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Help with VFD with Bypass on two speed motor

Help with VFD with Bypass on two speed motor

Help with VFD with Bypass on two speed motor

(OP)
I am an electrical contractor and need some help.

We have installed ABB drives on board a ship in three different types of applications, centrifugal pumps, centrifugal fans and vane axial fans. Previously, these loads had two speed starters, would first start in low speed for about 5-8 seconds before switching to the high speed windings.  The folks on board the ship are concerned that if the drive failed and we put it into bypass that the motor would have a problem starting from a standstill on the high speed windings.  These motors are approximately 15 – 18 years old, but are high quality and in other similar applications have withstood VFD application and run for years.

The most concern seems to be with the Seawater Pump (centrifugal) application where we have the ABB ACH550-UH-125A-4 E-Clipse drive connecting to the ABB pump motor Type HXR 280 SMB 6/8 1190/892 RPM.  Theses pumps were previously controlled with a two speed starter, where the low speed would start at 892 RPM on the low speed windings and then switch to high speed if required.  

We think that the drive bypass contactors in the ABB can handle this since they are sized for the high speed winding ratings of 59.6 KW.  What we are uncertain about is the motor and the existing power cables.  We have the option of just connecting the existing low speed windings to the bypass, but the real question is the high speed windings and cables...After all there is less than 300 RPM difference and as I understand things the real load is reached at the high end of the RPM on these loads.

What is the best course of action to determine if we are OK to use the byapss on the high speed windings?  Should I be more concerned with the pump application that the fan application?   
 

RE: Help with VFD with Bypass on two speed motor

Connect only the high speed winding and arrange the bypass as usual. No need to even know about the low speed winding.

The drive will not have any problem starting the motor. It will actually do a better job than the two-speed arrangement used before.

Don't let customer fear and superstition drive you. "The customer is not always right"

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Help with VFD with Bypass on two speed motor

Maybe their concern is that their power system cannot handle the Across-the-Line (DOL) starting current in bypass? If so, I wouldn't be too concerned for a 56kW motor, unless maybe they have a very small generator. If they do, propose a soft starter on the bypass.

Don't worry about the cable sizes etc.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Help with VFD with Bypass on two speed motor

OK, I see. It is very easy to check that. Just make it start in high speed and see if it works.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Help with VFD with Bypass on two speed motor

Hi hoppengarner,
You don't sound sure of how the controls work exactly! Is it a delay for that 5 to 8 seconds?
 If so, I dont think they would set up low speed then high speed for no reason, and would suggest leaving the old starters as the back up, or find a way to keep it as it was when accross the line. Note: I am a dumb guy so remember this is just a suggestion. Good luck, but there is no such thing as luck,so then have fun!

 moregas

RE: Help with VFD with Bypass on two speed motor

What are the motor speeds? Do they need a low speed for any reason other than starting?

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

RE: Help with VFD with Bypass on two speed motor

Could be, a part winding start, or two speed, but for what ever reason keep sarting loads down. need to know more of whats there.
 

RE: Help with VFD with Bypass on two speed motor

Given the speed ratio (10 pole/6 pole), this is a two winding motor, not a dahlander winding or a part winding start.

I would run a separate feed to the VFD and then run the VFD output to the high speed winding with a breaker in between the motor and the VFD output. That way, you can fall back to the original system if the VFD conks out. Of course, you need proper interlock so that the high speed winding is fed only from one source at a time.

Muthu
www.edison.co.in

RE: Help with VFD with Bypass on two speed motor

Quote:

The folks on board the ship are concerned that if the drive failed and we put it into bypass that the motor would have a problem starting from a standstill on the high speed windings.
It sounds like a legitimate question and I don't think there is enough info provided for anyone to conclude that it would work.

Quote:

but the real question is the high speed windings and cables...After all there is less than 300 RPM difference and as I understand things the real load is reached at the high end of the RPM on these loads.
imo your logic is backwards. Motor rotor heating is dependent upon slip.  Most of the heating is generally added during the high-slip period during the early part of the start. The fact that the normal starting sequence gets it to an intermediate speed that is only 300 rpm from high speed suggests that high speed winding has much easier starting duty in the normal sequence than it would starting DOL in high speed.  

Unless you have done some comparision of motor capabilities against load inertia and load torque based on nameplate or OEM info, how does one know the motor is capable to DOL start these load on high speed.  

Personally I would be leery of just trying it out (start on DOL fast speed) as a means to validate that the motor is suitable.  Possible outcomes:
1 - it might work fine and cause no damage
2 - it might trip and cause no damage
3 - it might cause motor damage or degradation... with our without trip. (the relaying is an unknown and we are operating outside of original design without anything to go on).  Small horsepower motors tend to be hearty in general, but this is not a typical one, so... do you feel lucky?

Quote:

Should I be more concerned with the pump application that the fan application?   
In general fan applications tend to have high inertia for a given horsepower.  Therefore without a lot of details one might guess the fan applications are more limiting for the scenario of DOL start in high speed.

Just some more to consider along with a lot of other good advice given by others above.

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RE: Help with VFD with Bypass on two speed motor

My post didn't mention that the ability of the motor to successfully start the pump also depends on the ship power system (as jraef alluded).  i.e. how high does it keep motor terminal voltage during the start.

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RE: Help with VFD with Bypass on two speed motor

It seems to me that the prudent way to proceed here would be to have a standard drive built with a softstart bypass.  Then connect the bypassed system to the motor's high speed connections only.

From what I can deduce from this posting, the slow speed winding in the motor was intended to limit inrush current only.

RE: Help with VFD with Bypass on two speed motor

I read that differently. The OP says: "the low speed would start at 892 RPM on the low speed windings and then switch to high speed if required"

So, if the pumping or ventlation work is small, you run at slow speed. And, if need increases, you switch to the higher speed. There are centrifugal pumps, centrifugal fans and axial fans so the relatively small difference between low speed and high speed will produce a noticeable change in flow.

The discussion has come to be centered around the ability to start an asynchronous motor at 1190 RPM while 892 RPM seems to be no problem at all.

Realistically, a design that is so marginal that a 60 kW motor can be started DOL at 900 RPM but not at 1200 RPM, would probably not pass classification. Also, the ship's electrical system should not have any problem handling the starting. If fans in the 60 kW range are present, it is no little vessel and no small generator(s).

Just go on and test a DOL start. You will not destroy anything and you will get the knowledge you need.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Help with VFD with Bypass on two speed motor

You just said what I was thinking, Gurra, but much better.

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

RE: Help with VFD with Bypass on two speed motor

OK. Se that other thread - we are even then?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Help with VFD with Bypass on two speed motor

Starting might certainly be the right thing to do.  

To flesh out the discussion some more.  I enumerated 3 possible outcomes in my post directly above but I forgot to mention they are listed in order of probability (imo) .  The first two give information without any harm done to the motor.  The third results in damage to the motor.  Is this an acceptable outcome or does the risk justify more research first?    That is a question best reserved for the original poster imo since he knows more details "on the ground".  For example when can the "test" be run and is a trip acceptable during that time (might not be acceptable at sea).... .. is another motor readily available (or is this more likely a specialty non-standard motor).... how accessible is the motor oem and how much do we know about the electrical system?  

Here is a simple excercize to compare total heat generated in two scenarios:
Scenario 1 – Start in two steps:
* 1A DOL start in slow speed (15HZ windings)
* 1B with motor running at 15hz, shift to fast speed (20HZ windings)

Scenario 2 – DOL start in fast speed (1200 rpm windings), then shift to fast speed (20HZ rpm windings)

We make a simplifying assumption that there is no load torque imposed during the start.   For many industrial applications where motor torque remains far above load torque during until the end of the start, this approximation gives only a small error in computing heating.

Now we can estimate heat input to the rotor as
Q = J * delta [ws^2]  where ws is slip speed = 2*pi*slip*sync speed
Use SI units:   speed in radians/sec and J in kg*m^2, then Q will be in Joules

Scenario 1:
Q1A = J*(2*PI)^2 *15^2 * [1^2 – 0^2] = 8883 * J
Q1B = J*(2*PI)^2 *20^2 * [(<20-15>/20)^2 – 0^2] = 987 * J
Q1 tot = 8883J + 987J = 9870J

Scenario 2:
Q2 = J*(2*PI)^2 *20^2 * [1^2 – 0^2] = 15791 * J

(Again if J is in kg-m^2, Q will be in Joules and we have not considered effect of load).

Compare the two:
Q2/Q1tot = 1.6

i.e. the heating during scenario 2 is 160% of that during scenario 1.

A normal NEMA design motor can handle two starts cold... so on a preliminary bases we conclude that we should be able to handle double the heat of one start and 160% should not be a problem.   But again this is not a normal motor and not necessarily started within voltage allowances of NEMA etc.

With Bill and Gunnar agreeing, it is probably a good bet to try the start. But now you have more info for consideration in your decision.

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RE: Help with VFD with Bypass on two speed motor

A reasonable followup question might be:  where does voltage come into the picture... it doesn't show up in any of the equations above.    The answer is related to the assumption of no load torque.  Load torque adds only a small amount when it remains far below motor torque in the early start.  But if you decrease voltage you decrease motor torque and threaten this assumption.  In extreme case motor torque is equal to load torque during start the motor stalls. Even if you don't reach this extreme, load torque coming close to motor torque substantially increases the heat production above what is predicted by the unloaded calculation above.

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RE: Help with VFD with Bypass on two speed motor

Clarification:
Scenario 2 – DOL start in fast speed (1200 rpm windings), then shift to fast speed (20HZ rpm windings)
should have (obviously?) been:
Scenario 2 – DOL start in fast speed (20hz windings)
 

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RE: Help with VFD with Bypass on two speed motor

Last Correction:  
Q = J * delta [ws^2]
should have been
Q = 0.5 * J * delta [ws^2]

Of course it doesn't affect the comparison, but just wanted to correct the equation.  It resembles a kinetic energy equation KE = 0.5*J*w^2.  In the usual DOL start case where one of the endpoints is s=1 and the other is s~0, it is kinetic energy.

I'm done now.
 

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RE: Help with VFD with Bypass on two speed motor

If you wanted to correct for effects of load torque, use
Loaded Heating = Unloaded Heating * weighted average of Te/(Te*Tm) over the entire range of speed, where s is the weighting factor (low speeds weighted heavier than high speeds).

It raises a question: what is the starting torque of the high speed configuration relative to the low speed winding configuration (lower starting torque creates larger correction for loading).  I'm not familiar enough with 4-pole / 6-pole dual winding motors to answer the question of how the 4-pole winding starting torque compares to the 6-pole winding starting torque.  



 

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RE: Help with VFD with Bypass on two speed motor

As Muthu mentioned this is almost certainly a two winding motor. The common denominator of 6 and 8 poles is 24 poles. I believe that reconnecting that many poles has been done in special cases, but those cases are for speed control over a wide range and electronic switching is used to develop almost all combinations of poles.
Here we have a two speed, two winding motor. Such a motor should be suitable for starting and for continuous running at either speed.
BUT, I'm not going to say that without seeing the motor. I suspect that others here feel the same way. There is always the chance that this may be a special motor.
However, the ABB motor site shows two winding marine duty motors with this frame and speeds.
However, if the customer has a real need for the lower speed, you may consider leaving in the existing starter and selecting between it and the new VFD with a transfer switch.  

A related question;
I always understood that the Dahlander connection was only usable for two to one speed ratios. BUT

Quote (ABBsite):

1200/900 r/min =6-8 poles 440 V 60 Hz Constant torque, Dahlander connection
Please tell me that ABB made a typo and I don't have to relearn Dahlander connections.

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

RE: Help with VFD with Bypass on two speed motor

Bill - I have read that in theory that ratios like 2:3 and 3:4 are possible with dahlander connection but in my 25 years, am yet to come across one.

I once had an interesting case of a 8/4 pole dahlander motor. The original scheme was to start the motor (compressor load) under 8 pole and then changeover to 4 pole. The client wanted me to eliminate the 8 pole operation. So I rearranged the leads at the terminal box and the motor would not start at all. On investigating, I found the stator slot/rotor slot ratio would not allow to start the motor as 4 pole but would allow it start it as 8 pole.  

Muthu
www.edison.co.in

RE: Help with VFD with Bypass on two speed motor

"Pole Amplitude Modulation" can apparently can achieve 2-speed motors with ratio's other than 2:1

http://books.google.com/books?id=_y3LSh1XTJYC&;pg=PT153&amp;lpg=PT153&dq=pole+amplitude+modulation+%28PAM%29+is+a+method+with+which+ratios+other+than+1+:+2+may+be+found&;source=bl&ots=iKUGYFxfuH&amp;sig=1v8BkoZHFZE46f55ObBKCPPtZ6A&hl=en&ei=GEa5S_KQDYb29ATFrszqAw&sa=X&oi=book_result&ct=result&amp;resnum=5&ved=0CBwQ6AEwBA#v=onepage&q=pole%20amplitude%20modulation%20%28PAM%29%20is%20a%20method%20with%20which%20ratios%20other%20than%201%20%3A%202%20may%20be%20found&amp;f=false

Quote (Design of Rotating Electric Machines):


There is also another method to create windings with two different pole numbers: pole
amplitude modulation (PAM) is a method with which ratios other than 1 : 2 may be found.
PAM is based on the following trigonometric equation:
sin pbα sin pmα = 1/2 [cos (pb − pm) α − cos (pb + pm) α] . (2.103)
The current linkage is produced as a function of the angle α running over the perimeter
of the air gap. A phase winding might be realized with a base pole pair number pb and
a modulating pole pair number pm. In practice, this means that if for instance pb = 4 and
pm = 1, the PAM method produces pole pairs 4 − 1 or 4 + 1. The winding must be created
so that one of the harmonics is damped and the other dominates.
But how it really works is a little beyond me.

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RE: Help with VFD with Bypass on two speed motor

pete - I have done only two PAM motors in my 25 years. Very complicated connections and tough to follow.

Muthu
www.edison.co.in

RE: Help with VFD with Bypass on two speed motor

Thanks Muthu and  Pete;
For the problem at hand, ABB lists quite a few two speed motors specifically for fans. Most are two winding motors. Only a few are shown as Dahlander connections.
A two winding starter is quite simple. It is similar to a reversing starter with two, 3 pole contactors for wye connected motors. 4 pole contactors may be used for delta connected motors to open the delta on the unused winding.

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

RE: Help with VFD with Bypass on two speed motor

(OP)
First of all, thank you all for the terific responses here...I have carefully read and re-read each response. I am new to this type of forum, so forgive me if I am not following expected protocal here.

In my orginal plea for help, I failed to mention that the voltage here is 440V...I also failed to mention that we are using the high speed contactor at the starter panel to power the newly installed VFD.  The reason for this was to maintain the orginal safeties, such as emergency shutdown and fire/smoke damper end switches.  We have configured the high speed contactor to be energized when the safeties are good.  The start/stop functions of the original starter were re-used to to provide a start/stop to the associated new VFD.  The bypass was an afterthought and added to the design late in the process.

We made the assumption that the motor's high speed windings would be able to handle a DOL start when the bypass was activated.  Since the new VFD/Bypass unit provides for motor and cable protection, the exsiting thermal overload protection(range 80-110Amp-seawater pump example)was jumpered out on the existing high speed contactor....the fused (200Amp-seawater pump example)disconnect at the starter was left intact. (Low speed fuses were 100Amp)

There was also concern about the high speed contactor being able to handle this DOL start.

You have all given me great insight into the issue.  Since the ship has redundency in place for each load, whether fan or pump, the odds of the ship employing any one of the bypasses is slim at best.  

At this point I am inclined to test the bypass DOL on one the fans and have a fast peak-hold ampmeter setup prior to testing.  I will make certain to follow up with the results.

Again, thank you all for your help here!
 

RE: Help with VFD with Bypass on two speed motor

Please don't take offense if you already know this;
You can't just close a bypass breaker across a running VFD as you can on a soft starter.
The VFD must be out of the circuit before the bypass is closed.
You probably knew that, but if you didn't it will save you a lot of money.

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

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