variable speed ac motors

A variac simply won't work for your application! You'll notice little to no response in speed variation when turning down the voltage, up until a certain point where the motor speed suddenly drops very rapidly. In some cases, you could get the motor running steadily at the speed you want, but when switching the pump off and back on, the motor just won't start: You would have to reset the variac to the normal voltage to get the motor start again.

For a while I've been experimenting with single phase 0.37kW motors (230V) on a 0.75kW Hitachi J50 VFD (230V single phase input, 230V three phase output):
I got to the point where I connected all three leads (U1+Z1, U2, Z2) of the motor WITHOUT the running capacitor to the three outputs of the VFD, and cut off the starter winding (Z2) at 5Hz or more, using the inverted 'RUN' signal from the VFD and a solid state. And this worked surprisingly well! But to be honest, this is way too much trouble.

If it's flow control you want, why not use an electrical valve? If it's a standard centrifugal (so NOT a positive) pump, you won't hurt it by closing or opening its output. In fact, closing the pump's outlet will reduce the motor current.

BTW, adjusting the pump's speed has a serious impact on water pressure, and this might become an issue...
 
Three possible solutions

Concerning, the connection of all three leads (U1+Z1, U2, Z2) of the motor...to the three outputs of the VFD, can you send a sketch of the wiring diagram?



On the other hand, the pump is a self priming electropump. And we need steady flow proportional to the production speed to avoid too much liquid waste. But I will discuss the valve issue with the mechanical engineer…



On the 3rd hand, I contacted our Siemens agent, and he indicated that we could use the Micromaster mm440 for our application (but it is not system tested, so no guarantee). I made a sketch for the wiring taking into consideration the protection of the motor using an lc filter and a transformer. What do you think of this? Will it work? (see attached)


When I have a bottleneck in my project, I like to have many plans for the solution…because the devil is in the details…
 
In an industrial setting, do not take shortcuts. Replace the motor and use whatever brand VFD (VSD etc) you are comfortable with.

There are many things that CAN be done, but few that should be done because shelves are full of products tested and known to do what you need done.
 
Kataeb said:
Concerning, the connection of all three leads (U1+Z1, U2, Z2) of the motor...to the three outputs of the VFD, can you send a sketch of the wiring diagram?



On the other hand, the pump is a self priming electropump. And we need steady flow proportional to the production speed to avoid too much liquid waste. But I will discuss the valve issue with the mechanical engineer…



On the 3rd hand, I contacted our Siemens agent, and he indicated that we could use the Micromaster mm440 for our application (but it is not system tested, so no guarantee). I made a sketch for the wiring taking into consideration the protection of the motor using an lc filter and a transformer. What do you think of this? Will it work? (see attached)


When I have a bottleneck in my project, I like to have many plans for the solution…because the devil is in the details…

That's almost funny, for heaven's sake spend the money on a 3 phase motor and save yourself a giant headache.
 
To the non believers, this is what I tried:

SinglePhaseMotorOnA3PhaseVFD.JPG


The inverted RUN signal from the VFD, with an RC delay of approx 0.3s, was used to disconnect the starter winding to avoid overcurrent in the starter winding.

To Kataeb:
Like I said before, it's quite a hassle... and in most cases, not worth your time! IF you're able to replace the single phase motor by a 3 phase type, then PLEASE DO! But most likely, this pump is a one piece diecast, so you're stuck with this assembly. Nevertheless, I strongly suggest using the (proportional) valve option! It will allow you to have total control over water flow. Water pressure might, and probably WILL, become an issue when changing the pump's speed.

I also took a look at your PDF... You do not need a transformer between the MM440 and the motor! All settings, including rated motor voltage, can be set on the MM440. If you're still thinking of going ahead with the VFD, you should be aware that a 0.75kW-230V single phase motor will consume around 5A at full load, you would practically need a 1.5kW-230V VFD to be able to deliver that current. It would not only become a matter of time and effort, it would also have a (serious) price tag. Anyhow, the valve still seems to be the right way to go...

To RSDoran:
No offense, but the fact that people tell me not to bother, just gives me that extra push to try it anyway, rather than taking an off the shelf solution. Although I have to admit, you're totally correct about not taking shortcuts in an industrial setting.
 
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Due to the pressure issue, we will finally opt for the valve solution (our mechanics will have more work to do!).



But, as our projects are either qualified as urgent or very urgent, and we might encounter in the future the case of speed variation of single phase motors, I just want to have more info on this subject.

Sparkz, concerning your circuit, will any damage happen if while running, the vfd tripped a fault (because the current will be automatically disconnected)?

On the other hand, in my circuit I added a transformer to use a vfd with lower power (reaching on its output 400v) to reduce the overall cost.



Thank you

 
Kataeb said:
Due to the pressure issue, we will finally opt for the valve solution (our mechanics will have more work to do!).
Wise decision, Kataeb! (Sigh!)

Sparkz, concerning your circuit, will any damage happen if while running, the vfd tripped a fault (because the current will be automatically disconnected)?
Not really sure if I understand your question correctly... Do you mean what happens if the starter winding is disconnected while running? Disconnecting a motor from a VFD should be done when the output of the VFD is not active. When disconnecting while running, the spark (read: high voltage) over any regular contact might and eventually will destroy the VFD.

That's the reason why I used a Solid State Relay in the first place: An SSR (or any thyristor for that matter) will 'open' as soon as its input is deactivated, BUT NOT before the current through the SSR drops below a certain cut-off point! This threshold is so low, that there's practically no energy left in the starter winding to create a dangerous voltage. Thus no damage to the VFD, ever.

On the other hand, in my circuit I added a transformer to use a vfd with lower power (reaching on its output 400v) to reduce the overall cost.
I noticed that you only used 2 of the 3 VFD outputs. The VFD's output capacity would still have to be 1.73 (square root of 3) times higher than the motor's rated power, since all power has to be delivered over just 2 instead of all 3 outputs. So, a 0.75kW-1Ph motor needs a 1.3kW-3Ph VFD. But there's something else why a single phase transformer won't help...

The reason why I use all three outputs of the VFD is TORQUE! If you start a single phase motor (leaving the starter capacitor in place) using just 2 VFD outputs, you will have practically NO torque below 30Hz. If you connect the motor like I suggested, you will have maximum torque even at 10Hz!

Ideal would be to only disconnect the starter winding at 25Hz and up, but I do not know of any brand VFD that has such a configurable signal output. Most likely, the 'analog frequency monitor' output (basically on any VFD, I think) can be used, along with a threshold switch, to activate/deactivate the SSR.

edit: In my application the motor had to run at approx 75Hz. All I needed was to make sure the starter winding got disconnected in time, so I just used a small RC delay circuit.

Knock yourself out! - ;o)
 
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The vfd that we are using, turn off their power transistors automatically when a fault occurs on the drive. So the current will be interrupt suddenly, wouldn't it?


In your circuit, the starter winding is supplied as long as the motor is running and for 0.3s after its stoppage, correct?
 
Kataeb said:
The vfd that we are using, turn off their power transistors automatically when a fault occurs on the drive. So the current will be interrupt suddenly, wouldn't it?
I don't see why this could be a problem... The VFD also does this when a 3 phase motor is used, so why would it be any different with a single phase?

In your circuit, the starter winding is supplied as long as the motor is running and for 0.3s after its stoppage, correct?
NO, this is not correct! I used the inverted 'RUN' output signal of the VFD and just delayed that for about 0.3s. In my application (with the speed set to approx 75Hz and an acceleration time of 1s) the starter winding stayed connected until 0.3s after start, so the winding got disconnected above approx 25Hz.
 
permanent split capacitor motor control

Hi Sparkz,



I have one question: our motor is with permanent split capacitor (20 microfarads, 450 volts), so normally when this motor is directly supplied by 220vac, its starter winding will be supplied as well. I think that in this case we could use your circuit without the ssr (see picture)?







On the other hand, siemens technical people suggested the following:



to use on the vfd output a 3 Phase Delta to Star 200V/220V Transformer instead of the single Phase one (the Phase to Phase voltage after filter is only 200V not 380V), this will symmetry the inverter load and compensate the lost of the filter as well as transient peak voltage for motor winding and permanent split capacitor. This configuration works with several customers’ applications The MICROMASTER 420 and 440 ore SINAMICS G110 can be used with single-phase motors.



What do you think about this?



Thanks







 
Kataeb said:
Hi Sparkz,
I have one question: our motor is with permanent split capacitor (20 microfarads, 450 volts), so normally when this motor is directly supplied by 220vac, its starter winding will be supplied as well. I think that in this case we could use your circuit without the ssr (see picture)
In order to get sufficient torque at lower rpm's the capacitor has to be removed and the starter winding must be fed directly with a third phase. The problem with any starter winding is that current through it has to be limited (normally by the capacitor), or at least limited in time (in my case by the solid state relay). You can leave the starter winding directly connected to the third phase up until approx 30...35Hz, resulting in maximum torque. At higher speeds the starter winding current would exceed its maximum value, so the winding has to be disconnected: At those higher speeds, torque would be acceptable/almost standard.

Kataeb said:
On the other hand, siemens technical people suggested the following:

to use on the vfd output a 3 Phase Delta to Star 200V/220V Transformer instead of the single Phase one (the Phase to Phase voltage after filter is only 200V not 380V), this will symmetry the inverter load and compensate the lost of the filter as well as transient peak voltage for motor winding and permanent split capacitor. This configuration works with several customers’ applications The MICROMASTER 420 and 440 ore SINAMICS G110 can be used with single-phase motors.

What do you think about this?
This is something I can't confirm. You could use a 3-phase 400/230V stepdown transformer to adjust the 400V output of a 1.5kW-400V VFD for a 230V 1-phase motor. But I'm starting to think you're trying to use a 0.75kW-400V VFD! I have serious doubts about just how symmetrical the load to the VFD will become with this transformer.
 
In the beginning I must have read something different or something wrong, not sure now. I could have sworn I read capacitor start.

Permanent Split Capacitor (PSC)
This design has an auxiliary winding with a "run" capacitor, but unlike the capacitor start / induction run motor, the capacitor and auxiliary winding remain in the circuit under running conditions. (There is no centrifugal switch on this type of motor). A permanent Split Capacitor motor has low starting torque and low starting current. PSC motors are generally used on direct-drive fans and blowers. They can also be designed for higher starting torque and intermittent applications, where rapid reversing is desired.

In this case you would not want to disconnect the capacitor in the ckt. I have no idea what would happen if you feed it with a 3rd phase from a VFD though.

The other aspect of using a VFD is attempting to use just 2 of the 3 phases, have not seen many that allow you not to maintain current equally on all 3 phases.

I wish I had the equipment to experiment with it.

If you are using permanent capacitor motors they make variable speed controllers for them: http://www.dartcontrols.com/products2.asp?Group=access

http://www.varidigm.com/products/prods_vsm.asp

These are examples so check with your local supplier(s)to see what is available. If using a VFD then I would get a supplier or the company to provide details and come in to set it up. Much of it may still depend on the type of PSC motor being used.

My earlier post had nothing to do with "can" it be done, its more of "should" it be done. When working for a company, especially industrial setting where time is valuable, it is more appropriate to use off the shelf items in case of future problems.
 
While I don't doubt that Sparkz's solution worked for Sparkz it is neither universal nor optimal. The winding to winding phase shift is greater than what you would get with a start cap. This will tend to decrease the available start torque.

In addition, the drives I use would get very unhappy about having a phase disconnected. Make sure the drive you use can handle this. Typically, the lower performance volts/hertz drives that monitor DC bus current to determine motor current will work OK in this application. Higher performance drives that monitor actual phase current will get unhappy.

The moral of the story is this can work, just do your homework first.

Keith
 

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