Reducing RPM 1486 to 900 of Induction Motor

Option 3: a 3:2 reduction gearbox.

Option 4: a 3:2 reduction belt drive.

I was not aware that changing the name plate would be an option in this case. I'd try this before the gearbox which would have been the obvious solution for me.

Well it will be far more easy to arrange 6 pole motor instead of gear box. i said motor cannot be replaced, gear box arrangement is as difficult as motor replacement

Do that, then.

I would think that changing the nameplate data is a bad idea. At a lower frequency and the same voltage, the motor will draw more current than it is designed for. (There is a reason why the motor parameters are fed into the VFD.)

However, reducing the voltage at a lower frequency (which the VFD does) reduces the power output. To achieve the same power output, you need to step down the speed with some sort of transmission, e.g., a gearbox or belt drive.

Rixter

Yes reducing the voltage at lower frequency reduces the output power which is what happening in our case, to keep the RPM near to set point the motor increases the current which tends to touch the Motor FLA, At half the speed motor runs on FLA which causes temperature rise, now to drive the same load at decreased current we need to boost some voltage and that voltage can be increased by altering the V/F curve. The proper solution is to arrange a 6 pole motor but we have to keep this motor for about 60 days.. Should I leave the motor to operate at 750 RPM drawing FLA for two months or should I change the frequency to get some voltage which is also not a proper solution but for 60 days I Need a solution.

VFDs limit the current at frequencies below the motor nameplate value and limit the power above the nameplate value. If you reduce the frequency to 60%, you will have the same torque (proportional to current) but 60% output power (proportional to current x voltage or speed x torque).

http://controltrends.org/wp-content/uploads/2010/10/VFFundamentals.pdf

How does the boiler react to the fan operating at only 750 RPM?

Tell us how this boiler/I D fan was originally installed. Constant speed? Direct drive? HP? RPM? What are the fan required HP at 750RPM? at 900RPM?

Feel lazy to analyze, Jraef is good at this.

See the manuals of the vfd and the motor first, there would be limits (minimum and max), guidelines and recommendation of its use, the experts are the maker. If unavailable, seek the manufacturers recommendation on this by email is best for liability documentation. Don't decide yourself on it, its a big deal, destroying such equipment, inclusive of your credentials.

Unless you have a need to constantly change the speed of this fan, why not just run the motor at its full speed and then size the motor and fan pullies so that you get the speed you require at the fan.

If this fan runs constantly at 900 RPM, the use of a VFD is way overkill.

Speed of the fan is constatly changing with respect to the amount of vaccum to be created in the furnance to get the required steam pressure. Earlier Dampers were used to change the amount of air flow which operated mechanically, now motor is controlled by VFD.

...which flies in the face of the claim of its being a new installation.

Is there a qualified Engineer at the facility - one who can work the problem through to completion?

Am I right thinking this is a new installation and it has not operated at 1486rpm?

You said

Motor was operated by limiting the mA signal following was the outcome:

At 12mA signal

At 12mA signal

Motor RPM = 750

Output Volts: 200

Amperes Drawn 265

Was this driving the fan? Amps drawn is what you'd expect, as when Hz and volts are reduced, power is reduced (all in proportion), and amps is constant. So increasing Hz to 30, volts to 240 is OK provided power is constant at 75kW (Hz ratio x nameplate kW). But if the fan draws 75kW at 750rpm it will draw a lot more at 900rpm, likely to be up by 1.2³ for an all-friction system. What does the fan actually draw at 900rpm?

I don't know offhand whether it's OK (for the motor) to keep (to a point) constant volts at reduced Hz, and hence constant amps and kW. Maybe somebody (or the motor manufacturer) can comment. If so that would be the answer, and I'm pretty sure the VFD can be set up that way.

Yes It is a new Installation and Has not operated at 1486rpm.

Was this driving the fan?

yes it was driving the fan. and Ampere drawn at this frequency that is 25Hz was not expected by me. We have not yet operated the fan at 900RPM, I think it will increase the current at 900 RPM havnt done practical though.

It seems bizarre that the new design has oversized the fan, and that this Band-aid approach to fixing it is in any way necessary.

But will it then have enough power to drive the fan? We need feedback from OP.

The motor is designed for receiving a specific ratio of voltage and frequency, has exceptions. If you run at > 50Hz, constant volts, constant power, the ratio falls.

Also Lenze have a scheme for getting more than nominal power from a motor. On a 400V supply, use a motor wound 400V Y, but connect it Δ and set up the VFD to give 240V at 50Hz, all good for rated power. The VFD can give 400V, so can increase Hz and volts by up to √3 x and get √3 x the power.

Typically in a centrifugal fan, the motor power requirement changes at the cube if the speed. So at 60% speed, one would expect that the power requirement would drop to just 21.6%. However, you bring up a good point. IF this is a replacement motor for a fan that was ORIGINALLY designed for a 900 RPM motor, and the power rating of the New motor is the same as the old one, then yes indeed, it might be too small.in other words if the original motor was 100kW at 900RPM, and the new motor is rated 100kW at 1500 RPM, then at 900Rm (60% speed), it will only deliver 60kW, which would not fit the original design intent. So yes, I hope the OP thought of that in selecting the replacement motor. But your igut, we would really need to know more, as per usual in this forum...

Not sure what you meant by your second statement. A motor is designed for what it is, the DESIGN of the motor V/Hz ratio doesn't change. If you give it something that it was not designed for, there re consequences. If you are referring to applying a frequency that is ABOVE the base motor speed, then yes, you enter into a constant power mode, meaning that as speed increases, but voltage cannot increase woth it, power cannot increase. That then means that torque is DECREASING as speed increases, and at some point you can stall the motor if the load remains constant. But tht has nothing to do with what is going on here.

in the scenario you describe with the Lenze drive instructions, you are changing the voltage connection of the motor, thereby changing the V/Hz ratio of its design. You do that (with any VFD by the way) as a way to extend the speed range, providing the the motor can handle the added speed interns of deceased efficiency of any cooling fans and the design of the bearings. But because of the cubed speed /power rule I stated above, it is almost NEVER appropriate to do this on any centrifugal load, and again, not germane to the problem at hand.

Your first parragraph is pretty much a very good reason as to why...

The pullies should be setup so that the motor turning at its design full speed only delivers 900 RPM to the fan shaft.

Agreed, we need more info. Implication is that it's direct drive, as if it's a V-drive there'd be no need to get involved with a VFD, just get the ratio right and job done. But as North of 60 says, even if it is direct drive the best solution could be to change it, specially if there's no need for vari-speed.

2nd para - OK I take your point about design volts/Hz ratio. I just meant that on a 400V supply, with a motor wound for 400V, 50Hz, in operation above 50Hz the volts can't rise above 400, so ratio volts/Hz falls (as I'm sure you know).

That's the point I was making about the Lenze scheme. As you say, anybody could do it, but I first read about it in Lenze literature, and it seemed a neat idea. But it doesn't mean changing the V/Hz ratio of the motor design. Motor can be 400V Δ, 690V Y, or 400V Y, 230V Δ. So you choose 400V Y, 230V Δ, connect it in Δ, set up the VFD for 230V at 50Hz and the motor is quite happy, as it would be on a 230V supply. But as the mains supply is 400V, can turn the volts up with increasing Hz, giving higher power. Naturally, need to make sure the loads and design of the drive train are appropriate. As you say, it's probably not relevant to the current problem, unless he wants to get more power out of his motor.

Lots of Information gathererd by putting my question on this forum.

Both of the methods that I stated to limit the speed can be done but there after effects are not good for the Motor as experts said.

Proper solution to my problem is to arrrange a 6 pole motor, or to add a 3:2 ratio gear box or belt drive mechanism.

Now we know the fan draws 75kW at 750rpm we're getting somewhere! With current setup - 200V, 50Hz, 75kW, you can't just raise Hz as the power available increases as Hz, but fan power as Hz³.

Fan power at 900rpm = 75*(900/750)³ = 130kW, well within motor rating.

I said earlier I don't know offhand whether it's OK (for the motor) to keep constant volts at reduced Hz, and hence constant amps and kW. If so, that would retain direct drive and be the easiest option, but it's unlikely, as if it worked OK nobody would bother turning volts down at Hz < 50, might as well keep it constant and take advantage of the higher power available.

You've now said you can install a V-drive, and as others have said, that's the best option. Ratio about 900:1500 = 3:5. Then at fan speed 900rpm the motor is OK at 50Hz, actual current ~ 265*130/150 = 230A as power is < rated. Actual fan speed slightly below 900rpm as motor speed 1486, but if it's critical can run at just over 50Hz as there is power in hand, or make the drive ratio slightly higher. Can turn the speed down no problem as fan power falls faster than motor available power.

Speed Variation is necessary but it is limited to 0-900 Rpm, Motor often operate at 700 to 800 RPM depending upon the setpoint

Try a poly vee drive(uk)-mechanical speed reducer.

By PIV drive speed could be varied according to your requirements

The Volts-per-Hertz (V/F) ratio must be maintained by the VFD in order to manipulate the motor RPM without damaging the motor windings from overcurrent and heating.

What you are experiencing is common issue with induction motors that are not designed to operate on a VFD.

It is imperative that the motor being used for the application is rated for and/or capable of, being subjected to the VFD control.

If you are not sure the of motor's VFD capability, a call to the motor manufacturer before proceeding would be a good idea.

Also:

Most VFD manufacturers provide a default basic set of control parameters based on voltage, frequency, and horsepower/wattage ratings.

It is up to the end user to tune the VFD control parameters to fit the motor application and therefore avoid damage to the motor.

The Siemens field service technical group can provide expert support in tuning the drive over the phone, via email, or with an on-site service call.