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Best motor starting method


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#1 djohnson

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Posted 20 October 2004 - 05:08 PM

I was pleased to find your site this morning while doing research on reduced voltage motor starting. I've read a ton of stuff which was very helpful. I'm in an agricultural area, using 3 phase 480 VAC motors for pumping water for irrigation from deep water wells. These motors range in size from 30 HP to 300 HP and almost exclusively use DOL starters. The problem is the voltage drop on the electric utility supply that can sometimes occur on startup. The utility requires PFC, but has no policy on starter type. Depending on the extent of the voltage drop, there can be some annoying problems for everyone involved. What would you recommend to avoid these startup problems?

#2 marke

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Posted 20 October 2004 - 06:30 PM

Hello djohnson

Welcome to the forum.
I have been heavily involved in this type of application for many years so I understand the problems.
In this country, we are required to use reduced voltage starters to reduce the starting current.
The standard accepted practice here now, is to use either a soft starter, or a variable speed controller.
The soft starter will allow you to start with minimum start current that will get the pump running, plus will offer a gentle start to reduce water hammer, and soft stop to prevent/reduce water hammer at stopping.
With submersible pumps, I expect the start current to be in the order of 250% to 350% of the rated current of the motor, in other words, about half the full voltage start current.
Power factor correction can be used with soft starters, but use a separated contactor to control the power factor correction and bring the correction contactor in once the pump has reached full speed. ( Use the top of ramp relay or the run relay of the soft starter to control the contactor)

In a rural environment, it is a good idea to use a line contactor with the soft starter so that it is disconnected from the supply when not running. This will prolong the life of the SCRs.

Variable speed control is used to maintain a constant pressure at lowest energy consumption and therefore save energy. The start current is less than 100% rated motor current, but due to harmonics, the power factor is not good. - you can not correct harmonic power factor with capacitors!!

I would recommend the soft start approach, but use a good soft starter.

Best regards,

#3 djohnson

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Posted 20 October 2004 - 06:43 PM

Thanks for the quick reply Mark. By the way, I represent the electric utility here. There is only so much we can do on the power supply side to avoid an excessive drop in voltage to the customer, so are investigating methods on the customer's side to avoid or reduce the problem. There are a few star delta starters in use in the 250 to 300 HP range, but I gather from some of your postings you believe they may actually be causing themselves more problems than they are solving?

#4 marke

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Posted 20 October 2004 - 07:02 PM

Hi

In the 50Hz world, this problem was recognised many many years ago and there have been regulations in place in most 50 Hz countries that require reduced voltage starters on all motors above a small size, typically 5HP.
The problem is that the reduced voltage starter was seen as the means to solve the voltage drop problems, but the result was not specified.
Consequently, there are many starters such as the star/delta used that are incorrectly applied and cause more problems that DOL. The regulation should have specified the end result rather than the method.
I have seen specifications such as "the start current shall be less than 200%" This is also a nonsense as you just will not start machines at this current unless you use a variable speed controller and that brings in harmonic problems. If the supply is so weak that the current must be held that low, then the harmonics will create major issues also.

Have a good day,

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Posted 27 June 2005 - 08:11 AM

hi ,can someone tell me the best starting method for a three phase 12 kw motor and why

#6 marke

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Posted 27 June 2005 - 08:30 AM

It depends on what you are trying to achieve and what problems you are experiencing.
The simplest starter is the Direct On Line (DOL) or Full Voltage Starter. This is very simple comprising a contactor and overload. The use of the DOL starter will result in a high start current, about 6 - 8 times the rated current of the motor, and a moderate starting torque, typically between 120% and 240% of the rated torque of the motor.
There can be problems associated with the high starting current or toque that can make the DOL starter undesirable.
The start current and start torque can be reduced by reducing the voltage during start. This is called reduced voltage starting. Reducing the voltage during start will reduce the current by the voltage reduction and reduce the torque by the voltage reduction squared.
If a reduced voltage starter is required, I would reecommend a solid state soft starter because they can give the best start voltage profile and do not produce transients.

Have a look through our web site at http://www.LMPhotonics.com for further information on traditional starters and soft starters.

Best regards,




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