Soft starter
Soft starter
(OP)
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?
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?





RE: Soft starter
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.
RE: Soft starter
The installation is new and the electric service will be new. I'll be building a new control center just for these 6 motors.
RE: Soft starter
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Soft starter
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.
RE: Soft 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.
RE: Soft starter
I'm quoted $7000 per motor!
RE: Soft starter
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.
RE: Soft starter
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
RE: Soft starter
RE: Soft 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?
RE: Soft starter
His comments are the reason for my question as to the type of pumps.
I agree also with DickDV's suggestion.
yours
RE: Soft starter
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.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Soft starter
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.
RE: Soft starter
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
RE: Soft starter
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.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Soft starter
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.
RE: Soft starter
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...
RE: Soft starter
Eng-Tips: Help for your job, not for your homework Read FAQ731-376