Motor Starting Method Selection 2

[Mahmoud-Abdullah]

Dear Every One;

I wonder which criteria i can select the type of starting method of the motor like direct on line, star/delta, soft starter, variable speed drive..
if there is any paper explain it, please share it with me...

Thanks

 

[jraef]

Lots of papers, but first, what size motor and what application? The starting technology is the same, but the reasoning may be different.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Mahmoud-Abdullah]

Motor rating like 110KW for water pump, there is standard combination when i use DOL or star/delta etc.. when i prefer a starting method than the others?

 

[waross]

Direct On Line is cheapest.
DOL is generally suitable for low inertia loads such as water pumps.
Factors such as excessive voltage drop, soft grid or power company requirements may mandate a reduced current starting method.

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

 

[jraef]

As a general rule, most power suppliers will want to see Reduced Voltage Starting of some sort on a 110kW motor. So since it is a pump, I always like using Solid Stat Soft Starters, because they offer the ability to use Deceleration control so that you can slowly reduce the pump pressure and allow any check valves to closed gently rather than slam. This avoids water hammer. No other starting method (other than VFDs) can offer this.

In addition, if you have ANY mechanical means of flow control on that pump such as a throttling valve, you should consider using s VFD instead. The potential for reducing energy consumption in stead of throttling control generally pays for the VFD cost within 18 months, depending on how much of the time you run at reduced flow.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[zeusfaber]

Depends a lot on how solid your supply is and how dip-tolerant any other loads on the system are.

If this is right next to the switchboard on a ship, DOL starting is going to be sensible, almost without question.

If this is out in the countryside at the far end of a very long bit of electric string which it shares with business-critical computer systems, you will probably come to a different conclusion.

A.

 

[Mahmoud-Abdullah]

Thanks alot for your support.

 

[GroovyGuy]

Hi Mahmoud-Abdullah,
Most people now use solid-state reduced-voltage (SSRV) starters, where inrush current needs to be limited. Obviously this is applicable to variable-torque loads, such as yours.
The cost of RVSS starter has come done significantly over the last few years, but will always cost more than a DOL starter.
Having said that I have DOL started motors up to 16,000hp, albeit with a 230kV feed to the plant. I have also used RVSS starters on 50hp motors on the end of a long 25kV rural feeder. Your Utility likely has restrictions on flicker (ie magnitude and frequency).
While a RVSSS starter will not provide any speed control, as with a PWM ASD, it will reduce the impact of water hammer on the line.
GG


I am forever learning and changing.
W.E. Deming

 

[Mahmoud-Abdullah]

Hi GroovyGuy
i faced many of pump starters for the same rating either star/delta or soft starter so, i was need to know the difference between them in the pump operation as i am far away ffrom any site to know it.
Thanks alot

 

[jraef]

There is a significant difference between Star-Delta and Solid State as far as the pump STARTING and STOPPING, but the term "operation" implies running. Once the motor has accelerated to full speed, there is zero difference between the two methods.

On STARTING, a SD starter will reduce the torque and current at first, but when it transitions from Star to Delta, there is a second "hit" and both the current and torque will jump significantly. In the case of "open transition Star-Delta", that transition spike can actually exceed the DOL spikes of both cuttent and torque, and has been know to shear the shafts of motors. S-D starting is also a fixed value two step method; if the 33% initial torque value is insufficient to accelerate to near slip speed, or is more than necessary and causes surging, there is nothing you can do about it. With RVSS starting, there is a smooth gentle increase in torque and current, infinitely variable to match load and site conditions.

On Stopping, the RVSS can ramp the pump motor back down to alleviate water hammer as previously mentioned. The S-D starter cannot and you must either use complicated expensive pump control valves, or suffer the consequences of simpler swing check valves slamming shut and creating pressure transients that can destroy piping and fittings.

[pre][/pre]Generally speaking, the savings in electrical equipment costs from not using an RVSS are simply passed down to the mechanical side of the project, either in increased component costs or in long term repair costs.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Mahmoud-Abdullah]

Hi jraef;
Thank you for the detailed explanation but i still have a question. The solid state soft starter is simply a thyristor at start and then by passed by a contactor. if any thing happened in this electronic circuit, it means total shutdown for the motor unlike the star/delta starting-if it well sized- it has a little worry than soft starter. So, how i can protect the soft starter from that issue?
Noting that if i faced any problem with the contactors or overload, i can replaced them quickly from the market. the soft starter may be not like ease.

 

[waross]

Are Auto-Transformer Starters still available Jeff. More important, in the rare event of failure, would a replacement autotransformer be readily available? The contactors should be available.
A closed transition auto transformer starter is as robust as a star-delta starter and has a much more manageable transition surge.
Auto transformer starters were sometimes called Compensators.
In my experience, motor failures are more common than failures of starters of any kind.

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

 

[GroovyGuy]

All,
If I remember correctly, a wye-delta starter has a couple of disadvantages;
1) Cabling between the motor and the starter requires 6 conductors (ie cabling costs can get pricey if the motor and starter are a significant distance apart)
2) the motor has to have all six wires brought out to the junction box (this is not always the case)
I would have no issues recommending a RVSS starter for most variable torque loads that require means to reduce inrush current.
GG

I am forever learning and changing.
W.E. Deming

 

[davidbeach]

I'd never consider an open transition wye-delta starter; even back when it made any sort of sense. Now the RVSS starter is so much more robust that it was when I was first considering options regarding the starting of large motors. As an issue of large motors, 6 leads vs. 3 leads wasn't that much of an issue in that you were already getting close to the point where multiple smaller conductors might well be easier (thus less expensive) than fewer smaller conductors. It is absolutely correct that the motor had to provide enough leads at the junction box, not all did/do.

I quit doing that type of work over a decade ago, about the time that wye-delta stopped being a reasonable alternative to the RVSS starter. I'm surprised to be seeing this type of discussion today. The RVAT starter that Bill mentions was always superior to the wye-delta, but also noticeably more expensive. But today, why have two steps when you can have an infinite number?

 

[jraef]

RVATs still have their place, but fewer and fewer people sell them, plus you are right Bill, replacement availability is now an issue. Their remaining benefit however is that because of the transformer action inherent in the design, you can get higher motor amps PU of line amps that any other method. The trade off is that you have three steps to chose from (by moving conductors) and if one doesn't work, you must go higher, which might be too high. They are also physically very large, heavy and hot.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Mahmoud-Abdullah]

Hi David
I don't understand what make you nervous to discuss star/delta as a starter method. Every thing has a dark side which make no thing is perfect. RVSS is already a great devise which provides many advantages for controlling, protecting and capable of communication with a monitoring system but if a fault happens and RVSS blows up. i lost that motor until replacement unlike star/delta as i explained above so, the continuity of operation is not achieved at this situation. The question is what are the precautions should i take when using the soft starter?


Keep Moving Forward

 

[jraef]

When I was in that business, I used to conduct training on troubleshooting soft starters. To make it easy for general electricians to understand, I offered the following:

If you have an Across-The-Line (DOL) starter and it is not working, what can be wrong? It's either a bad coil or welded contacts, correct? With an RVSS it's either a bad PC board or shorted SCRs. You can think of them the same way. In fact most RVSS units will TELL you that an SCR is shorted, whereas the ATL starter will not. If it's a bad board, you replace the board. There is no need in dissecting it in the field, just swap it out.

As to designing for success, the main issue I found was in addressing your external risk factors. If you are in an area where lightning strikes are common, I recommend having an isolation contactor ahead of the soft starter so that when it is off, there is an air gap separating it from the line. That's just prudent for any electronics in that situation. Other than that, 90% of problems in the field boil down to excess heat. Most people do not pay enough attention to that. Yes, having a bypass contactor reduces the heat created by the soft starter, but there is still ambient heat to consider. Most electrical equipment is designed around 40C operation, even electro-mechanical devices like contactors. Contacts are more tolerant, but that heat eventually shows up as coil failure. So if you ignore ambient heat, it will not matter what type of equipment you use, failure is eminent. Only the time factor changes.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

If this motor is so critical that it can't be down then you should stock spare parts, be it spare parts for the soft-starter or ATL or wye/delta starter that you are using.

If you just don't want to use a soft-starter because it's too "difficult" then that's a personal choice.

If you're working with a local soft-starter manufacturer or supplier and they can't get you replacement parts or a replacement soft-starter within a few days then they're not much of a soft-starter supplier or manufacturer. It's very rare that we couldn't get replacement parts or a complete replacement soft-starter to a customer anywhere in Canada almost as fast as airplanes fly and vehicles drive between our location and theirs. Sometimes, a service tech even accompanies the parts.

 

[HCBFlash]

jraef is right-on (again). You find plenty of people staying clear of Wye-Delta starters specifically because of their complexities and maintenance issues, as well as the additional wiring involved.

Today's solid state devices are far, far sturdier and more reliable than those of 20 years ago. Additionally, you can use your bypass contactor without the SS functioning if you need to. Lionel's comments about keeping spare parts for critical equipment are NOT just a wisecrack! Perhaps you should order spare parts along with your initial material purchase for something crucial.

As far as papers go, some of us are relatively old farts, and still use the term "paper" literally. Most of my education was anything but paperless, and, well, I'm just too lazy to scan somebody else's work and upload it, especially when, just like you, I'm able to Google something useful pretty quick and easy.

.


Me wrong? I'm just fine-tuning my sarcasm!

 

[Mahmoud-Abdullah]

Dear HCBFlash;

you are bull shit.

thanks.

Keep Moving Forward