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Motor Cotroller


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#1 pw_lock

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Posted 24 March 2006 - 03:52 PM

Dear All,

In general a motor controller will have at least the followings:

a. Starter (be it DOL, AT etc)
b. Contactors

My question in regards to starter is as follow:

a. Why do we need a starter? Why not start the motor directly, DOL. Lets put the money thing aside. Will there be any technical problem (e.g. damage to motor or motor cuold not start)?

b. I can only think of using a starter is to avoid dipping the voltage in the supply system (i.e. upstream of the motor). Is this right? If yes, what is the lowest limit we can dip the voltage to (any idea)?

My question on the contactor is as follow:

a. Contactors require to be latched on. Sometime are achieved through electrical means. If there is a voltage dip while starting the motor, will this not latch the contactors?

The reason why I asked this is that I have a motor controller with burned contactors. The controller is for 300hp motor. According to the supplier of the controller is that there is a voltage dip as a result the contactor could not hold due to dipped in holding voltage. My opinion is that if this is the case, the controller is having the wrong starter. The starter should have the voltage being maintained at certain level to at least hold the contactor allowing the motor to complete its starting sequence. Right?

Any idea, everyone. Your help is very much appreciated.

Best regards,
;p;

#2 jraef

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Posted 24 March 2006 - 07:57 PM

Your posting is confusing. It seems as though maybe you meant SOFT starter instead of starter, but you didn't say that.

As to the problem you posted, if the power souce is not stiff enough to support a DOL start without a significant voltage drop, then it is very common for the contactor coil to chatter, which quickly burns out the contacts. You most likely need a soft starter to avoid having that happen.
"He's not dead, he's just pinin' for the fjords!"

#3 Murph095

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Posted 24 March 2006 - 09:12 PM

Please define how drastic these burned contactors appear. As contactors open and close an electrical arc is produced across them. Eventually, the contacts become blackened with burn marks and pitting made by the electrical arcs. This is not a reason for contact replacement. In fact, this black deposit (Oxide) helps them to make a better “seat” to conduct the electricity. However, contacts do need to be replaced once the surface is badly corroded or worn away.

As for your contactors opening because of voltage sags, this is a very common problem affecting manufacturing sites today. There are many products on the market to help combat this such as UPS, Dip Proof inverters and a new one we market called the WaveSync which provides benefits above and beyond the others listed. I am not trying to sell anything here just staing there are solutions out there to help mitigate these issues.

#4 pw_lock

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Posted 25 March 2006 - 03:50 AM

Thank you for your reply. How to make the supply stiff? By having a stiif supply will reduce the voltage dipp during starting?

I thought voltage dipp is due to motor starting not related to the supply. We must dip the voltage enough to have high current to get the motor going. However sometime the dip is not desired as it may affect the voltage levels in the system. Therefore starter like soft start is provided to ensure the system voltage is not being dipped below what the utility supplier guarantee.

Am I right? Pls help.

Thank you all.:)

#5 jraef

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Posted 26 March 2006 - 11:54 PM

Usually you cannot easily make the supply stiffer, it is dependant on a lot of factors such as the transformer size, cable size and distance and even the generating capacity of the utility! When all those factors combine in an application where you want to start a large motor, the voltage drop created by the motor starting current may be so much that the coil of the motor contactor will no longer hold in. When it releases, the current draw on the power grid goes away, and the voltage goes back to normal. When it does, the contactor pulls in again, which again drops the voltage. This can go on indefinitely and very rapidly, sometimes so fast that all you see is the arc flash in the contactor and you hear a loud chattering sound as the contactor armature bangs in and out. The arc that you see represents a melting of the contact surfaces, something that normally occurs, but normally there is only one instance per start. When chattering, the burning process is greatly accelerated and the contact life can be consumed in a matter of seconds instead of years.

You are correct that this is one very good reason to use soft starters. By reducing the current at start up, they usually avoid or at least lessen the flicker effect on the power supply and avoid all these problems.

You are correct in that
"He's not dead, he's just pinin' for the fjords!"

#6 joj101

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Posted 29 March 2006 - 05:31 AM

Hi pw_lock,

1. Motor starter is not limited to the "formal" starter that we all know. Even a simple plug or a switch can be classified as motor starter. We are using starters to control (Start or Stop) motors at the same time protecting the motor from overload.

There is no problem with starting the motor direct-on-line for a long as the power distribution system can take the high inrush current that can cause voltage dip without affecting the other loads attached to the system. Otherwise we have to use reduce voltage starters (AT, Y-D, SS, etc.) to limit the voltage sag. As to what level we have to limit the voltage sag, this again depends on the type of loads the system is serving and the tolerance of those loads to handle v. sags.

I hope I was able to contribute something.

#7 pw_lock

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Posted 29 April 2006 - 04:24 PM

Thank you everyone. How do we measure the voltage drop? This is so instaneous that a volt meter will not pick it up right?

#8 GGOSS

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Posted 02 May 2006 - 01:25 AM

Hello joj101

By definition a motor starter is a device that provides three functions.

1. Start control for the motor.
2. Stop control for the motor.
3. Protection for the motor.

Plugs and switches cannot be classified as starters in any sense of the word. When a motor is connected or disconnected from the supply it will draw a large arc. Standard plugs and switches do not contain suitable arc quenching chambers. If used to start and stop motors, standard plugs and switches will fail prematurely. They also present a potentially lethal hazard to the operator. Even a contactor alone cannot be classified as a starter. Add a thermal overload to contactor and now its beginning to resemble a starter.

Your comments regarding the use of reduced voltage starters are only correct in part. People use reduced voltage starters for 2 primary purposes. The first (which you have outlined) is to reduce starting current which in turn reduces voltage dips during starting. The second is to reduce mechanical stress. In many cases the decision to purchase/install a reduced voltage starter has absolutely nothing to do with reducing start current or voltage dips.

When a motor is started DOL it draws a high current from the supply, and it produces a lot of torque. This torque is transferred to the load via some sort of coupling. Therefore the coupling and the load are subjected to torque levels that can be damaging. At very least they will REDUCE the mechanical service life of the coupling and load.

By installing a reduced voltage starter, the starting torque produced by the motor is also reduced thereby EXTENDING the service life of the coupling and load as compared with a DOL starting arrangement.

I trust the above helps.

Hello pw_lock,

If the voltage dip is very short duration and can't be measured with standard analogue or digital multi-meter, you may wish to consider a power monitor, chart recorder or osciloscope. As an alternative, if you have all supply and load information available to you, you could calculate voltage drop. If that's an option you want to pursue, you might try doing a google search for 'voltage drop calculations' or similar. There are a number of good papaers & software tools available as free downloads from the net.

Regards,
GGOSS




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