Performance of Fluid Coupling With Different Motor Starting Methods

I take it you didn't like the answers you got here:

http://cr4.globalspec.com/thread/69401/Performance-of-Motor-With-Soft-Starter-And-Fluid-Coupling

Yes.

Only electrical aspact of question is discussed in that forum.

If you have noted the question is little different what have being asked in Electrical section of CR4

I thought to try my luck with Mach. Engineers

1. Motor subjected to lower starting stress due to Soft starter
2. Coupling subjected to less severe service.
3. Probably more no. of starting per hours for equipment train.

We have used fluid coupling for long travel motion of the EOT Crane but without soft start. We were very happy with performance of the fluid coupling. I do not think if there is any need to add soft start along with fluid coupling as it allows motion to start very slowly without any jerks. If it is added for electrical reasons, please let us know the same.

When Motors is started Direct on Line (meaning full supply Voltage applied to motor terminals) it takes 6-7 times its full load current during starting.

Depending upon magnitude of inrush current (Motor Size) the supply voltage drop (Ohm law Voltage drop = IxR) in electrical System.

When such motor are started on DG set they require bigger size DG set to start them satisfactory.

Soft Starter reduces the starting voltage applied on Motor which gradually increases as per setting of Soft Starter resulting in reduced starting current.

When Motors is started Direct on Line (meaning full supply Voltage applied to motor terminals) it takes 6-7 times its full load current during starting.
Your assumption of 6-7 times may not be correct along with the fluid coupling. Chick with fluid coupling manufacturer. Fluid coupling is nothing but a mechanical soft start, generally used where driven machine has very high moment of inertia.And I do not see any problem using both of them.

Dear Pritam,

Exactly this is what is said in marketing fluid coupling 'Mechanical soft starter'

But in reality, The starting current of motor remains same as when not using fluid coupling. Only difference arise in the rate in which this maximum current falls to normal value.

The fluid coupling gives soft starting (mechanical) to driven equipment. It also starts motor at almost no load at starting and motor quickly reached full load speed resulting in smaller starting time for motor

Dear Happy singh,

In case of starting of 3 phase induction motor with full rated voltage ( DOL start ), the motor takes 6to 7 times of rated full load current irrespective of satrting in no load condition or fully loaded ( driven equipment is directly connected ) condition. The difference is the sustaining period of inrush current. In case of fully loaded condition starting, the time period will be much higher, depending on the load GD square value of load. In case of starting through fluid coupling, the motor is started in no load condition and locking with load is happend at about slip speed and locking torque is about 120 to 125 percent of full load torque. So this is a great advantage of starting through fluid coupling. The torque - speed curve of motor should match with torque - speed curve of fluid coupling for proper locking of load.

Manindra

Thanks Manindra for clarifying. Still I feel there should not be any problem in using both soft starts as Mr. Happy Singh wants. He is happy but want to be more happy (with due respect).

Can some one share Torque speed characteristic for Fluid coupling ..(If one exist)

This available in the technical manual of fluid coupling manufacturer please.

Manindra.

I searched web and come across many manufacturer broucher, but could not get 'Torque speed characteristic for fluid coupling'