Soft starter 1

[LAsludge]

I’m currently designing a commercial version of a high pressure pumping system. This is a scale up project. I currently have a single pump with a 125 hp motor. The pump came with a soft start and I use it to limit the startup current to 180 amps.
I am now scaling the system up to 6 pumps, each with a 250 hp motor. The pumps all start up at low pressure and are ramped up over 15 minutes to their final setpoint so the start up loading on these motors is minimal. Also I control the startup sequence so I can avoid starting two pumps at once.

I’m I just wasting money by specifying soft starters for these pumps?

 

[buzzp]

Perhaps not. Your doubling the HP so I would be doing a little more investigating as to the ability of the supply to handle this large of a motor starting direct. It may cause lights to dim or other problems with equipment or it may not even start.
I can't imagine the low pressure starting is going to help you much with the motor starting current in a pressure pump application.
When you say the pumps are ramped up, I assume your NOT talking about a variable speed drive (just want to check as this will limit the starting current by starting at lower freq/voltage-programmable). However, I can not envision what else you could be talking about.

 

[LAsludge]

When the pump is started the discharge pressure is zero. No VFD. After the pump is up to speed the discharge pressure is slowly ramped up by means of closing a downstream valve. This is done independently for each pump.

The installation is new and the electric service will be new. I'll be building a new control center just for these 6 motors.

 

[jraef]

Starting the motor loaded or unloaded will still involve high starting current (unless you use a VFD), and most power utilities will require some sort of reduced voltage starting at that HP anyway. So then your choices are electro-mechanical or solid state soft starters. Part winding is the cheapest, but requires a special motor. Y-Delta is the next cheapest elec-mech, but also requires a special motor and comes with a host of other problems. Autotransformer starting is simple, but very big, heavy and now more expensive that solid state. Solid state starters offer the best value; standard motors, lower cost and smaller footprint, plus many now come with very good motor prptection systems equivalent to expensive Motor Protection Relays.

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[itsmoked]

Would it be worthwhile to share one VFD with all six motors? Seems this would cost less than (6) 250HP soft starters. And provide the best of all solutions. Would also allow modulation of the last pump in case you need 325HP for some circumstance or even 85HP for another.

It might also help with power factor allowing motors to run at full load and the last motor at fractional load but via the VFD.

 

[DickDV]

Using a softstarter is a good idea for two reasons. First, you limit inrush current with a starter that likely will not cost significantly more than a mag starter.

Second, you will lengthen motor life considerably again for little additional cost.

At 460V 250hp, a VFD is a needlessly expensive solution unless you really need variable speed.

 

[LAsludge]

"Little additional cost?"

I'm quoted $7000 per motor!

 

[waross]

I'd check with the local utility. They may have restrictions on the size of motor you are allowed to start "Across the Line."
If the utility requires reduced current start, then re-evaluate your starting options. If the utility has the power available in your area for 6 x 250 HP motors, starting them one at a time should be possible across the line. I'm assuming that your pumps have a reasonable inertia. If you have a large flywheel affect, you will have to use reduced current start regardless. BTW, what type of pumps are you running?
Large motors were being started, many of them across the line, for decades before the Soft Start or VFD's were even thought of.
That said, Soft Starts are now available and should be given serious consideration.

 

[Marke]

Hello LAsludge

Firstly, if you are concerned about minimising the start current, then a soft starter is a very good way to go. There are other alternatives, but each has it's limitations.

If the start current is the major concern, then you need to look carefully at the motors you use. There are major differences between the starting efficiencies of motors. The minimum current required to start an induction motor is dependent on the load torque requirements and the ability of the motor to convert amps into newton meters (torque). Modern high efficiency motors are particularly bad at making torque from current.

The other issue to consider, is the start torque requirement of the pumps. You mention that there is no pressure on the pumps at start. In some cases, this can be the worst starting condition as it requires the highest start torque. If the pump is a standard centrifugal type pump, the lowest start torque occurs when it is started against a closed valve. If the pump is a positive displacement pump, then the open valve is the lowest start torque option.
Unfortunately, with the information I currently have, it is difficult to be more definitive, however the driven load determines the torque required to start the pump. The motor determines the current required to develop that torque and the starter regulates the voltage or current to minimise the start current. The soft starter is able to do this well, other electromechnaical starters have limited control perhps only two steps and usually provide a higher start torque.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[cbarn24050]

HI, if you only need to ever start 2 motors at once then you only need 2 softstarters. It's very easy to share a softstarter with several motors unlike a vfd.

 

[DickDV]

I've sold softstarters for years (Benshaw and ABB) and around 250hp, 460V, the cost comes very close to a mag starter.

Sounds to me like someone is trying to hose you or, more likely, is heaping all kinds of options on a basic starter that you may well not need.

Check out a basic solid state softstarter and see what it costs?

 

[waross]

I concur with Marke,
His comments are the reason for my question as to the type of pumps.
I agree also with DickDV's suggestion.
yours

 

[jraef]

I agree with DickDV, $7000 for a 250HP 460V soft starter seems excessive, even for a full combination starter with a circuit breaker and bypass contactor. There are some "big name" brands out there that attempt to get you to pay a lot more for their name, but that seems high even for them.

Someone either is trying to cut a fat hog on you, isn't buying right, or has no idea what they are doing. I urge you to go out for competitive bids. There are several companies that specialize in soft starters that are not going to make you pay extra for a particular logo on the box.

Also, make sure when comparing against Across-the-Line starters that you are comparing the same package, i.e. with or without a circuit breaker, NEMA enclosue etc. etc.

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[LAsludge]

Thanks for all the feedback.

While I’m usually focused on the control side of things, I’m also building the MCC for this project. As I said I’m scaling up from a single 150 hp pump to 6 - 250 hp pumps. The pumps are belt driven three cylinder reciprocating compressors. Normally these pumps will run 24/7 with a weekly scheduled shutdown. When the pump is off the drive pulley is easily turned by hand. The only loading at startup is inertia.

When I started to pencil in the budget for this job I had to ask myself if it was justified to spend an extra $40K to soft start something I was only going to start once a week. Based on your feed back I surmise that using soft starts is the generally preferred practice for these motors and they’re not as expensive as I’ve been quoted.

I guess I’ll be getting some additional quotes.

Thanks again.

 

[waross]

Some years ago I specified one MCC section to add to an existing MCC.
My supervisor called the manufacturer for a price and and then told me that we would have to cancel his pet project. No way the budget or the cost benefit analysis would stand $20,000.
I suggested that we could mount the section as a "Stand Alone Unit", and asked him to get quotes from some other manufacturers. I suggested that he go back to the original manufacturer and explain that this was new work and we were getting competitive quotes. The price dropped from $20.000 to $5000. The boss got his pet project.

By all means get competitive quotes, and never ask any supplier for a "Replacement Price."
yours

 

[jraef]

Waross makes an excellent point.
Often there is a premium for adding soft starters into MCCs. MCCs are for the most part a "loss leader" for motor control mfrs. The sell price is much lower than they would like because of competitive pressure for "white bread" MCCs. As soon as you add something out of the ordinary, such as a Soft Start, VFD or PLC, they now think they have a proprietary item and attempt to get a premium for it. They know that if you want a different brand of soft starter inside of the MCC, it can ONLY be theirs because the UL listing for MCC equipment is different that for stand alone. In essence, they have you by the short hairs and they will use that to their advantage.

By simply moving the soft starters, drives and PLCs over to the wall next to the MCC, you can save a lot of your budget for other things. What most smart users do is put the feeder breaker in the MCC and buy a non-combination soft starter.

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[mc5w]

AT one place where I worked in July-October of last year we had to rig the bypass contactor of a 300 HP soft start for across the line starting. This was because silent or invisible lightning caused the soft start to go senile. The longest that the soft start would run the motor was 5 minutes.

The power supplier was Cleveland Public Power and the primary voltage was 6,928Y12,000 volts unigrounded and a substantial distance from the 138,000 volt primary substation transformer. So, you might get away with across the line starting but the minimum transformer KVA is about 3 time motor HP. This is because larger oil transformers and most dry transformers are about 5.4% impedance.

Actually, one of the benefits of a part winding starter is to use a pair of say 200 HP contactors to run say a 350 HP motor for less money. A 300 HP 480 volt motor controller is about 3 to 4 times the price of a 250 HP controller. Part winding is not really a reduced voltage starting method and the second contactor typically must close within 2 seconds of the first.

 

[ScottyUK]

silent or invisible lightning caused the soft start to go senile

Is that you or the soft start?

Joking aside, is this a real phenomena? I assume that 'silent lightning' is a colloquial name - what causes it? It's not something I've encountered over here. My plant is full of gremlins and demons masquerading as operators which screw up equipment without the aid of lightning!


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I don't suffer from insanity. I enjoy it...

 

[jraef]

Damage to soft starters by lightning, silent/invisible or otherwise, s typically in the form of shorted SCRs, just as contactors can weld under similar circumstances. I would venture to say there was some sort of infancy failure on your unit. Any reputable mfr. would have replaced or repaired it. There is no reason to live with such a situation.

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