VFD Motor Types for pump application ( Constant vs Variables)

[bmw318be]

I would like to know how we determine the application pf the pump is variable or constant torque ?

Under what circumstances that inverter duty motor os selected ?

Is standard VFD motor is capable to run under constant torque application ?

Is there a rule of thumb or guide to set motor range, assuming 4 poles ?

Eg: 4 poles at 50 Hz which is 1500 rpm is it capable to be run at below 1450 rpm or lower. How low it could be stretched ?

My understanding is that standard VFD is also capable of lowering the speed (50%) of 1450 rpm. Will the motor burn if using the stdrd VFD to drive at lower speed and high torque ?

I encountered a formula : rpm =120 x hz / pole. DOes this formula determine the minimum the motor could run assuming the Frequency is 30-50 Hz.


Does the VFD vs inverter duty, inverter"s benifit exceeded standard VFD ?

 

[micalbrch]

Positive displacement pumps are pumps which require constant torque. Variable torque applications are typical centrifugal pump drives. "Constant torque" means a constant torque from nominal frequency (50 or 60 Hz) down to 5 Hz. Whether or not a standard 4 pole AC motor is suitable for VFD control and how far down depends upon the load. The heat which is generated by the motor is a the limiting factor. You can oversize the motor and slow it down to low speeds or you select a motor with a forced driven cooling fan. In general motor and VFD shall be properly and correctly sized because speed range, torque, altitude etc. are all factors which affect the size. Have a look at the attached Danfoss paper which gives a good overview about the different VFD applications.

 

[bmw318be]

Hi, yes indeed it is pd pump. Why calling it constant torque ? . My application is various chemical with viscosity varis from 1 to 500 cps and the temperqture is constantly changing.

Is this called a constant torque ?

If regardless any poles I could varies the frequency from 50 to 5 Hz, is that means the lowest speed is = 5 hz * 120 /4 = 150 rpm

I doubt it can be tuned down to this for standard vfd motor ?

Is all motor could be driven by VFD ?

 

[BigInch]

That's probably a reasonable application for VFD, but do not count on it operating over the whole range; maybe from 250 to 500. Depends on how head varies, not viscosity or temperature, except as how those affect head. In extreme circumstances, 1300 cP at 20C and 100 cP at 90C, I've had beter performance by switching from series to parallel operation.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[hydtools]

A positive displacement pump is not constant torque unless the output pressure is constant. Otherwise it is variable torque as the output pressure varies.

Ted

 

[Oldhydroman]

I can't resist being super-pedantic here; a positive displacement pump also requires the specific capacity (cubic inches per revolution etc.) to be constant as well as the "differential" pressure to be constant in order for the torque to be constant.

Except the torque does actually vary with speed (because of increased mechanical and fluid friction losses at higher speeds) and it also varies with viscosity (again because of increased churning losses with the thicker fluids).

By the way Ted, can I take this opportunity to wish you and yours a happy Christmas and a peaceful new year. I've very much enjoyed contributing to the same questions as you - but you usually get there first and beat me hands down on brevity.

DOL

 

[hydtools]

DOL, Merry Christmas and a Happy New Year to you and yours, also.
I enjoy and appreciate your more detailed responses.

Ted

 

[bmw318be]

Hi, as far as I knoe the diff Pressure of PD pump when pumping different types of liquid is that viscosity varies,at different Temp, would we still get constant torque ?

Would motor manufacture specify the stdrd VFD frequency ? Assuming it could be VFD driven at 30-50 Hz.

Am i right to say, i cant drop the motor below 30 Hz ?

 

[ScottyUK]

The constant torque v's variable torque thing is probably some marketing guy's attempt to succinctly differentiate between centrifugal fan / pump loads and 'the other types'. It's certainly not a precise engineering definition as you've all clearly stated. The difference is really down to how much overload capability is built into the drive: a 'quadratic' drive might have only 10% overload capability, where a 'standard' drive will probably have 50% overload capability. In both cases the overload can be sustained for only a short period.

You can, subject to the limitations of the driven pump, run the motor down to a virtual standstill. To do this you'd need a separately cooled motor. In small sizes these are typically provided with a small fan mounted on the NDE cowling and which runs from a permanent auxiliary supply. To know the lower speed limit for a standard motor you would need to know details of the load, and then consult the motor manufacturer, not the VFD manufacturer.

 

[BigInch]

http://www.eng-tips.com/viewthread.cfm?qid=328365

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[bmw318be]

Hi scotryuk, I agree with your statement, motor stdr vfd could be overloaded by 50% and how do we determine the period of overloading the motor assuming 50% lesser than the 1450 rpm for 4 pole, could i drive it at 960 for few hours
Operation?.

 

[micalbrch]

How far you can slow down the motor depends upopn the load and the motor size. The motor manufacturer can tell you that. A 1:10 turndown is not a problem for a motor with forced driven cooling fan. Without a 50 % speed reduction should still be o.k.

Regardless what other valued members say I still say that you should look for a constant torque drive for a PD pump. The terms "constant torque" and "variable torque" are related to the drives and have something to do with the speed torque characteristic which can vary with the square of speed.

 

[bmw318be]

Hi mical, variable torque in pd pump seems ambiguous, could anyone gve me an example where pd us used in variable torque application.

 

[ScottyUK]

micalbrch,

The drives are no different - it's a sales 'trick' to allow you to buy, say, an 11kW drive and use it to run a 15kW centrifugal pump. The drive can do this by giving up most of its overload margin, in the reasonable assumption that a centrifugal pump or fan load is not going to suddenly increase. The drive is not built differently - the 'quadratic' drives are just working a lot closer to the edge of their safe operating area. Some drives are dual-rated, one being the continuous capability with only 10% overload capability and the other being the continuous capability with 50% overload capability.

Merry Christmas!

 

[micalbrch]

Got it Scotty! I did not read your first post thoroughly.

 

[hydtools]

This link may help.

http://www.motorsanddrives.com/cowern/motorterms7.html

Constant torque means the torque required by the load is independent of speed. The particular torque for a particular fluid would be constant over a range of speeds. Change the fluid and the required torque may change but remain at the new constant at various speeds.


Ted

 

[bmw318be]

Merry chrismas and Happy New year and thanks for the contribution to this threads !