Dahlander motor control

[BK32]

Hi all,

I need some assistance with some investigation into a dahlander motor winding failure. The motor drives an axial fan via a direct coupling. Based on anecdotal reports the fault appears to be occurring in the following scenario:

1. The fan is operating in high speed;
2. The fan is then stopped (I.e. the high speed, low speed and star contactor are all opened);
3. Then the fan is started in low speed.

The winding failure then occurs on restarting in low speed?

Notably, the issue has not occurred when moving from low to high speed. Therefore I am assuming that the star contactor is switching prior to the high speed contactor as required.

Also in order to switch between high and low speed a selector switch is used which must first travel through the OFF position prior to switching into high or low speed. On this basis therefore, I don't believe there is a scenario whereby the low speed contactor and star contactor are both closed resulting in short circuit of the low speed winding.

Certain parties believe that the issue is that the inertia of the fan is such that when switching from high to low speed a certain time delay is required to allow the fan to slow down and avoid an overload scenario on the low speed winding? I.e introduction of a time delay relay (decelerating relay) into the control circuit has been proposed.

Has anyone had similar issues before? Any advice anyone could provide would be appreciated.

 

[BK32]

Just following my initial post, is there anyone that may be able to pass on any advice/info that may be applicable?

 

[Parchie]

BK32,
Add a speed monitor-sensor to prevent drastic changeover from high to low speed control. Even if you cut out the high speed contactor, that doesn't mean your fan also stopped turning. Once you have a speed monitor, you can prevent the low-speed contactor from closing on a fan rotating near twice as fast as its low speed value.

Please remember, the motor slip will become negative once the actual speed becomes greater than the rated synchronous speed-> meaning your unit is generating. With the Dahlander connection's speed ratio at 2, the negative slip approximates that figure (s ~ -1). A little less than the amount of power the fan draws when in low-speed is then pushed back to the source, but the frequencies may be off as well as the voltages.

 

[jraef]

As Parchie said, when re-connected in Low after running in High, you end up with negative slip AND when the motor regenerates, it is out of phase with the line, which causes a severe voltage and current/torque spike that can cause all kinds of damage. I have seen it shear off motor shafts.

Typically, a Low-to-High speed transition is controlled by a timer to prevent this scenario from happening if the load is high inertia or has a long coast time. The speed monitor is better, but often impossible to implement. So you use a deceleration timer and observe how long it takes for the motor to coast down to a speed slower than the Low speed before allowing the Low sequence to engage. It's basically just an Off Delay timer that is energized only when in High speed, but with its NC contact in series with the Low speed selector switch. So when starting from a stop, the contact is closed and it can start in Low. But after running in High, that contact is open and stays open until the timer times out, which you have set from observation at being the Decel time, plus a fudge factor to be sure.

Word of warning (based on unfortunate experience);
Use a timer with retentive memory, meaning if power is cut, it does not reset the time. Many simple electronic timers are like this, so the lazy operator, who does not like the delay, will discover that if they open the disconnect and kill power to the control panel, the timer resets and he can re-start sooner. I liked using older pneumatic timers for this reason, you can't trick them. But some electronic timers now have this feature, you just need to make sure.


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

 

[BK32]

Thanks parchie and jraef. Your responses were very helpful. I think I will need to do some reading on induction generators just to understand the principle in more detail

 

[ScottyUK]

If it helps at all, an induction machine's performance curve is pretty much symmetrical above synchronous speed and below it, so all the things you're already familiar with like the speed-torque curve, slip, etc all behave the same way when the machine is acting as a generator.