Dahlander motor - high to low speed

[jimmy2times]

We've had some failures of control circuits of dahlander motors. I believe it was as result of someone fiddling with delay off timer to ensure high speed to low speed does not occur unless motor has first come to standstill.

I'd like to find an authoritative write up of this issue, textbook or link to website/literature that anyone is aware of? Or if someone has the time to develop a good discussion of the problem.

Maybe I'm not using correct search terms but i'm googled out!

apart from finding hits to other eng-tips threads on problems with these motors (which has been valuable) the best i could find was this

http://www.moeller.net/binary/schabu/SB2011-8_online_EN.pdf

From other links i'm aware motor runs as generator and negative slip.

is it a case that a more robust motor (if specified) could tolerate this under worse case conditions, i got that impression reading the eaton stuff.

In our case We are seeing failure of terminations on bottom of high speed contactor (happened on two separate fan starters) and in another case terminations of motor terminal box.

Not discounting poor installing and joints, as it is fairly new system, but the fact that there is rumour of timer being changed at some point makes me think this could be reason. No record of timer setting to compound matters but Ive told them to check the run down time from high speed mode.

 

[LionelHutz]

Well, switching directly from high to low will cause high currents as the motor is decelerated. But, the connections/cabling should be capable of withstanding any starting currents long enough to trip the overload protection, meaning you shouldn't get connection failures. From what you describe, if the fan was jammed and you tried to start the connection would fail before the overload would trip.

 

[edison123]

Dahlander windings are meant for changing from one speed to another on the fly. Why would one want them to start from zero speed?

Muthu

http://www.edison.co.in

 

[waross]

For what it's worth, I experienced terminal failures on two different projects that were widely separated and not in any way related.
The common factor._
The same crew of inexperienced installers did both installations.
This was new work in both cases.
An experienced electrician will tighten his cable connections and then attempt to twist the cable in the terminal or connector. This will tend to straighten the strands and the connection will loosen. The connection may then be easily tightened more. The twisting-tightening process may be repeated two or three time before the connection is truly tight.
Seven strand cable is the worst case.
As a demonstration, I have asked electricians to tighten a connection on seven strand cable as tight as they could and then I have given the cable a twist and dropped the cable out of the connector.
Multiple failures may be the result of improperly tightened connections.
Too long a time setting between high and low speed should not be a problem.
Too short a time setting between high speed and low speed, may cause an electrical issue but also may cause damage to driven machinery.
It depends.
Hoisting equipment is a special case.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[315kVA]

I haven't done many starters in my life, but have checked some drawings from more experienced colleagues. My job is in shipbuilding where the most usual application of Dahlander motors is for dual speed pumps such as firefighting (high speed) / bilge or ballast (low speed), or for certain ventilation fans with high and low speed. In these schematics I have't found any time delay between the two speeds. Basically we are talking about 50-100 ms, just the time for the outgoing contactor(s) to open and the incoming one(s) to close as allowed by the interlock between them. The same is when we go from high to low or from low to high speed.

 

[panter]

We have a two-speed 2 MW , 6kV motor and when it going from higher on the lower one tachometer measures speed and give a condition for turn on breaker of lower speed when it is slightly below lower working speed.
Also time setting between low and high speed is one second.
Good luck !

 

[jraef]

There is a reason why you sometimes do not see a timer between speeds. In some applications, you select EITHER speed before starting, so there is never a transition from one to the other. The example above of the marine pumps being bilge or fire fighting would be an example (although technically you could be pumping out the bilge when a fire breaks out). Sometimes even if there is a speed change, the selection is only made with a manual selector switch which has an Off position in between the speed selections. That means there is always a slight delay between the speeds in which all contractors are open. In general however, you need something beyond just the mechanical interlocks on the contactors.

My suspicion here, given the description of it being CONTROL system failures that you are experiencing, is that the application IS automatically changing speeds or stepping through them, but there is no transitional timer. So the controls are relying solely on the mechanical interlocks and aux contacts associated with them. That will work when brand new, but as the components age, the slight changes in timing as a result of mechanical wear can have detrimental and progressively worsening consequences.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[electrolitic]

This subject was discussed recently. It was explained the reason of a time delay during high speed to low speed switching.
The example of Panter (to see above) 2MW motor, Dahlander connection, it means "0.5MW/2,0MW motors" (1 to 4 power ratio if variable torque/Y-YY connection)L/H speeds -typically fans/pumps. This is the reason of a tachometer interlock - more safe than a simple time delay. A "500kW motor" trying to reduce the speed of a load that was accelerated by a "2000 kW motor" without any form of control it is not a good idea.
edison123@"Dahlander windings are meant for changing from one speed to another on the fly. Why would one want them to start from zero speed?" A typical application is the ventilation fans of diesel engine rooms off shore vessels that depends of ambient conditions. It can be Man/Auto switched from 0 to Low speed, 0 to High speed (better if in 2 steps), L to H, H to L (idle time with tach supervision or time delay) L to 0, H to 0. Other intentional delays are refining of design. Hope it helps.