Need soft start recommendations.

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iwire

iwire

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I am working in a customers building that has a 400 amp 208Y/120 service. They also have a 15 HP reciprocating air compressor with a FLA of 40.

When the compressor starts it is noticeable throughout the building, Talking to the employees this has been a long annoyance and sometimes makes the HID site lights drop out.

I want to up sell them on a solution for this. I am thinking a VFD would be overkill so perhaps a more basic soft start?

Anyone have any suggestions on models, makes, what to look for etc?

BTW, the unloader is working fine and the motor, controls and compressor look to be in good shape.

TIA, Bob
 
This is one instance where I really don't have a brand preference. I've used or worked on probably a dozen different brands, I can't tell much difference enough to have a preference. Some brands are quite expensive for all they are (SqD, A-B). If you made me pick one today, I guess I'd probably pick ABB for the price. I guess if this is an important motor, it's probably worth getting something that you can get locally, off the shelf though.
 
I can provide a little more info. I put a recording meter on the 400 amp service.

After one day I had the following readings.

Current

Average 106 amps

Max 370 amps

Voltage

Average 209 volts

Min 180 volts
 
ptonsparky said:
Would a power factor correction capacitor be of any help in this situation?
Click to expand...

I don't know, I would like to learn. :smile:

A drive or a softstart in this size would not be very expensive in any case.
Click to expand...

I asked our purchasing agent to get prices on three different manufacturers soft start.


My experience is limited but as I have no need for speed control I felt a drive was over complicating it and less reliable in the long term. But I could be wrong.
 
I'm a fan of Benshaw. I've used them many times on all sizes of motors from 5hp to 400hp. They are really easy to setup and install. Very simple to operate.
I'm from central MA and the dealer we used was Ruby Electric in Worcester. They are great guys and if you tell them motor specs, application, and the issues you want to resolve they will be able to sell you what you need, whether it's a soft start or PF correction or something else.

check out http://www.benshaw.com
 
Last edited:
080816-0803 EST

iwire:

A very crude estimate of the source impedance is 19/264 = 0.07 ohms. I might expect that starting current of the motor is 6 to 7 times FLA or 240 to 280 A, and this somewhat correlates with the measurements you observed.

I would hope that your source impedance would be lower for a 400 A service. My home with 200 A service with probably a 25 KVA transformer has about 0.05 ohms source impedance measured from neutral thru one hot side. A big part of this is the transformer. And maybe more appropriate for my calculation below for your customer is to calculate the source impedance as (19/1.732)/240 = 0.04 ohms because this makes line current more in phase with the voltage. Still I am dealing with a wild guess from your data.

A simple soft start device using impedance will marginally lower the starting current but lengthen the start time. There will still be a big voltage drop during startup.

Suppose everything was linear and we assumed resistors. 240 A at 120 V is 0.5 ohms. Assume the motor could start at 2/3 of 120 or 80 v, then the current would be 240*2/3 = 160 A. Thus, you might see a change of line voltage of 19*2/3 = 13 V and this might prevent dropout of lights but not greatly reduce flicker of incandescent lamps.

If instead of soft starting the motor you could reduce the source impedance to 1/2 of whatever its current value is, then the drop would be about 9.5 V.

Are the supply transformers too small, or too high internal impedance, or the service lines too small. Compared to changing these a VFD is probably cheap.

.
 
gar said:
A simple soft start device using impedance will marginally lower the starting current but lengthen the start time. There will still be a big voltage drop during startup.
Click to expand...

How long it takes to ramp up is of little concern in this case. It is the compressed air supply for a truck repair garage.

My only concern is trying to lessen the VD at the service.

If I had to I could live with a VFD and ramp it up over a couple of minutes but the mechanical load being what it is the unit is always going to need lot of starting torque so even with a VFD there will be some inrush.

Are the supply transformers too small, or too high internal impedance, or the service lines too small.
Click to expand...

My guess from just a quick look at the pole is the transformer is to small and the overhead drop is also to small. However both those are power company issues. The customers equipment is of adequate size and fairly short.

The meter showed an average PF of around 98.

But it also showed 3 to 5% THD? To be quite honest I am not that familiar with the meter and I don't know what the 3 to 5% is telling me.
 
080816-0919 EST

iwire:

If you could get the power company to change the source it would be the best starting point.

Assume 208 V is nominal, then a 10% drop makes an allowed low voltage of 187.2, and your low reading is really a drop of 13.5%.

A pump will have a torque requirement of friction plus compression force. Torque does not need to be any higher at 0 RPM than running. If the pump is unloaded at startup, then the torque required is for friction.

Here is a quote from Bailey & Gault p197. This is Chapter 13 Induction-Motor Computations. 1951 McGraw-Hill Alternating-Current Machinery.
4. A locked test is made, usually at reduced voltage to reduce the power drawn from the line and prevent overheating the motor. The test results are converted to equivalent values at rated voltage, assuming that torque and watts vary with the square of voltage and that current is proportional to voltage.
Click to expand...
An example on the first page of this chapter had a 35 HP motor FLC of 43.5 A and a locked rotor of 252 A, a 5.8 to 1 ratio. The locked rotor torque for this motor was 208 #-ft and from 35 HP and 1732 RPM I calculate full load torque at 106 #-ft.

Using the square of voltage as the relationship of torque, for this motor you could drop to 71% of rated voltage and get a starting torque equal to the running torque. But, if required starting torque is considerably lower than running torque, then the starting voltage could be lower yet.

.
 
gar said:
080816-0919 EST

iwire:

If you could get the power company to change the source it would be the best starting point.
Click to expand...

If the customer wants to take that up with the power company they are free to do so.


I am looking to provide alternative solution and make some money.

Assume 208 V is nominal, then a 10% drop makes an allowed low voltage of 187.2, and your low reading is really a drop of 13.5%.
Click to expand...

You may have misunderstood me, the 180 volt was seen only during starting. Once up to speed the voltage returns to 208 at the service.


A pump will have a torque requirement of friction plus compression force. Torque does not need to be any higher at 0 RPM than running. If the pump is unloaded at startup, then the torque required is for friction.
Click to expand...

Yes friction which is relatively high with a piston in a bore and the heavy flywheel on the end of the heavy crank.
 
080816-1136 EST

iwire:

If your pump has no back pressure, then friction and inertia primarily define torque. The inertia load is a function the acceleration rate.

15 HP at 1750 RPM is 15*5252/1750 = 45 #-ft. If you measured the torque to turn the pump by hand you would have a good estimate of the alsolute minimum starting torque. A spring type fish scale at a known distance would make an easy way to check static torque.

It takes about 6 #-ft to rotate my 5 HP two stage pump. Therefore maybe 18 #-ft for your 15 HP unit.

If you could assume for starting torque 1/2 of full load torque, 1/4 of locked rotor at full voltage, then maybe half voltage would be adequate starting voltage.

Yes, I had assumed that the 180 V was due to the startup current of the pump.

A VFD or vector drive should have current limiting and thus you could set this to the minimum to start the pump. To make money on the job and allow easy adjustability go for a vector or VFD, but add in play time.

.
 
caps mentioned..

caps mentioned..

Addition of a power factor correction capacitor as close to the motor as possible may be all the system needs.
Not that the system really needs it if the pf is measured to be 0.98, but it would raise the starting pf of the motor, and thus increase the voltage.
A reasonable rule of thumb for a 0.8 pf motor is 25% of the size of the motor, or in this case ~ 10kVAR.
If it's a 0.9 pf motor, perhaps reduce that to 5-7.5.

If that didn't work, my experience says use a soft-start with a low-level current limit, like 150%.

I believe the 13.5% V drop value reading, because the motor is pulling as much as 40*(5-6) = 240 amps. But that in itself is not excessive for startup of the motor, since NEC 695 alows 15% for fire pumps during startup.

I'd be more comfortable with the pf caps addition if the pf was lower than 0.98, but I don't believe the overall system would see any adverse effects since the caps would be switched with the motor contactor, thus only on when the compressor was running. And perhaps the compressor runs infrequently, thereby not contributing much to the average pf of 0.98.

John M
 
In a compressor (assuming reciprocating), a VFD is not always a good choice because they are more complex and more expensive, but the advantages of varying the speed are usually not there.

Any soft starter will likely be fine as long as you size it properly. My advice s to talk to your favorite trusted supplier and see what they have to offer AND SUPPORT. You will not likely need support, but it's nice to have if you do. I was in the Soft Starter business for 25 years, I can tell you that most of the crappy brands didn't survive, meaning that for the most part, products from the major players are all pretty good or they would not be in the business.

Siemens, Schneider (Sq. D), Allen Bradley and ABB are all good products from the majors. Motortronics and Benshaw are specialty manufacturers who survived the fallout by having good products as well. Toshiba brand-labels the Motortronics so those are of course just as good. Other smaller players with good products in the US are Emotron, who brand-labels to Cerus, and Solcon, who sells to Magnetek. SAF Canada makes a good starter as well, although they are hard to find.

I would stay clear of the Cutler Hammer IT starter, they redesigned a few years ago to go after the "cheap" market and did some questionable things. But if you oversize them, they are OK.

Saftronics, once a powerhouse in that market, stopped manufacturing and started brand-labeling a product from New Zealand named Aucom, as does Danfoss and a few others. Aucom's low-end products have some questionable design issues. Teco was brand labeling from Saftronics, so same there.

GE is all but getting out of the industrial control market and since they bring their soft starters in from Agut in Spain, who knows how long that will last.

Absolutely stay clear of any soft starter products advertised on the Internet as "energy savers". Those are all scam programs so the manufacturers have no stake in making a quality soft starter.

But for a compressor starter on something that small, a soft starter will almost definitely solve your VD problem.
 
iwire said:
I don't know, I would like to learn. :smile:



I asked our purchasing agent to get prices on three different manufacturers soft start.


My experience is limited but as I have no need for speed control I felt a drive was over complicating it and less reliable in the long term. But I could be wrong.
Click to expand...

Try Carlo Gavazzi (RSHR2245CV20 @ $785) or Eaton. I think both will be better in pricing that either AB or SqD. Eaton IT is a superb product regardless what others may claim. They old drives sucked.....to be nice.......:D . Danfoss is highly reliable product.

In the case of air compressor, you would need some sophisticated control to get the benefits and probably be an overkill on this size to get the differential paid off. Logic would dictate that a less complex equipment would be more reliable. On the other hand the ASD market is just wastly larger than soft start so more reliability was a strong push from the users. The price difference is much less than one would expect and it can easily paid of by energy savings even at small needs for variable drive on the application. Ex. if a cyclic need v.s. 80% constant speed are the requirements the 80% ASD may be an economic solution.
 
If there is an existing FVNR starter take a look at something like this

http://www.us.telemecanique.com/products/Motor_Control/Soft_Starters/Panel_Starters/ATS01/index.html

This is a module placed in series (down stream) with the existing FVNR starter, that will provide the soft start with adj ramp. Not a feature rich device, but should give you what you need. If the compressor starts under a high head pressure you might want to think about using a module rated fro 20hp for the additional starting torque.

The 15hp @ 460 lists @ $799.00
 
I think that the recommendations as to _which_ soft start to select are far better than any recommendations that I could give about _which_ variable speed drive to select; I don't _buy_ variable speed drives even though I work with the basic technology.

But I think that I can expand upon the point that Laszlo was making as to _why_ you might consider a VSD for this application.

Right now you have a classic system where the compressor charges a tank to some pressure that exceeds the final use pressure. When the pressure in the tank gets too low, the compressor cycles on, and when the pressure is restored the compressor cycles off. Both of these limits exceed the use pressure. A pressure regulator on the output drops the tank pressure to the pipe pressure; or the variable tank pressure is fed through the system and pressure regulators at the point of use provide the final output pressure.

In addition to the electrical problems caused by cycling the compressor on and off, this pumping air to a higher pressure than necessary is wasted work. By using a VSD to run the motor _continuously_ at reduced speed, you could lower the pressure in the storage tank, and for the same total cubic feet, peak CFM, and pressure delivered to the tools, you could put fewer KWH into the system. Sit back and calculate how much it costs to run a 15 hp motor for a year, and you can see that even a small change in system efficiency can have a big payoff.

The difficulty here is _not_ the VSD itself, but the control system that sets the VSD to a speed that makes sense and actually saves energy. The VSD itself won't cost much more than the soft start, and will provide _better_ soft starting (a VSD can provide full load torque at zero speed, consuming perhaps 10-20% of full rated power, whereas a soft start producing full load torque at zero speed will consume more than full rated power)...but the control system could easily double the cost.

-Jon
 
Bob just a thought

You said the unloader was working Ok.Just the check valve and unloader doesn't work that good.

On some larger truck washer pumps I have worked on we used a 2" pneumatic water valve and a time delay to hold the back water pressure off the pump head long enough to let the motor get to full speed before turning the water loose.

If you thought it might work with the air compressor you could take a couple regular valves and pipe them in to see if it would work before you went too the expense of ordering a 1-1/2 or 2" pneumatic valve.
 
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