VFD overfrequency 4

[electrolitic]

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?

 

[Skogsgurra]

The standard way of doing it is to use a motor that has rated voltage equal to mains voltage at the desired speed.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DickDV]

Unfortunately, few NEMA motors bring out both ends of the motor coils. The wye or delta connections or at least a few of them are made internally and cannot be changed outside the motor.

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.

 

[jraef]

If it is a small dual voltage (230/460V) motor, you can strap it for 230V and apply a 460V drive, so it will reach maximum output at 230/60, then when you go over speed, it maintains constant torque above base speed up to 120Hz (at which time you are at 460V). But if your load HP requirement does not go down with increased speed, you need to double the size of the motor or you will smoke it.

 

[waross]

Hi Gunnar.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Does this mean that you have to order motors for wye/delta starts extra? Can't be done with standard motors?

Not that I recommend wye/delta starts. But it seems to be used also in the US.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Gunnar,
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.

 

[waross]

Aside from the recent offerings of Toshiba, the most common 3 phase motor in the North America has been the 9 lead motor. These were usually star, occasionally delta, but not suitable for star delta. Next most common, in Canada, was the 3 lead 550 volt motor. Star delta and part winding start motors were 6 lead motors, often special order.

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

 

[PaulBr]

Let me understand this. If I have motor with the nameplate:
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???

 

[gepman]

No, as jraef said in his original post you will need to double the horsepower of your motor, which in this case would be a 7.5hp. You can't get something for nothing.

 

[jraef]

Right. The HP rating of the motor remains the same, so if you over speed and overexcite, you are trying to get more HP from a motor not designed for it. Motors are "dumb animals" and will attempt to deliver (so to speak), but will go up in flames in the attempt.

 

[Skogsgurra]

Sorry guys. Do you not believe what you said earlier?

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

That's what I thought Gunner, same torque output but double the speed gives double the hp. As long as you run the motor at or below rated torque (which occurs at rated current) then it should not fail. Of course, the cooling of said motor can be an issue above 60hz and below 60hz.

 

[PaulBr]

Why cooling of the motor connected as described above would be an inssue?

 

[waross]

The same frames are used for 1200 RPM, 1800 RPM and 3600 RPM.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

I agree that with Gunnar.

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

 

[ozmosis]

electrolitic
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.

 

[sreid]

And the iron losses go up.

 

[waross]

Possibly the most common motor in North America is the 1800 RPM (minus slip) motor. Typically the same frame is used for 1800 and 3600 RPM motors, however the 3600 RPM typically produces double the HP. The motor develops the same torque, but is spinning twice as fast.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[DickDV]

waross, checking my NEMA frame assignment tables, I find the following:

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?