soft starter inrush 3

[bam55]

Is there any point in putting a soft start for a motor which only starts at no load? (The application is a magnetic coupled adjustable speed drive controlled fan which starts it at no load) Is there not a spike whether I have a soft start or not?

Also- will the soft start let through the spike?

Any comments appreciated.

 

[itsmoked]

Oh there is still a load...The inertia of the actual fan. If the fan is large it could amount to a lot because of its moment arm/s.

Probably a soft starter is not needed in a fan's case but really a lot more info would be needed to make the determination. Such as how rigid your supply is. How big this fan is. How many horsepower. Are you having dimming problems now on other things?

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[CJCPE]

The magnetic coupling adjustable speed drive probably has controlled acceleration which provides a soft start. If the motor is started prior to accelerating the load with the magnetic coupling, the inertia while the motor is started is only the motor's rotor inertia plus the inertia of the input member of the magnetic coupling.

 

[DickDV]

A softstarter on an unloaded motor will be able to reduce the starting inrush from about 6 times nameplate to about 2.75 times nameplate.

If that reduction is useful to you then I would go ahead with it.

If, however, the desired purpose is to reduce the starting torque delivered to the load, then the existing coupling should be good enough for that.

 

[jraef]

Starting current with Across-the-Line (DOL) starting methods is identical regardless of load. The only thing the load component adds is time to full speed. When a motor is started unloaded, the starting current may pass through it's peak so quickly that you don't see it without a scope, but it is there nonetheless and as DickDV pointed out, it is typically 500-600% of nameplate FLA. So using a soft starter will definately reduce that starting current surge, even if you don't notice it. Inrush current is technically not the same as starting current, because it is the 1-2 cycle magnetization current of the stator windings, and can be as high as 1300% of FLA, or even 2000% on new small HP energy efficient motors. Soft starting also reduces that inrush current as well.

The other thing that happens when the current surges is that the shaft torque surges as well. In a Design B motor, that means 160-180% of Full Load Torque at startup from a dead stop. When the load is not yet moving, that torque is mostly excessive, above and beyond the load torque and accelerating torque necessary at that moment. This means that the excess torque is absorbed as stress by the mecahnical components in your system, the couplings, belts, shafts, gears, journals etc., even the motor stator windings themselves. Mechanical stress always finds the weakest component and over time this will lead to failure. Soft starting not only reduces the starting and inrush current , but also reduces the torque stress during startup by reducing torque to levels just sufficient to accelerate the load. Their best value is actually in prolonging the mechanical life of the motor and driven equipment.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

Gads..how did I miss the magnetic coupling? Doh!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bam55]

I think I'll go ahead with a soft start as it is the same price as an across the line starter anyways. If it can pull down the spike to 2.75 times starting or less than 13 times inrush, then that will be the benifit to the electronics in the building. The utility won't have any questions which requires reduced voltage starting on motors over 40HP,600V.
So there will be a reason for putting essentially two softstarting devices on the motor.

Bye the way the magnetic coupler ASD is for the fan part to vary its speed and eliminate the harmonics as well. We are using it to increase the CFM without changing the fan size,, but increasing the motor size and varying the speed according to demand.

Thanks for all,

 

[waross]

Hello jraef
I think that I have probably misunderstood you. If I have misunderstood you, others may have also.
Re; Starting inrush and starting current. I understand the inrush or surge as the magnetizing inrush in the first 1/4 cycle.
This is partly dependant on the point in the cycle at which the current is applied. It varies widely and with a mechanical starter is usually different on each phase. I have found it to be so short and to contain so little energy as to not be a problem. Years ago we had a problem with a main frame computer at 120/208 volts when a 600 Hp chiller was started across the line on a 4160 tertiary winding on the same supply transformer.
We put a scope on the input to the computer and could not detect any trace of the magnetizing peak making its way to the computer. We did see a severe voltage drop. We used a memory scope and were able to examine the first few cycles in detail.
The significance was that a constant voltage transformer was 20% of the price of a power conditioner.
The starting current.
This is easily seen with an analog ammeter. Part of standard trouble shooting procedure in the analog days was checking starting current for anomalies with an analog ammeter.
When the digital meters came out, The rolling numbers as the starting current reduced made the meters unreadable for starting current. The first digital ammeters had a peak lock that would lock on the magnetizing inrush which would be much more than the 6 times running current we expected and vary widely from start to start and be pretty useless for trouble shooting. I haven't used one but I imagine that some of the digital meters with the bar graph will show the starting current.
respectfully

 

[machmech]

Thank you jraef and waross for response, waross an analog ammeter??? I think mine is due for a battery change once I get past the dust on the cover, It's not sleek and sexy in comparison to my Fluke however comes in handy now and then.

 

[jraef]

machmech,
That's a good plan, but one issue bothers me a little. You said that the soft starter costs the same as the A-T-L starter. That has me a little concerned because that's not normal. Either you were paying way too much for the A-T-L starter, or you are getting an extremely cheap soft starter, which is the part that concerns me. You get what you pay for in that industry, and going too cheap can often set you up for failure. In your particular case it may work out anyway because the motor is almost completely unloaded, so there is probably no need to cancell any orders. I'm stating this mainly for the benefit of future readers. As a general rule, low voltage solid state soft starters are less expensive than Reduced Voltage Auto Transformer starters, but more expensive than Across-the-Line starters. When it looks otherwise, there is probably good cause for caution. It doesn't save money when you have to buy it twice.

waross,
Point taken. I guess I only brought it up because I'm a semantic snob when it comes to the use of the word "inrush" to mean "starting current" because technically they are distinctly different definitions. Like so much of or language however, common use eventually becomes the defacto standard, so I probably need to learn to let that go. If I confused someone like you who does know the difference, I probably confuse a lot of other people as well.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

Waross: I agree that the transient inrush current may appear to be insignificant in impact to other equipment in the system, but it contains adequate energy to trip the magnetic trip element in a molded case circuit breaker or mag-only breaker in the starter if the trip is sized too low. I know you know this, but I don't want others to be misled into thinking this transient current can always be ignored.

The inrush does depend on the voltage phase angle when the contactor closes, so this nuisance tripping oftens seems quite random in practice. It is also one of the main reasons that you typically find all instantaneous trip elements in motor starters set to MAX

 

[waross]

Hi jraef;
I thought that I understood your terms but it was the comment "The starting current may pass through its peak so quickly that you don't see it without a scope". That made me wonder if I had misunderstood which was inrush and which was starting current.
I am often struck by how much time is spent in forums explaining the use or meaning of a word or phrase. If we were having coffee together it would only take about 3 words for us to realise that we were in agreement.
Hi dpc.
I agree with you also. Same comment about a cup of coffee.
respectfully

 

[jraef]

Oops. Guilty of my own pet peave!