87Hz Delta Motor 2

[elinBG]

Hello!

Last week in our workshop came a motor with a "strange" nameplate (attached you will find a photo of it).

I will be glad if someone wants to discuss this "technique"

 

[elinBG]

http://www.lenze-actech.eu/Portals/45/Documents/LenzeDS22-en.pdf

http://cache.automation.siemens.com...rating_motors_at_higher_frequencies_V2_en.pdf

 

[LionelHutz]

It's just maintaining the V/Hz ratio on the motor. 230V/50Hz = 400V/87Hz

In theory, the power could go up to 90kW * 87/50 = 157kW but the special application this motor was rated for likely only required 106kW @ 2600rpm.

 

[elinBG]

When I read the 2 links from Lenze and Siemens, it is now much clearer.

And the most important and main conclusion which I made is that:
"frequency inverter can increase the frequency above 50Hz, but cannot increase the voltage above 400V" And that's why the 87Hz's used - to maintain constant V/f and torque

Is this correct?

 

[Skogsgurra]

This is standard practice. You get more power out of the motor if you run it faster.
For some reason, some people regard this as "cheating". I have asked them a few times why they think so. Never got any good answer. I guess it is the old saying about free lunches that influence them. Engineering knowledge it is not.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

It does trade bearing life for increased power, but that may be an acceptable compromise if space or weight is a factor. No such thing as a free lunch!

 

[ozmosis]

Yes, you are correct.
However, usually, the main benefit is when used in conjunction with a gearbox as it provides a more cost effective means of producing the same amount of torque but using a lower cost unit by having the ability to increase the gear ratio(hence increase the output torque)with a reduced size motor.

It is only possible if you have a dual rated motor, the voltage available and a VFD to achieve 87hz.
It is very often used on motors sub 7.5kw where it is typical to have 230/400 D/Y windings and common to have 400V 3ph available. Usually, above 7.5kw, motors are typically wound 400/690v D/Y but not that common a 690v supply is available. If it was, in theory you could connect 690v and connect in delta and get the same benefits, assuming you have a VFD rated for the voltage.
It's all about saving money by getting more out of your motor than 'normal' but, you will only save money if you already have a VFD. If you don't (I.e. You do not need variable speed), it is an expensive way to get more power out of a motor.

 

[Skogsgurra]

I have a different view of the bearing life thing.
VFD=>EDM
Low speed=thin oil layer=more EDM=short bearing life
High speed=thick oil layer=less EDM=long bearing life
So, there are free lunches.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I have heard rumors of air compressor skid manufacturers using 230:460 Volt motors on the 230 Volt connection and supplying them with 120 Hz, 480 Volts to double the HP of the motor. If the customer wants a VFD, why not take advantage and cut the motor size in half?
When I first read the post I thought of LeTourneau, but he used 72 Hz. so that he could run his diesel engines at the more powerful 2160 RPM compared to the 1800 RPM required for 60 Hz.

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

 

[controlsdude]

To me this is just asking the mechanical engineer if what is attached to the motor can take the increased speed.
Generally 90 hz is the limit everytime I ask mechanical.

 

[elinBG]

more on this thread...

Today I was thinking...
What will happen if I calculate and rewind a 50Hz motor which was designed for 400V/Delta to 100V/Delta.
Does it mean that I can run it up to 400V/200Hz?
I will have the same torque (V/HZ ratio) up to 200Hz ==> so I will increase the power 4 times?

...so what will happen if I rewind it to 40V?

Where is the border?

http://www.eldvigateli.com

 

[mikekilroy]

ElinBG, yes, you get 4x the power - at 4x the current required too.

HIgh speed machine tool spindle motor mfgrs play this game all the time. What's the low end limit? Physics and what you want from the motor. Lowering the voltage at typical 1500 or 1800rpm speed allows you go higher voltage at higher speeds; typically this is only done when you need a wide constant HP range; by winding for lower voltage, you can play a game and continue to increase to the available voltage and thus extend the constant HP range. We have many motors rated like this one and we often call them 1500rpm base speed at the 90kw rating yet let the v/hz continue up to the 87 or higher hz to then have even higher constant 90kw hp range. Remember that as you go up in speed in constant hp range you eventually run into the max torque the motor can put out (breakdown torque). So by artificially playing with the voltage like this it lets a motor designer run that 90kw motor up to way higher speeds before they hit the breakdown torque.

Where is the low voltage limit? Application dependent. Of course the lower the voltage the higher the current and thus the more expensive the drive to run it also. So to lower the voltage without a reason (wide constant hp) is just a waste of money.

I think a potential clarifications should be made on the free lunch discussed here tho....

There is indeed no free lunch on the higher speeds due to bearing life: this is why the generic ac motor mfgrs generally limit their speeds to 90Hz max: bearings. Sure, you will get the argument thrown back at you "but their 2 pole motor goes to 3600rpm - equiv of 120hz in same 4 pole size..." Yes, BUT: if you need the 120hz on the 4 pole motor you will find the mfgr generally makes a special part number - why? they put in the 2 pole motor higher speed rated bearings. A lot of people will argue this point, but it is often a fact.

When we rate a spindle motor to 10,000 or 12,000rpm, guess what? The bearings go from $ 25.00 each to $ 500.00 each. No free lunch.

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

What will happen if I calculate and rewind a 50Hz motor which was designed for 400V/Delta to 100V/Delta.
Does it mean that I can run it up to 400V/200Hz?
I will have the same torque (V/HZ ratio) up to 200Hz ==> so I will increase the power 4 times?


Should temper my previous reply of yes 4x.... A given motor size can produce a given power; so if you do nothing other than change the voltage, that same size motor will still only produce the same power as before, just at a lower voltage.

Since you did not change the number of poles, it will produce that same power at the same 'base' speed.

Example: say you have a 400v 4 pole (1800rpm @@ 60hz) 50 amp motor rated 40hp (120#-ft). If you put this on a vfd 400v drive, assuming mechanics could handle it, you can probably continue the freq up to about 140hz before you hit the breakdown torque limit (v^2 reduction limit).

So now you take out windings, rewind so now it is a '100v' motor. It is still 4 pole, so now at 60hz (1800rpm) it will want 100v and thus 200amps to produce SAME power (& 120#-ft) as before you rewound it. You just changed volts for amps.

But since you have 400 v AVAILABLE, You can continue increasing the voltage up 4x higher as you increase the frequency. Means you stay on v/hz curve and do NOT run into breakdown torque motor limit all the way to 200hz, and then can continue to increase freq at 400v limit up to breakdown limit (probably around 400hz. So you got 4x the HP up to 200hz then can continue in constant HP range up another 200+ hz.

So it did not get you any more torque, just more speed doing this.

http://www.KilroyWasHere<dot>com

 

[waross]

I think that you may be a little off on the breakdown torque. This is related to the current and the Amp turns.
If a motor is wound for 40 Volts, and then the voltage and the frequency are increased in the same ratio the torque will stay the same as the HP increases.
If anyone wants to try pushing a motor I suggest starting with a NEMA 230/460 Volt motor. These are typically connected in star. If you are able to locate and break the star point you will have six windings (two per phase) rated for 230V/√3 = 133 Volts, or 2.2 Volts per Hz.
With the star connection changed to delta and 465 Volts and 210 Hz the motor will theoretically put out the same torque at something over 6000 RPM and 3.5 times the HP.
There are a host of second order effects that will have varying effect. The fan will probably be starved for air and not following the basic fan laws.
If someone wants to do some hands on experimenting this may fill the bill a lot easier than rewinding a motor for a lower voltage.

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

 

[mikekilroy]

I did not consider changing the std 230/460v Y motor to D! Worth a star for that thought! We have a scheme where a few times per year we take various high performance Reliance 230v only spindle motors and go the OTHER WAY: We show the local motor shop, near the machine we are retrofitting with new 460v spindle drive, how to reconfig those 4 windings per phase (in these cases) to double its voltage to 460v. Let's us cut the spindle drive size in half, which is good for various reasons.

http://www.KilroyWasHere<dot>com

 

[LionelHutz]

To answer your questions today, yes in theory it can work for speed you wanted. In practice, things like the rotor exploding or the motor/load bearings or other parts of the system not handling the high speed will limit your speed.

Think RC motors - the brushless controllers and motors do more or less the same thing and they run 50k rpm. They get a lot of power for a motor whose weight is measured in grams.