Help with VFD with Bypass on two speed motor

[Hoopengarner]

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?

 

[Skogsgurra]

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

http://www.gke.org

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[jraef]

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.


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[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

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[moregas]

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

 

[waross]

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

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

 

[moregas]

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.

 

[edison123]

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

http://www.edison.co.in

 

[electricpete]

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.

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?

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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[electricpete]

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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[DickDV]

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.

 

[Skogsgurra]

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

http://www.gke.org

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[waross]

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

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

 

[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

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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[electricpete]

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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[electricpete]

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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[electricpete]

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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[electricpete]

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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[waross]

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

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