VFD & AC motor RPM question

[hub]

Hi,
I think I have this figured out correctly, but I'm not sure..

So, my motor is rated 50Hz, 2850 rpm.
My VFD has 0-120Hz.

If I set my VFD at 100Hz, that would double the rated RPM to 5700 rpm. Is that correct?
And the max rpm (at 120Hz) would then be 6840rpm?

Please confirm me right or wrong I need to know the rpm based on VFD Hz setting.

Thanks,
Hub

[Al_The_Man]

Basically correct.
2850rpm would indicate you have a 2 pole motor, the rpm of a AC induction motor is 50hzx60(sec)/pole pairs.
you have a 2 pole (1pair) motor. The number of 2850 and not 3000 is because of a thing called slip where the rotor can never run at the same rpm as the rotating field.
You may be exceeding the rpm the motor is capable of mechanically however, as I believe a motor manufacturer, especially a non-vector rated motor, may design for the highest rpm, i.e. 2 pole.
The 4 pole motor being half the rpm of a 2 pole can usually be ran a twice rated rpm.
Running the higher 2 pole at 120hz may exceed the bearing and rotor balance extremes.
Some thing to keep in mind.
Al.

[hub]

Thanks Al!

It runs fine at 120Hz. Stable rpm, no apparent problems (this is by ear/feel, I haven't measured anything)

But yes, I don't know what bearings it has & what rpm they will take. But we have used these kind of motors (where I work) connected to VFDs for years and never had a motor bearing fail so far. Other shaft bearings have failed, belts too etc. But never a motor bearing so far. The motors are mostly running 60-80% of VFD max frequency. Most are used pretty much 24/7.

[mike_Kilroy]

Please tell us the brand & model of motor that works so well!!!!!!

[hub]

We use a few different models and a couple of brands..
But the one I'm now testing is a 3phase ABB 1.1kW.

I found some guides from the manufacturer regarding speed:
(My motor is size "80" so it should run well up to 6000rpm according to specs)

High speed operation
In a frequency converter drive the actual speed of the motor may deviate
considerably from its rated speed. For higher speed operation, the maximum
permissible speed of the motor type - or critical speed of the entire equipment
must not be exceeded.

Guideline values for the maximum
speeds of standard motors:
Frame size Speed r/min
71-100 6000
112-200 4500
225-280 3600
315, 2-pole 3600
315, other pole numbers 2300
355, 2-pole 3000
355, other pole numbers 2000
400, 2-pole 3600
400, other pole numbers 1800
450, 2-pole 3600
450, other pole numbers 1800

When high speed operation exceeds the nominal speed of the motor, the
maximum torque and bearing construction should also be checked.

Maximum torque
In the field weakening area, the voltage of the motor is constant, but the motor
flux and the capability to produce torque reduces rapidly when the frequency is
increased. At the highest speed point (or at any other duty point in the field
weakening area), the maximum (breakdown) torque must be not less than 40 per
cent higher than the load torque.
If filters or additional reactances are used between the converter and the motor, the
voltage drop of the fundamental voltage with full load current must be taken into
account.

Bearing construction
There is a limit to the speed at which rolling bearings can be operated. Bearing type
and size, internal design, load, lubrication and cooling conditions, plus cage design,
accuracy and internal clearance, all influence the permissible maximum speed.

[hub]

Originally Posted by mike_Kilroy Please tell us the brand & model of motor that works so well!!!!!!

I had to look up the type we use most:
BROOK crompton 0.18kW U-DA63SG-4+B14A/FT75

[mike_Kilroy]

These are typical max speeds most motor mfgrs allow today. When you said you are running 6800rpm I thought it was a larger frame motor than that. I have an application right now for 15hp@1750rpm with 5500rpm max needed and so far no one has offered a motor that can do it other than Siemens or Indramat - with high cost and LONG delivery times. Baldor advertises they can go upto 11000rpm but that is a special and it is turning out to take them 2 weeks to get it even quoted. I'm always on the lookout for a source of high speed 5-30hp motors at a reasonable price and quick delivery. I will check out ABB closer. Thanks.

[hub]

Yes, the 0.18kW one is tiny! And the ABB 1.1kW is a lot bigger, but still in the mini league & easily lifted by hand.

I tried to find an AC 15hp that would run @ 5500rpm. No luck, looking at standard motors. Yes, ABB is worth checking out. I know ABB has very high quality on their stuff.

From the ABB guide (I attached it for reading):

"In general, the limit is set by the operating temperature with respect to the lubricant
and bearing component. Changing the bearings and/or lubricant may enable higher
speeds. However, if this is done, the combination should be checked by ABB."

"The balancing accuracy and mechanical strength of all rotating parts should be
checked if the standard motor speed limit is to be exceeded. All other parts
mounted on the motor shaft, such as coupling halves and pulleys must also be
carefully balanced."

"All rubbing shaft seals (V-rings, oil seals, sealed bearings RS, etc.) have a
recommended maximum speed limit. If this is below the proposed high speed
operation, non-rubbing labyrinth seals should be used."

"In the field weakening area, the voltage of the motor is constant, but the motor
flux and the capability to produce torque reduces rapidly when the frequency is
increased. At the highest speed point (or at any other duty point in the field
weakening area), the maximum (breakdown) torque must be not less than 40 per
cent higher than the load torque."

So, basically they are saying, if you want high rpm: change the bearings, change seals and balance it. And you will loose some torque at high rpm. IF I understood it correctly.

[keebler303]

Originally Posted by mike_Kilroy These are typical max speeds most motor mfgrs allow today. When you said you are running 6800rpm I thought it was a larger frame motor than that. I have an application right now for 15hp@1750rpm with 5500rpm max needed and so far no one has offered a motor that can do it other than Siemens or Indramat - with high cost and LONG delivery times. Baldor advertises they can go upto 11000rpm but that is a special and it is turning out to take them 2 weeks to get it even quoted. I'm always on the lookout for a source of high speed 5-30hp motors at a reasonable price and quick delivery. I will check out ABB closer. Thanks.

I bought a 20HP Baldor Vector Duty motor for a project here a couple years back. The standard product has max speed of 5000 RPM and runs just fine at that speed for us.

Matt

[mike_Kilroy]

I think key word here is "couple years back:" Baldor app engineer yesterday told me point blank they have NO 7.5 or 15 or 30hp motors, Ac OR Dc (servo) that can run 5500rpm or close to it today.

[keebler303]

This is the exact motor I bought: Products: AC Motors: ZDM2334T: Baldor Electric Company, a leader in energy efficient electric motors, linear motors and adjustable speed drives industry

1765 RPM rated 5000 RPM max, says right on the name plate.

Matt

Edit: Seems most of their Vector duty motors are rated 5k max, think the apps guy might have been blowing smoke. New guy who goes by the rated speed only??

http://baldor.com/products/specs.asp...g=40CMB%2DCONT

[mike_Kilroy]

funny how you cant rely on the specialists at some companies.

thanks! that listing showed 1/2 dozen possible choices we can add encoder to to do the job!