Multiple motor starting techniques 1

[abernier]

I know this probably sounds like a really easy question to some people but I am not really a "motorized" person. I am a new engineer and a little green when it comes to motor controls.

I have a project which requires me to start three or four 60 to 75 HP 3Ph 460V motors starting large fans at approximately the same time. The fans are in close proximity to each other (inside something like a grain bin) and if one is started for too long, the others will start to spin backwards and cause problems. How do I make the fan motors start within a certain time (1 to 2 seconds) or at the same time with out overloading it? I am slightly familiar with Star-Delta (wye-delta) but am still trying to grasp the total concept of it. Is there a way to do this with out using expensive star-delta controls?

Thank you so much for your help!!! =

 

[Danfossegypt]

hello abernier

i think ur solution will be variable speed drive, it will control the fans and their run time and thiir speed all of this will be carriedd out by only one speed drive, 60 hp, i hope u know that power consumed by a fan is related to speed as follows, P=n^3, so any decrease of speed will decrease power to much and save energy.
i really work for danfoss speed drives, so you can contact your suplier, go to

http://www.Danfoss.com

to know ur supplier,
you will order a drive VLT6000 series, i think it will be better and safe and it contains PID controller so if u are seeking for a certain temperature you can control it, and safe energy(your fans will not operate all the time).

i hope that can help you
best regards

 

[busbar]

One solution for backspin is barometric dampers.

Unstated is the characteristic/sensing device intended to control fan-motor operation.

Whether they can be started simultaneously or not depends of stiffness of the power source. If needed, simple cascaded time-delay relays can be used to stagger starting of the four motor controllers (typically individual magnetic starters.)

 

[ScottyUK]

Solid-state soft starters are well suited to fan applications. Mark Empson's website which is referenced elsewhere in Eng-Tips -

http://www.lmphotonics.com

from memory, but please check - has a lot of useful info on the subject. The cost of a soft starter is probably comparable with a star-delta system, but gives smoother results and more flexible control of the acceleration.

I personally like the Siemens Sikostart range of soft starters because they are robust and reasonably well protected electrically. There are numerous other manufacturers of soft starters; I don't think there is any need for the expense of a variable speed drive as was proposed earlier.




-----------------------------------

Start each new day with a smile.

Get it over with.

 

[jbartos]

Comment on the original posting:
If the HVAC system is more complicated, motor start timings are important and governed by the HVAC system. Some fans may require as many as 17 seconds to accelerated with full voltage starters (DOL). Reduced voltage starters (star/delta) or soft starters may be applied. However, all will cost more than DOL. Once a motor is integrated with the fan, there is very little it can be done to start it faster than over DOL.

 

[BJC]

I think Busbar has the only solution.
If one motor stops while the other three are still running you may have to shut down the other three to restart the system.
If one motor stops while the other three keep running the efficiency of the system will go down. The three remaining motors will waste energy recirculating air through the stopped fan.
I had a similar problem once with some containment recir fans on a nuke. If one fan started the other would not. Plant management was determined to fix the problem by first changing the motor protection settings and then with dynamic braking. It was not an electrical problem, the solution to the problem was to repair the seals on the dampers.

 

[Barry1961]

Could you use a brake motor instead? Also there are disc brakes where the disc is bolted to one half of the coupling. The disc brake would run around $3500 so a brakemotor would be cheaper in it would work.

Barry1961

 

[Marke]

Hello abernier

This is not an uncommon problem.
If you start all three (or four) fans at the same time, then you are putting a higher stress on the electrical supply.
One solution is to use inverter drives to control the fans. This is quite a common practice these days as there can be energy to be saved if you do not need all the air flow all the time. The inverters will start the fans at less than their current rating.
Another solution is to use a reduced voltage starter, and the best of these for this application is the soft starter. Typically, to start an unshuttered fan with a soft starter, you will need about 2/3 of Locked rotor current for 15 to 20 seconds depending on the inertia of the fan, and the motor characteristics. This may be sufficient current reduction to overcome your problem and the soft starters are much cheaper than inverters!! You could alsostart sequentially using the soft starters. The last fan to start will no doubt be spinning in reverse when you go to start it, but with a reduced voltage starter, this is not a problem except that the start time will be increased. If the fan was spinning at 50% speed in reverse, then the start time will be increased by about 50% which would not be a problem provided that the thermal inertia of the motor was high enough for this period of start time. I doubt that a fan would be accelerated to more than 20 - 30% speed in reverse in the time taken for the other fans to start, so the thermal effect on the motor would not be all that great.

If you can provide some sort of mechanical or electrical braking system on the motor, or shutters in the airflow, then the reverse speed will be reduced even further.
How about applying 24VDC to the stator winding of the motor that is not being started? this will prevent the motor from turning in reverse with the windmilling effect.

If you employ an inverter, reverse spinning motors are a major issue. With soft starters and other reduced voltage starters, the effect of a reverse rotating motor at start, is to prolong the start time.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: The following principle of operation might be considered:

http://www.magnadrive.com