VFD overfrequency
VFD overfrequency
(OP)
Sometimes, caused by motor + gear ratio specified , the AC motor needs to run above its base speed without torque reduction. This means more output power. Which is the criteria to specify the VFD?
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RE: VFD overfrequency
The European 400 V motors are often Wye connected. If you change that to Delta, you will have a 230 V motor that can run at 50 Hz. But, since available voltage still is 400 V, you can also run it above 50 Hz and still maintain the correct V/Hz ratio. You can do that until you reach 400 V, which is at 50*sqrt(3) = 87 Hz, which is your new base speed frequency. Above that, you are in field weakening aka constant power.
Not quite sure how your 460V/60 Hz motors are connected. But if they are Wye connected, you can do the same thing. Your "base speed frequency" will then be 104 Hz.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overfrequency
That means that a 460V 60hz motor is going to reach its hp label at 460V and 60hz and that's it. If you want more torque at more speed, you need a bigger motor.
Having said that, you can sometimes squeeze a bit more out of a 460V motor if you have an AC service that runs a bit high in voltage, say 500V. You could set the base speed for 500V at 68Hz or so but the extra won't be much. And, if you supply voltage droops, your hp will go away too.
RE: VFD overfrequency
RE: VFD overfrequency
NEMA motors, as DickDV says, have either hidden star points, or in the case of the odd delta wound NEMA motor, hidden corner points. The voltage change is always 2:1 with series or parallel connections.
The NEMA solution is to buy a 230/460 volt, inverter duty motor, connect it for 230 volts and supply the VFD with 480 volts. Now your new base speed frequency is 120 Hz.
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD overfrequency
Not that I recommend wye/delta starts. But it seems to be used also in the US.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overfrequency
Can be Y-Delta or not, we can have it both ways now fairly easily. Toshiba US started offering 12 lead NEMA design motors as standard a number of years ago and dominated the market for a while, until others followed suit. 12 lead motors are designed for dual voltage, Y-Delta start or just Delta start. Either way, that would not be germane to VFD applications anyway.
Tanget issue: I for one never recommend Y-Delta starting because I am of the opinion that it solves one problem and creates 3 others, but some people are cheap. So to me, the availability of that option in the motor is of no value.
RE: VFD overfrequency
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD overfrequency
3hp, 230/460VAC, 4/8A, 1769rpm and I will connect leads of the motor as 230V and connect it to the 480VAC drive then my motor will be able to run at twice speed of 3538rpm producing the same torque as at 1769rpm therefore producing 6hp of output power, correct???
RE: VFD overfrequency
RE: VFD overfrequency
RE: VFD overfrequency
Keeping the V/Hz allows the motor to produce the same torque all the way up to the new base speed.
Since that speed now is twice what you had before, the motor delivers twice the horsepowers.
At least our EU motors deliver 73 % more power when run at 87 Hz instead of 50. And I think that yours are similar animals - even if they are dumb animals.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overfrequency
RE: VFD overfrequency
RE: VFD overfrequency
That is, a frame that produces 5 HP at 1200 RPM will produce 7.5 HP at 1800 RPM and 15 HP at 3600 RPM. I don't think that cooling will be an issue in that speed range.
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD overfrequency
Freq goes up, frame size gets 'electromotively' bigger. That's why 400Hz motors in "itty bitty" frames put out shocking amounts of HP.
You still have to consider shaft size, fan function, and centrifugal strength of the rotor though.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: VFD overfrequency
another area to watch is the mechanical impact of increasing the speed, especially where gearboxes are concerned. Doubling the speed on a 2 pole motor that is driving a grease filled g/box could have a detrimental effect on lubrication; likewise certain oil filled boxes may not get the required lubrication to bearings etc if the input speed is too high.
The induction motors are generally ok for 2 x rated speed mechanically but check the 2 pole motors and motor manf.
RE: VFD overfrequency
RE: VFD overfrequency
I see no problem with using an 1800 RPM motor at double voltage and double frequency to do the same thing.
There have been a few suggestions on this site that some compressor skid manufacturers are using this technique to reduce the size of the motors required for their compressors.
I would not rule out running a 3600 RPM motor overspeed either, but I would do some more investigation and possibly change out the bearings, maybe not, my investigation would supply the answer.
Anyone who spends 15 minutes cross checking frame sizes, RPMs and HPs in a NEMA frame chart will quickly see a pattern emerge and will be less hesitant to worry about cooling, lubrication and torque when motor speeds are changed.
You can take an 1800 RPM 10 HP motor and rewind it as a 3600 RPM 20 HP motor, or you can use a VFD and double voltage to do the same thing.
Respectfully
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD overfrequency
5hp 4 pole 182T frame 2 pole 184T
10hp 4 pole 213T frame 2 pole 215T
50 4 324T 2 326T
125 4 404T 2 405T
This is hardly a doubling of the size of the motor when the speed has doubled.
Or am I misunderstanding what you are saying?
RE: VFD overfrequency
So, if: 1) I have an medium size AC motor, 6 poles, 60 Hz,
220/380V (easily found in my country), P [kW}, Tn[Nm] nominal torque;
2)The available reducer gear ratio requires motor to run up to 1420 rpm keeping the nominal torque to match the process load requirements.
So, if I solve the motor overheating applying motor frame forced ventilation (separated fan) and feed motor(380V connection) through a 460V, 72 Hz inverter drive, I can get around 1,2 times P[kW}. basically without problems.
Am I correct?
RE: VFD overfrequency
Pick a frame size and check the HP at various speeds. It has been a few years since I went over the tables but although the same frames are used for TEFC motors at 1800 RPM and 3600 RPM you will see an increase in frame sizes for a given HP at the lower speeds. eg; 1800 RPM vs 1200 RPM vs 900 RPM.
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD overfrequency
Voltage will essentially be linear from 0 to 460V from 0 to 120 Hz since it is a constant V/Hz. Can someone say what the current to the motor look like as a function of speed from 0 to 120 Hz? I have my hunch but I don't want to show my ignorance.
As I have posted before, the publication at this link gives the maximum overspeed for NEMA motors on page 15. http://www.nema.org/stds/acadjustable.cfm#download
It looks like you can overspeed motors up to around 50hp, bigger than that at least NEMA doesn't like it.
RE: VFD overfrequency
The cooling can be an issue when running at a low speed and at rated torque. Some fans can "stall" and quit moving air at speeds well above rated. Also, it is possible the fan will put more load on the motor when running above rated speed. Remember, the power to turn the fan has to come from the motor.
RE: VFD overfrequency
RE: VFD overfrequency
Will it work for drive in sensor vector mode?
What is the current of the motor running at 120Hz connected for 230V and drive at 460V if the load torque at or less than rated motor torque?
PaulBr.
RE: VFD overfrequency
It should work even better with encoder feedback.
The current will be approximately the rated motor current at all speeds if the load on the shaft is the rated motor torque. So at 120hz you still get rated motor current when the load is the rated motor torque.