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
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
RE: Overspeed with a VFD
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
RE: Overspeed with a VFD
RE: Overspeed with a VFD
RE: Overspeed with a VFD
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
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
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
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
Oops...
"Will work for (the memory of) salami"
RE: Overspeed with a VFD
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
RE: Overspeed with a VFD
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
RE: Overspeed with a VFD
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
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
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
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
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
...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
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
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
www.KilroyWasHere<dot>com
RE: Overspeed with a VFD
RE: Overspeed with a VFD
RE: Overspeed with a VFD
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
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
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
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
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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Overspeed with a VFD
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Overspeed with a VFD
"Will work for (the memory of) salami"