. On the other hand, if the motor is nearly fully loaded at its design speed, not much extra speed is going to be possible before the motor becomes overloaded (that is, load hp exceeds motor hp). Under those conditions the current will rise over nameplate full load amps and, assuming the drive is programmed right, the overload fault on the drive will protect the motor by tripping out.

So, as can be expected, you can't get more out of a motor than it has in it and the drive enforces that rule.

Having said all that, there are a couple of tricks that can be played under desperate production pressures. Remember that motor nameplate limits are based on an assumed ambient temperature of 40 degrees C (104 degrees F). If you find that the motor is not too hot at its fully loaded point, you could trick the overload function in the drive by entering a higher motor full load current than stated on the nameplate. You may end up sacrificing the motor by doing this but, in some cases, you could save a production process and become a hero! Imagine that! Imagine all the praise and wealth that will be heaped upon you! (I've got to stop this!) You get my point!

You could also place additional cooling fans on the motor to keep its temp down or, as I've seen numerous times in paper mills, you could run a little cold water over the motor to keep it cool.

If you examine a NEMA Design B speed torque curve, you will see that, without the thermal limitations, a motor can develop about twice its nameplate hp. Again, this is a high risk game and you may find its simply not worth the risk to play. Another very important consideration is machine safety. You must be absolutely sure that the extra machine speed is safe or forget everything I've told you about overspeed. It is not worth getting someone hurt over a little extra production, I don't care how ugly the production manager gets. A good example is any motor driving a flywheel as on a stamping press. If the press nameplate says max strokes is 50 per minute, do not be tempted to increase it even to 51. A disintegrating flywheel makes bad company, if you get my drift!

 

[Lancie1]

DickDV,
Now I am agreeing totally... The last situation that this came up for me was with a fan about 6 weeks ago. We needed a little more air flow to get the process right, so I stepped the drive up to 90 HZ, having checked that the drive overload setting agreed with the motor nameplate amps. The unit ran for about 2 minutes, then I got an overload trip. It turned out that I was bumping against the maximum rated current of the drive, so I was afraid to go beyond that point. Basically we need a bigger fan and a bigger drive. By trial-and-trip, I was able to move the max speed down, and finally get it running at 70 Hz, but we were still low on air pressure. It seems that our designers tried to push the ragged edge of the envelope when sizing the equipment. I find this to be true in most of the projects that I have worked on in the last 5 years. Profit margins seem to be stretched to the low limit.