Maximizing Power From AC Induction Motor With VFD

You don't need gearing if you are using a VFD. Thats what the VFD is for. It can also maintain required the torque.

By gearing I mean that I can select the sheaves (pulleys) so that the motor runs at a desired speed range. If I select the sheaves so that the motor only runs at 50% to 60% of the rated speed, I think that then only 50% to 60% the rated power would be available at full torque. But if I select the sheaves so that the motor speed is 100% to 120% of rated speed, I think that then the Motor can deliver the full rated horsepower and that I'll have more power and airflow available by overspeeding the motor than if I underspeed.

With a VFD you can reduce the speed to what ever you want and still at the max torque. It is programmed into the drive. I don't know how you select the speed but you can get feedback to the drive so that it controls the speed and still maintain the torque.

I need to maximize power. Max Torque at reduced speed is not max power.

It's a little late for me to go looking for my books, however if I was setting it up without any additional information I would set full load amps at 60 Hz. Technically the motor can maintain full load output to 90 Hz, after that point it must be Derated.

However these are generalizations, the first thing I would do is contact the motor manufacture. Running the motor at full output at less than rated rpm could lead to overheating, or this larger motor may be sensitive to over nameplate rpm . To me it would certainly be worth the effort to contact the manufacture, they are the experts on their product.

Thanks. That's good advice. I'll see what I can dig up from Siemens.

There's no free lunch, but I want to ensure that I get a full serving! All I want to pull off is maximizing airflow for short periods of time. Currently the fan is operating at about 89 hp and 565 rpm with the motor running on 60 hz at 1775 rpm. Tehr should be power available to increase airflow by the cube root of 115/89, or by a factor of about 1.09. If I use a VFD I hope to be able increase the drive frequecy to 65 hz to speed the motor up to 1933 rpm where 115 hp will be required. This will proudce peak airflow about 9% higher than the exisitng setup, That doesn't sound like much, but it makes a big difference in our process.

I've gotten some input that it's better to change sheaves on the motor so that the motor does not run above 60 hz. I'm quite sure that this would not allow us to run at 115 hp. The motor would run at maximum torque, but at reduced speed and therefore reduced power.

As a rule of thumb, leave gearing as it is and run to 60Hz and beyond whilst monitoring motor current. If the current stays inside Full Load current and you are a at 66Hz or less (10% more speed approx), don't think there's much to worry about and you'll be delivering more than original design air volume / pressure (power).

The load should rise roughly as a cube of the speed (as already outlined) and 10% more speed should also stay inside design limits of materials (tip speed) and bearings etc for the driveline. Audible noise will incraese quite alot, though!

Thermally, generally the motor is not too challenged as long as it doesn't overheat, the extra speed will introduce extra motor cooling with windage etc, as long as the motor doesn't feel steady state too hot (when you get chance to feel / monitor it!!) it's OK.

Obviously the fan efficiency will vary with speed a little, so its best to monitor load whilst testing but I'd reckon you'd be pretty unlucky for this not to be alright from what you've described.

Hello Jreaf:

A Detailed Explanation, you saved me pulling out the books this morning. Personally I've never found a reason that I had to run a motor over nameplate frequency, I may be a little old-fashion but I rather change the sheave. As I mentioned in a previous post I generally set full load at the rated frequency.

My concern at running the motor at full load at a significantly lower rpm than nameplate is that if the motors only source of cooling is its internal fan there wouldn't be enough heat dissipation. When possible I set the full load amps without adding the service factor in. I don't have any problem running within the service factor . However there are transitory conditions with AC units that can lead to (wide-open economize while the VAVs stabilize for example) increased amp draw. Setting full load amperage for the worst possible scenario can leave you with insufficient airflow under normal conditions. So if possible I use the service factor for these transitory conditions.

So as to my question and comment. My 90 Hz comment is derived from what I've learned and read from various classes. It is my memory faulty?

Never say never I've seen motors fail at under full load amps from overheating when in environments that are at excessively high temperatures. I've also witnessed dramatic examples of what cooling a motor can do when I found motors running in excess of two times their nameplate rating in a refrigerated air stream without problem. Or I should say without mechanical problems, it did create problems for the service personnel that didn't do a proper startup. So if you could enlighten me with your thoughts on overheating motors by running them at full load at less than nameplate frequency you might save me ia few trips to the motor shop in the future.

I have always admired someone that could do left handed-one handed push ups.

But I really get a kick out of anyone that can explain something in this much detail to the point were the thread initiator does not even understand what you said.

I for one am better for your imparted data.

Thanks

Mike

Your assumption is wrong. I understand every word and concept expressed.

Perhaps you still can gain some power.

If the connections in the terminal box of your motor are changed in such a way that the motor can run at a lower voltage (per example 230V) and your VFD still is supplied with the 460 Volt, then there is a way out.

The most important thing is to keep the ratio V/Hz of 7.67:1 at the same level. If the V/Hz level is correct, then the motor is enough magnetized and can deliver the rated shaft torque.

The motor output power is equal to the multiplication of shaft speed and shaft torque.

The shaft speed can be increased by increasing the output frequency of the VFD.

The shaft torque can be hold at the rated value if the V/Hz ratio is the same.

This means that if you increase the motor speed and at the same time you increase the output voltage, you can gain power. The big problem is that your VFD is connected to a supply of 460V and will never give out a higher voltage than 460V. The solution is to reconnect your motor for a lower voltage, lets say 230V and tell your VFD that it must give 230V output at a frequency of 60Hz.

In this case the V/Hz ratio is 230/60 = 3.83

When you increase the frequency of the VFD to 100Hz the output voltage is at 383V.

The ratio V/Hz is still the same 383/100 = 3.833

The motor runs at its maximum power when the output frequency is 120Hz and the output voltage is 460V

The ratio V/Hz is still 460/120 = 3.833

Warning, keep in mind that :

1) The motor is running at double speed, is it mechanically strong enough for this speed ?

2) You have to buy a VFD for a 200 HP motor. The motor current at 230V will be much bigger than at 460V and this current must be delivered by the VFD.

3) The electrical input power will also be twice as high.

4) The connection cable between motor and VFD will carry the double current.

5) The level of the thermal protection in the VFD has to be increased.

6) When your motor is running faster, your load needs more power. In the case of a fan the power increases to the cubic power. ( look at what JRaef has written.)

So theoretically you could run the fan at 2230RPM or 126 % of the speed

Information is given without any liability !!!

From the field;

Install your VFD.

Install a pressure switch (with control device) in the down stream duct work that will produce a 4-20 output signal.

Put your new drive in hand.

Drive it up to the max setting that you need.

Program the VFD to look at max amps first then to produce the duct pressure that you need.

If you only need it for a small porition of the year and your local temp. is ok this will work.

It takes X horse power to product X air movment.....To support the best answers on this thread NO FREE LUNCHES.

Good luck

Mike

Your questions have pretty much been answered so I won't even try adding my own answer (I'll just be repeating what's been said anyway). I just want to say that the production are always looking for that extra that you've mentioned.

We installed a pump/motor with VFD that were properly matched. A year or two later, we needed to increase our output so they asked me if I could set the motor's maximum speed higher. I gave a rather long explanation about this and that, ending with the statement that you can't squeeze any more power out of the motor than it was designed to give. Better to order a new pump and motor that were properly sized.

About a week or so later, I began to see nuisance trips from this pump/motor. By nuisance, I mean the motor would trip for no reason, or so it seemed. I put it in the schedule for replacement in the next maintenance cycle but it burned out before that. Then I learned that production had gone behind my back and had one of my technicians make the change anyway.

You've done the smart thing by asking first. Good for you.

OK, so now the pump is running overspeed and delivering more power than design.

No-one has looked at the wiring yet. If the motor is delivering more power then the wiring is carrying more power, and it may not be adequately sized. So the next thing that happens is the supply fuses rupture and they are replaced with, maybe, a larger set in the best case or even three rusty nails in the worst! Then at some point.......fire insurance companies usually take a dim view of the claims made ofter such a thing....

So the next thing to do is to check whether the supply cabling, and indeed the whole upstream supply, is actually capable of supplying the additional load.

No-one has asked why more air flow is required. Would modifying the duct shape so as to deliver a smaller jet of air at higher velocity do instead, while maintaining the existing motor and controls? Just what is the parameter that is process critical?

So stop sodding around with the VFD and get back to basics!

Good point on the wiring. we're checking it out. We are also implementing improvemetns to the air handling system ( fan, not pump) by installing turning vanes , by eliminating sytem effects at the blower inlet and outlet and by minimizing flow restrictions. However, models indicate that an additional 8-9% peak flow increase (cube root of 115 hp/ 89 hp) by using a VFD to go into the service factor of the motor for about 400 hrs per year is huge, huge, huge to us.

If you intend to try and get more HP than rated 100HP this would affect the power drawn at supply side with its implications on current, cable sizing, fusing etc.

However, remember that VFD is a 6 pulse uncontrolled rectifier and the input current will be related to supply reactance and the VFD components resulting in total RMS input current.

What is the supply voltage available and what is the motor rated voltage. If the supply is greater, you have an easy answer to more power being available (see my original comment re motor rated current also, you need to monitor motor current to avoid heating problems to start with).

If the input current is getting higher than co-ordinated cabling / fusing, you can add inductors to DC link of VFD (best) or AC supply side, reducing RMS input current, harmonic content and heating effect, of course.

Increased power implying increased RMS input current isn't necessarily something you can't do something about relatively easily

"VFD is a 6 pulse uncontrolled rectifier and the input current will be related to supply reactance and the VFD components resulting in total RMS input current.If the input current is getting higher than co-ordinated cabling / fusing, you can add inductors to DC link of VFD (best) or AC supply side, reducing RMS input current, harmonic content and heating effect, of course."

I don't understand this technically, but I'm working with a guy who can evaluate the idea. My guess is that the input wiring is oversized.

This means that when you are looking very carrefully at the input current of the VFD (by means of an oscilloscoop per example), you will see that the input current is not a smoot sinus. In stead it is a signal with a lot op spikes.

This is because inside the VFD there are capacitors that need to be reloaded every periode of the current. (in your case 60 times per second). The capacitors are only reloaded when the input voltage is almost on the top of the sinus. The momentairy suplly voltage must be higher then the residential voltage on the capacitors.

These spikes on the current can give a lot of interference with other equipment in the surroundings. The effect is worser when you have signals cables with low voltages in the neighbourhoud.

You can minimize these effects by installing 3 big coils (inductors) in the supply line of the VFD. The inductors will act as a low pass filter and smoot the current.

On the same way you can install one big inductor in the DC power circuit of your VFD.

When the IGBT's of the VFD are fired, the big inductor will help the capacitors to supply the necessairy current for your motor.