Single phase motor Torque / Speed Curve.

[Pumpsonly]

We need to use a single phase motor for a magnetic coupled centrifugal pump instead of the normal 3 phase.
The supplier is concerned about the starting / run up torque, if too high can de-coupled the magnet during the run up.
But can not get the motor supplier to provide the torque / speed curve.
Is the torque / speed curve for a single phase motor much different than a 3 phase?
The motor is only 1.1kW. 4 pole 50Hz.

 

[waross]

How about a single phase VFD driving a three phase motor. Then you can set a safe torque limit or a safe acceleration ramp.

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

 

[ScottyUK]

Depends on the type of single-phase motor. A split phase motor has fairly low acceleration torque. Capacitor start / induction run motors have more torque, and capacitor start / capacitor run type have more again.

Single-phase motors are generally unfavourable compared to their 3-phase cousins: larger, heavier and more expensive for a given output, more vibration, less reliable. Using one to overcome a basic design problem seems a poor idea.

 

[waross]

I wouldn't be concerned with the starting torque for at least two reasons.
1> Starting torque is also known as locked rotor torque. Centrifugal pumps have an inverse square speed torque relationship. At slow speeds and when starting from rest, the pumping load of a centrifugal pump is negligible. The major load is the inertia load.
2> The starting torque is generally less (possibly 1/2 or less) than the pull-out or breakdown torque.

You should be concerned with the breakdown torque. This typically occurs at about 80% or 85% of synchronous speed in a design B motor. (The most common torque curve of induction motors.)
What does this mean?
As an example:
The starting torque may be about 150% when the pumping load is near zero. Pump friction and pump inertia are the major concerns.
The breakdown torque may be about 200% at about 85% of synchronous speed. The pumping load may be about 70% to 75%. The motor torque may be about 200%. Inertia may add to the load on the coupling.
Rant-on
Starting torque or breakdown torque. In the present context it may (or may not) be obvious that we want to know the maximum torque.
No problem if we neglect the pumping load.
Little errors such as this are of no concern to the lower case group.
But, when you ask the sales rep for the starting torque, or if you ask the sales rep for the maximum starting torque and he asks tech support for the starting torque you may get a figure which is not related to your problem and which is seriously low.
I make mistakes, ( I am still embarrassed about the time I sloppily listed the capacity of a generator as milli-Watts mw instead of in mega-Watts MW.) but I try and I do understand the need for accuracy in the presentation of values.
Rant-off.
Use a single phase VFD and a three phase motor and program a suitable acceleration curve.


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

 

[waross]

For some good general information on motors see the Cowern Papers.

http://www.baldor.com/pdf/manuals/PR2525.pdf

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

 

[jraef]

... But can not get the motor supplier to provide the torque / speed curve...

You need a better motor supplier by the way... There is no way this info does not exist (unless the motors are counterfeit or clones or brand-labeled so they don't want to expose that by showing you data that has the real name on it). More likely though, your supplier is just being too lazy about tracking down that information.

"Will work for (the memory of) salami"

 

[Compositepro]

The concern about starting torque with a magnetic coupling is valid. It works similarly to a stepper motor. When the torque rating is exceeded, it will decouple and after that the net torque transmitted is zero. You have to stop the motor get recoupled.
A vfd does seem a good option.

 

[Brad1979]

"milli-Watts mw"

Actually, the correct abbreviation is mW.

 

[waross]

I said I was embarrassed. grin

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

 

[Brad1979]

And I was just pointing out that mw is neither megawatts nor, as you said, milliwatts.

 

[electricpete]

Aside from transient torque during starting, steady state conditions may be something to think about.

During steady state load conditions, the electromagnetic (em) torque developed by a 3-phase induction motor is constant, while the em torque developed by a single phase induction motor in contrast oscillates at twice line frequency (2*LF). Specifically, every 16.7 ms the torque goes to zero twice and the torque reaches a peak twice (for 60 Hz power). The peak electromagnetic torque reached is twice the average (load) torque (!).

What fraction of this oscillation is seen at the coupling will depend on the torsional inertias and torsional spring constants of the system. For example: if motor inertia very large and load inertia very small and the coupling acts torsionally rigid at the frequency of interest, then the motor inertia blocks the torque oscillations from being transmitted to the coupling. On the other extreme: near torsional resonance the 2*LF oscillating component of transmitted coupling torque can be many times the 2*LF component of em torque (which was already twice the average load torque).

For anyone who feels the urge to correct my abbreviations, grammar, spelling etc in a condescending manner: please consider redirecting your energies to the grammar forum unless the meaning is unclear.


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(2B)+(2B)' ?

 

[electricpete]

Brad - rest assured my comment was not directed to you.

=====================================
(2B)+(2B)' ?

 

[Brad1979]

Good. I was being facetious but there is always the worry that people won't realize that. Anyway, I hope people realize that yes, you need to be clear and precise in your communication with others but being fastidious with respect to spelling/grammar/etc. can be just as bad for communication as being unclear.

 

[Pumpsonly]

electripete , thank you for your very insight info on the torque variation.