1500 RPM Motor using 50 HZ motor with VFD driven by 60 Hz

[Schnaubelt]

Basic issue is that I have 1500 RPM loads that are direct drive all the time. I would love to not derate motors by being so far off rated rpm. So can I use a 50 HZ motor driven by 60 HZ and a VFD? Looking at Direct Driven loads matched with motors; in the US for 60 HZ we have 900, 1200, 1800 RPM motors in the USA. But there is a gap at the 1500 RPM range that I want to fill. Maybe with a 50 HZ motor and VFD. Thoughts? Thanks in advance.

 

[waross]

A belt or drive will be much cheaper and more efficient.
Driving a 60 Hz motor at 50 Hz will be the same torque times less speed resulting in 83% of the HP.
With an 1800 RPM motor driven at 1500 RPM, cooling is only an issue if the motor is continuously at or near 100% HP at or near 100% of maximum rated ambient temperature.
That is a fairly small window, more likely to be encountered in the tropics than in the Snow Belt.

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

 

[crshears]

If I understand the question clearly, you wish to drive a 50 Hz motor that normally rotates just below 1500 rpm with a VFD supplied by 60 Hz power. I'd say yes, you can do this; but the V in VFD stands for variable, and if all you're ever going to be looking for is a steady 50 Hz supply there must be a far cheaper static frequency converter you can get.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[itsmoked]

Probably not CR.. Due to volume. If this is a sub 2hp task you could probably find $50 VFDs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

Fair point, Keith; then again, OP never stated number or total kVA of motors to be driven...

One of numerous possible scenarios is a European vessel in North American ports for an extended period of time...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[jraef]

The 60Hz (or any frequency) input to the VFD is insignificant in this; the input just gets rectified to DC. The thing that matters is the input voltage and the voltage rating of the motor. A VFD can artificially lower the voltage, but it cannot increase it (leaving out AFEs for keeping things simple).

Once you have a motor and a VFD, you can run the motor at virtually any RPM, so just get a motor that is easily available where you are that matches your available source voltage. Using a 50Hz motor in a 60Hz country will result in not being able to get a replacement in a reasonable time frame.

All that said, I agree that using a VFD to just permanently run at one speed is not usually a cost effective way to get what you want compared to simpler means like belts and pulley or gear ratios.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Schnaubelt]

Thank you for your comments. Here are some more details behind my initial question.

So I am working on an application right now that is not uncommon in the Heating Vent Cooling Industry. I have an Air Handler with (3) 150 HP motors. It will in fact by a Variable Speed load with the fans that the motors are driving. The whole fan industry is converting to Direct Drive Fans, as they are more efficient and no one wants to maintain belts and sheaves as the cost of VFDs come done. Also the 90.1 energy code is really demanding that fans be variable speed always in building applications.

So that is my background application. I am a mechanical engineer now that is being thrust into electrical application of these direct driven fans with VFDs. My fans are usually most efficient and quietest around the 1500 RPM range. I have come to the understanding of the # poles and the Hz of the utility effect the speed of the motor. Also recently learned that I have to derate my motor 1500/1800 in HP, so a 150 HP motor is really a 125. That really stinks! and there are a ton of poeple installing these motors in the mechanical world that are missing this completely.

So my goal is direct drive, 1500 RPM variable loads, that I don't want to have to derate. It seemed then using the VFD with the 50 Hz motor is the right move?

 

[davidbeach]

Why go to a motor that will be nearly impossible to find quickly in a repair situation? If the 150HP motor can drive the fan at 1800rpm it will have no problem at all at 1500rpm.

 

[waross]

Read my lips:

Driving a 60 Hz motor at 50 Hz will be the same torque times less speed resulting in 83% of the HP.

Fact of life.
When a 60 Hz motor is operated at 50 Hz, the HP drops in the ratio of 50Hz/60Hz.
If you don't want to de-rate, then use belts or gears to drop the speed, or live with it.
By the way, if your fan needs 150 HP at 60 Hz, it will need less than 125 HP at 50 Hz.
But, if you size the motor below 125 HP for use at 50 Hz, you will not be able to go to 60 Hz.
If, in the future, someone re-sets the frequency to 60 Hz, the motor will quickly overheat.

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

 

[waross]

By the way, the main physical difference between a 125 HP, 50 Hz motor and a 150 HP, 60 Hp motor is the speed and the nameplate.
The motors may be physically interchangeable.
Some of us, over the years, have been called upon to convert motor driven machines from 50 Hz to 60 Hz and from 60 Hz to 50 Hz.
The 5/6 HP ratio is not so much written in stone as it is cast in copper and iron.
Some loads accept a frequency conversion, some do not.


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

 

[Schnaubelt]

OK thanks. If they don't really make the 50 Hz different than the 60 Hz, then it sounds like I can't get away from the 5/6 derate anyway, then I agree there is no reason to goto a hard to find motor.
My load is 119 HP at 47 Hz, so I am ok, just not alot of safety factor which is what led me to trying to understand this derate situation better.
Thanks again. Ed

 

[waross]

Your motor will be overloaded by about 1.5 HP.
As long as you are not in the tropics in the summer it should be fine.
Even at maximum allowable ambient temperature the expected life of the motor may be slightly reduced.
A 150 HP @60 Hz motor with a Service factor of 1.1 or 1.15 would be safe and allow the occasional short time overload.

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