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Overspeed with a VFD
4

Overspeed with a VFD

Overspeed with a VFD

(OP)
Hopefully this is an easy question. Which motor is capable of going overspeed, induction or synchronous? How? What would be the maximum increase allowable (%)? Does service factor figure into the equation, ie, does a motor (power) SF of 1.15 limit the speed also to 1.15 x synchronous speed, or some other factor limit?

RE: Overspeed with a VFD

MKMason
Firstly, it would be useful to know what your application is. Do you know the load (torque) characteristics?
Secondly, why do you want to over speed?
This is a good starting point.

RE: Overspeed with a VFD

(OP)
Oh dear, it seems I've over-simplified the question. The driven equipment is a high flowrate slurry pump, ~500 HP, and the client is attempting to vary the developed pump head to match an increasing downstream restriction that there is no other way around. One could overspeed to raise the pressure head and still control the flowrate. If you could see the (confidential) process diagram, you would agree with the design. However, I need to go from 50% of rated head to somewhat over the rated head to overcome the slowly increasing downstream restriction. I need to know what type of motor will get me there, assuming I'll be using a VFD either way.

RE: Overspeed with a VFD

If you're picking the motor then pick a motor with a lower rated voltage then the line power and VFD rated voltage. That would allow you to maintain the V/Hz ratio above line frequency which means you continue to get a constant torque and linear HP beyond the motor rated frequency. For example, a 380V motor on a 600V source allows you to run to 158% of the motor speed and get 158% or the motor nameplate HP.

As for motor type. I would expect most any induction motor to be capable of some over speed, but I wouldn't expect the same from a synchronous motor. Most induction motors seem capable of about 150% over speed but it's always best to check with the manufacturer.

RE: Overspeed with a VFD

(OP)
Thank you Lionel. You are obviously not in the USA. Will this idea you have also work for 480V on a 4160V system? It's a much bigger delta-V. I calculated my required overspeed, and it will likely be 125%.

RE: Overspeed with a VFD

I know this idea has been explained in other posts, but I'm still struggling to understand. I get the constant V/Hz and torque plus higher RPM, thereby higher HP, but doesn't the extra power equate to extra heat and wouldn't that cause the motor to overheat? Maybe I'm just a pessimist, but it seems too good to be true.

RE: Overspeed with a VFD

Yes, running a motor over sync speed does create more issues. Many times you could also be running into a critical frequency not too far above the sync speed, in addition to greater heat and other losses.

RE: Overspeed with a VFD

At the current of a loaded motor is closely proportional to the torque produced. The greatest heat is I2R losses in the windings.
By maintaining the V/Hz ratio and supplying an over voltage at over frequency we get more HP due to the greater speed and voltage at the same current.
HP is dependent on current and voltage.
My first choice would be a couple of small dry type transformers in open-delta auto-transformer boost configuration.
My table shows a typical current for a 500 Hp motor at 460 Volts as 590 Amps.
If you use a 25% boost you need to add 120 volts. Your transformer size will be 590 Amps x 120 Volts. or 70.8 KVA. A pair of 480:120, 100 KVA transformers will do the job nicely. The transformers must be rated for boost operation.
My second choice, if this is a new installation and a transformer must be purchased, consider a 600 Volt secondary. 600 Volts is a common Canadian voltage and most manufacturers will be able to supply a 600 Volt transformer.
Another third option is to have the motor reconnected in delta. That will let you go to 173% of base speed.
Another option may be to oversize the motor. A 600 HP motor with a 1.15 service factor will be the equivalent of 690 HP.

What is the maximum speed and the HP required at the maximum speed?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Overspeed with a VFD

Bill,
Sorry if I appear dense (heck, maybe I am). If you're saying heat is from I2R and the current stays the same, then it follows that there won't be much, if any, extra heat loss even up to say twice the nameplate HP? Why aren't motors just built this way in the first place? Is it just the fundamental limit of number of poles and 50/60 HZ?

RE: Overspeed with a VFD

MKMason

Based upon your comments:
You have a 500HP, 4kV induction motor driving a slurry pump.

Going to assume motor is a 1780RPM (4pole mtr) to a gearbox and then to the pump.
You want to drive the motor from the 50% to 125%. (885 to 2225 RPM)

Typically slurry pumps (centrifugal type) are considered variable torque applications.
So between 885 to 1780RPM a typical 4P-500HP motor can operate without any issues.

From 1781 to 2225 RPM, the motor become Constant HP unit, meaning HP stays the
same through the overspeed, but torque decreases as the speed increases.
IE: at 1780 RPM motor will produce approx. 1474 lb.-ft. of torque.
At 2225 RPM motor will product approx.. 1179 lb.-ft. of torque.

You will then need to talk to your slurry pump mfg. and find out what is the motor torque
requirement at 2225RPM. Also ask pump mfg. about operating the pump at the higher
speeds. If there is a gearbox in between pump and motor, same question to the gearbox mfg.

This is just the starting point. There will be many other issues to consider.

Good luck
Mac

RE: Overspeed with a VFD

(OP)
WOW. I'm sure getting a lot of great advice here. I don't think the client will like the transformer idea, and has already stated that 600V is not available. It needs to be a VFD, they say. I TOTALLY forgot about the critical speed issue. That could be the achiles heel. Thanks for the reminder. It has no gearbox, and will likely run at 1750 rpm. The pump curves are running up from about 450 HP to 600+ HP depending on flowrate. I was hoping to limit the HP to 500, but I can see now that won't be possible. I will need to talk this over with the pump supplier concerning the torque -- constant or not? What am I after besides more pump head and flow? Only the process engineer can tell me.
To help clarify my inquiring mind, just what features of how an induction motor is designed allows it to go overspeed, while a synchronous motor cannot?
Again, thanks to all of you for this phenomenal thread.

RE: Overspeed with a VFD

If we ASSume that the motor size was chosen based on full speed of the motor connected to that pump running at that speed, increasing the speed, on a CENTRIFUGAL pump will increase the POWER required by that pump by the cube of the speed change. Increasing the speed and voltage (as suggested) also increases the output mechanical power of the motor, but not at the same rate. So at 125% speed, that same pump will require 1.253 power, or 1.953X the HP. If you increase the motor terminal voltage appropriately to match the new speed, your 500HP motor is now capable of delivering 625HP at the shaft, but your PUMP will now require 976HP at that speed.

Oops...

"Will work for (the memory of) salami"  
 

RE: Overspeed with a VFD

Meant to add this... ohhh for an edit function....

The affinity laws work in all directions, not just for reduction of speed.

"Will work for (the memory of) salami"

RE: Overspeed with a VFD

An induction motor speed is based on the frequency of the power, thus a 2 pole motor will run at 3600 RPM at no load and 60 Hz source (3000 at 50 Hz). At load the RPM actually drops as the load increases. A synchronous motor is designed to run at a specific speed that stays at that speed even under an increasing load.

RE: Overspeed with a VFD

The issue with synchronous motors has to do with the field more than the motor. A synchronous motor may be started on a VFD to reduce starting current because a synchronous motor starts as an induction motor. When the motor is close to synchronous speed the field comes on and the motor locks onto the frequency. Also, synchronous motors are often used for power factor correction. That may become complicated if the frequency is varied very much.
You have noticed that induction motors always run a little below synchronous speed. The speed difference, translated to frequency is called the "Slip frequency". The slip frequency is what induces the currents in the rotor to magnetize the rotor.
For a motor rated at 1760 RPM the slip is 40 RPM. 40 RPM/1800 RPM is a ratio of 1/45. 60 Hz x 1/45 = 1.33 Hz. The slip frequency stays much the same regardless of motor speed.
Another option may be to connect the motor for 230 Volts. Now you can go to 200% speed and 200% HP. You may want to change the bearings out for the bearings that would be used on a 3600 RPM motor.
Take this back to the pump group. For the price of a 500 HP motor you can give them 1000 HP at 200% speed. (You will have to use a 1000 HP VFD)
You can have the impeller trimmed to bring the head at 200% speed into your desired range.
Are there any winders following this thread? Are there any issues rewinding a large 3600 RPM motor for 1800 RPM? That would take care of the speed issues.
The pump load drops off so much with reduced speed that cooling fan efficiency should not be an issue.
Anecdote: We had a submersible sewage sump pump that was grossly overpowered. When it started the pressure forced a gravity connection apart and there was stuff everywhere. The mechanical folks decided to trim the impeller to reduce the head. They calculated the amount to take off the impeller and sent it to the shop. The instructions got mixed and the impeller was trimmed on the face instead of the diameter. It worked until a solid or gas bubble went through and then it would cavitate and stop pumping totally. Glad to see the end of that job and the accusations that there was something wrong electrically when the pump stopped pumping.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Overspeed with a VFD

Rewinding a 2 pole to a 4 pole is an issue. For one, the same horsepower requires a larger frame. In the fluid filled motors I deal with, a 75 Hp 2 pole is an 8" motor and a 4 pole is a 10". Second is the torque, a 4 pole creates about double the torque at the same Hp. Depending on the lamination design used in the rotor and stator, the performance would change.

RE: Overspeed with a VFD

No, you can't run a 480V motor on a 4160V drive. You likely can run a 2400V or 3300V motor on a 4160V drive. You have to pick the drive to match the full-load current of the motor.

You really need to define the speed and power needed and then use that to figure out the motor HP and rpm required at 60Hz and the over-voltage ratio required.

For example - 750HP @ 2200rpm, assuming a 1750rpm motor will be used. 2200rpm/1750rpm = 1.26x over speed. 750hp/1.26 = 595hp. 4160V/1.26 = 3300V. So, you need a 600hp, 60Hz, 3300V motor to achieve 750hp @ 2200rpm when using a 4160V drive. Make sure the drive current rating matches the FLA of this 600hp motor.

RE: Overspeed with a VFD

Lol, I feel as though I'm watching a debate on the upholstery in the lifeboats of the Titanic when the fact that there aren't enough boats is immaterial...

Whether or not you successfully get to the higher speed with that motor is probably still going to leave you about 300HP short of what the pump is going to demand at that speed.

"Will work for (the memory of) salami"

RE: Overspeed with a VFD

(OP)
Yes, jraef is correct. The pump speed N1/N2 CUBED is = the power ratio (HP1/HP2), I could easily jump from 300 HP to 2000 HP going from 1800 rpm to 3600 rpm.
Sooooo... I will use an 1800 rpm motor with the VFD, run normally at 25% less, cut it down 50% more when I need that lower pump head (pressure) and forget about overspeed. It was a bad idea, sounds like. I'll be content with the torque I get, and the process engineers will just have to get off their ivory pedestals and learn how to use valves.

RE: Overspeed with a VFD

LOL, well I think it's funny someone is so worried about the motor being overloaded when it doesn't even appear that MKMason has the motor yet....

On the flip side, if the flow and head require 976HP then you're going to need a motor capable of 976HP, even if you change the pump so it's giving the flow and head at 1800rpm.

RE: Overspeed with a VFD

There was a similar, recent, thread titled '87 hertz motor' in which the idea was suggested that a motor could be operated at a constant volts/hertz ratio above the design frequency to produce a higher than designed power output.

As an example, it was suggested that a dual voltage 230/460V motor could be operated on the 230V connection with an input power of 460V/120hz to produce double the rated power output. Since the insulation rating would be suitable for 460V power and the volt/hz ratio was kept contant, the idea was that this would result in a 'free' increase in power output that was double the original design output.

While in principle this seems possible, the fact is that in practice that this will not work. The reason is that while the load current, and therefore the motor I2R losses, will remain constant with a constant v/hz frequency change, the core losses will change in proportional to the change in frequency. At a constant volt/hz ratio, a lower frequency results is a proportional lower core loss and a higher frequency results in a proportional higher core loss. In most cases, if not all, the increase in core losses as the constant volt/hz frequency increases above the design value will result in an increase in temperature rise that will exceed the designed temperature rise. The result will be overheating of the motor and a reduction in service life.

RE: Overspeed with a VFD

While in principle this seems possible, the fact is that in practice that this will not work. The reason is...the core losses will change in proportional to the change in frequency.

...In most cases, if not all, the increase in core losses as the constant volt/hz frequency increases above the design value will result in an increase in temperature rise that will exceed the designed temperature rise.

The result will be overheating of the motor and a reduction in service life.


We should apply real world values to these statements.

So you are saying to run to 120hz in constant v/hz mode will cause the motor to overheat, even no load?

www.KilroyWasHere<dot>com

RE: Overspeed with a VFD

MKMason
I think you need to get back to the basic requirements, all the way up the chain to the core requirements of the application.
Forget a VFD for the moment, forget what a motor can and cannot do, forget your voltage supply.
You are pumping slurry. You need to change what you have and this demands a view on a few basic questions such as flow rates and required head. This will determine, in part, your system curve on your pump. I'm no pump expert( this is the stage guys like biginch and littleinch jump in and tell us electricals what a load of cr@p we have been discussing...) but I do know that if your pump selection is not correct, then you will be in a whole lot of trouble down the line with everything else. Once you establish your pump curve, then you can design the electrical equipment to support your mechanical requirements.

RE: Overspeed with a VFD

(OP)
Ozmosis:
I know my pumps pretty well, and understand the issues around pump curves. It's the Process Engineers that do not understand what can and cannot be done and there is my battle. Sometimes I must re-rate pumps with VFD's on the motors, sometimes buy new pumps, and other times the process demands more flexibility. Slurry is a constant torque application. I do know this: a 1.15 SF on the motor HP will only allow for an almost 5% increase in flowrate due to pump affinity laws. It won't be enough for me, I needed 25%, and I need to consider a higher speed system if I intend to get the flowrate up. My electrical engineer here, and I, are reluctant to run an existing pump into overspeed.
This has all been a very interesting discussion, I'll agree. The Reality is, for a refinery type application, long life and reliable service are the governing factors, not "what tricks can I pull?"
Once again, thanks to all of you for your input: you've outlined the project limitations remarkably well. Hats off!

RE: Overspeed with a VFD

We who apply vfd drives for a living say 'throw away your SF 1.15 rating - it no longer has any meaning when a vfd is used:' assume a SF=1.0 - no extra overloading allowed over nameplate no matter what your particular motor says. It is a good rule of thumb you should consider using since you have referenced SF more than once....

www.KilroyWasHere<dot>com

RE: Overspeed with a VFD

I second mikekilroy. As a motor manufacturer, a VFD automatically drops it to a 1.0 service factor on the motor.

RE: Overspeed with a VFD

(OP)
OK, I'll accept that. This means I really can run the induction motor to 150%(-ish) speed? And not freak about overheat if I'm staying at constant torque?

RE: Overspeed with a VFD

OK, I'll accept that.

great!

This means I really can run the induction motor to 150%(-ish) speed? And not freak about overheat if I'm staying at constant torque?

Sorry, not sure how you concluded this from SF=1? Two totally separate issues.

Others already went thru the 150% speed and torque available there.... I will summarize all those answers and you can go back and reread the great posts above for details why.

I really can run the induction motor to 150%(-ish) speed?

YES

not freak about overheat if I'm staying at constant torque?

YES if you keep rated v/hz curve all the way.
NO if not

www.KilroyWasHere<dot>com

RE: Overspeed with a VFD

MKMason
I didn't mean my comment to be disrespectful in any way. It was just a comment to highlight the need to keep focused the basics in the application, and that always starts at the prime mover.
This is also coming from someone who is in the VFD business for a living and needs to ensure the application fits any recommendation for drives, if it is to be used correctly.

RE: Overspeed with a VFD

I think what we can all agree on is that old axiom, "There is no free lunch." You cannot get something for nothing, including getting more from a motor than it is designed to give.

But here is where the trick mentipned earlier can make it LOOK as if you are. So let's start with this to maybe help you understand, and taking your word for it that this is truly a constant torque application.

For the most part, torque and current follow each other.

Torque in an AC motor is also directly related to the ratio of voltage and frequency applied to it, the V/Hz ratio.

If I want to run a motor at 150% speed then, I need to accept two principals:
1) If I apply the motor rated nameplate voltage at 150% frequency, I am running it in Contant HP mode, meaning I begin to LOSE torque once I get over base designed speed. This is because my V/Hz ratio is dropping as Hz increases with voltage staying the same.
2) If I want to maintain the same torque at 150% speed, I must also then increase to 150% Voltage, so that my V/Hz ratio stays the same.

So under #2, since I am maintaining my V/Hz ratio, I am maintaining my torque, which means I am maintaining my current as well (in theory). Motor heating is mostly the result of current based losses, so I am not really over loading that motor in that sense, because my current is still within design limts. "HP" is just a shorthand notation saying xxx torque at yyy speed, so with the SAME torque and a higher SPEED, I am actually getiing more HP from that motor. But in reality, I am really still getting the same torque, which in the case of a constant torque application, is what I need.

Now the theory part. A little over half of the losses in the motor are associated with current, but not all. Friction and windage losses will increase with speed, and iron losses, which are based on applied voltage, will increase as well. So this robs your motor of capacity in that sense, because running at the higher speed will mean the non-current base losses become a greater percentage of the total. A good Vector drive however will be capable of optimizing the motor operation in terms of the some of the voltage related losses, so that will help. Still, a 1.15SF may not be enough if your motor torque requirement for that pump is right at the limit of what that motor can provide. If on the other hand you already had a 20% cushion in the design, plus a 1.15SF, you may be able to pull it off.

But still, it comes down to you being able to supply 150% voltage at that 150% speed.

"Will work for (the memory of) salami"

RE: Overspeed with a VFD

In regards to higher voltage: This is in relation to motor rated voltage.
Another option is to keep the applied voltage the same and to reduce the motor rated voltage.
Star delta; If a star connected motor is reconnected in delta, the effective rated voltage drops to about 58%.
If a 460 volt star motor is reconnected in delta, the effective rated voltage is now 265 Volts. With a 480 volt supply, (460 Volt utilization voltage) you have enough voltage to go 173% over speed and over voltage.
Dual voltage; If a 460 Volt rated motor is reconnected for 230 Volts, a 480 volt supply will allow the V/Hz ratio to be maintained up to 200% speed and 200% voltage.
The point is that it may be easier and cheaper to reconnect the motor than to adjust the supply voltage.

I would determine the maximum HP needed at the maximum flow and head. Then I would consider trimming the impeller to give the best pump match for the best available motor speed.
Consider going oversize on the motor.

A question in regards to the losses jraef; When the frequency is increased, do the iron losses increase for the whole motor or just for the stator? I am thinking that the slip frequency and the rotor frequency may remain the same. Comments??

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Overspeed with a VFD

Also remember that although the losses DO increase SOME (it is NOT a major amount) at higher speeds, these losses are EASILY taken care of with the shaft mounted cooling fan going faster and blowing a lot more cooling air.

Someone a few posts back suggested the losses are so great this is not even possible to do; I would not want to have to be the one going back to my customers where we have run 230v motors upto 460v@120hz (and then on up even higher) and tell them it isn't possible! We probably have 400 various size motors (5-100hp) in the field over the last 30 years this way!

I have never run into a motor run this way, even with a separately excited fan, that had any heating issues maintaining rated torque above base speed upto 2x voltage.

www.KilroyWasHere<dot>com

RE: Overspeed with a VFD

Thanks for the information Mike. I suspected that was so. It is good to hear it from someone with first hand experience.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Overspeed with a VFD

4160 volts was mentioned. At the HP we are looking at a 2300 Volt motor on a 4160 Volt VFD may be a good option.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Overspeed with a VFD

MKMason, what is the voltage level, ie Medium Voltage or Low Voltage, that you are intending to use for the motor and drive? You mention having 4160V available, but is that a viable option for the user? A MV drive will be a lot more expensive for the drive, but eliminates the transformer and losses in the transformer in terms of operating costs. But in some cases, the user may not want that because they have nobody on staff qualified to even open the door of MV equipment, let alone service it because in some jurisdictions, working on anything above 600V (the definition of MV here) requires special qualifications or certification. So if we are to offer suggestions, it would be nice for you to narrow the field a bit if possible.

"Will work for (the memory of) salami"

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