Problem / Failure Analysis PLEASE HELP!!

[noel0589]

Hi all,

My problem is in a hydraulic elevator system with a electromechanical wye delta starting unit. The problem is that there is severe burning at the contacts on both the wye and delta side, but noticably more on one side than the other. I am not sure which side it is. Does anyone have any ideas? I have considered the ratings, wye-delta timer, and wiring configuration for leads. I would like help in terms of lead wire configurations. Thanks!!

 

[noel0589]

Actually, the lead wire config. does not seem like a problem. I am at a loss here.

 

[aolalde]

The problem with wye start-delta run motor is the transition created at the instant of reconnection.
The motor is wye connected and full voltage applied, if you have load connected the motor will reach a speed considerably lower than the synchronous speed, then when the starter switches to the delta connection the impedance of the machine is very low, to make the situation worst, if the reconnection is fast enough the motor has a remanent voltage which make an out of step reconnection creating a high current surge.
This schema will destroy the contactors and even the motor windings.

Closing the wye-delta transition with resistors properly added to the circuit will reduce the problem.

 

[rbulsara]

I would simply replace the wye-delta starter with a solid state reduced voltage starter. They are very affordable now-a days.

Technically, aolalde is correct..If you have to use a wye-delta units, use a closed transition type.

 

[mc5w]

Are you sure that you even need reduced voltage starting? The power system may be able to handle an across the line start. That is, you skip the wye portion of the starting sequence. I do not know of any hydraulic pumps that cannot withstand full voltage starting from a torque standpoint. Large air compressors are a different story because it is cheaper to put in a wye delta starter than it is to put larger gears into the gearbox.

If you are using a soft start, remember to use a bypass contactor that engages from signal from the soft start before activating the hydraulic circuit. Some soft starters have this built-in.

There are exceptions. If you are only going 8 to 16 feet vertically and only have 2 landings and it is an occasional use elevator, a bypass contactor will not help efficiency.

Mike Cole, mc5w at earthlink dot net

 

[mc5w]

I forgot that motor controllers need to be oversized for frequent starting duty, jogging duty, and plugging duty. Applies to both NEMA and IEC controllers. This also applies to wye delta starters.

Also, your wire size and AC-1 (resistance heater) rating of your motor controller need to be 140% of full load current per National Electrical Code table 430.22(E). Also consider using a disconnect, supply circuit breaker, and so forth that are 140% of full load current. The reasons for the 140% conductors are:

1. Voltage drop control which is more critical for frequent starting.

2. Frequent starting generates more heat.

3. Elevators, rubbish compactors, and so forth are subject to deliberate overloads that tend to spot weld contacts, which will turn a motor controller or circuit breaker into something that isn't.

Mike Cole

 

[jraef]

noel0589,
aolalde is right, but a closed transition Y-Delta starter may not be appropriate for your specific application (because they are in special fire safety zones, elevator electrical systems have certain special rules about heat generating things like the resistors used in closed transition Y-D). Elevator mfgrs solved this problem years ago but going with solid state soft starters. There are several soft starter manufacturers who have units dedicated for use in hydraulic elevator starting applications, and even provide wiring diagrams for all of the major elevator mfgrs. If you are an experienced electrician familiar with Y-Delta connections that may not be too important. But many elevator mechanics are not, and even if you are, the next guy may not be. Having them connect exactly like the one they take out can be very beneficial. Furnas / Nordic (now Siemens) were the first ones to do this in partnership with Dover Elevator, but almost all elevator mfgrs have them now so several other soft starter mfgrs came out with similar products. Do a google search for "elevator soft starters" including the quote marks so it filters out the mish mash.

"Venditori de oleum-vipera non vigere excordis populi"

 

[noel0589]

Thank you everybody for your posts. I have confirmed that it is the delta run contacts of the starter unit which is experiencing considerably more burning. This makes me heavily consider aolalde's post. My understanding is that there is a surge in current during the wye to delta switch which will affect the delta contacts. However, I do not understand one portion
"then when the starter switches to the delta connection the impedance of the machine is very low, to make the situation worst, if the reconnection is fast enough the motor has a remanent voltage"
Does this mean that at the switch from wye to delta, the inductive impedance drops? Is this because at that instant of transition the rotor will momentarily stop because there is no current? Also, should it read "if the reconnection is NOT fast enough" ??
For the elevator system in question, the contactor/starter is rated for 105 Amps where full load amps is at about 130Amps. So it is somewhat being misapplied. However, all of our systems seem to be undersized somewhat and we do not have problems there. In this case, for all possible reasons, it looks like the misapplication is much more evident, although I do not know why.

 

[motorsdirect]

Hi Noel0589,
The impedance will drop due to the reconnection of the motor windings from y to delta...It is in effect, a kind of series / parallel type of connection.

jeff

http://www.motors-direct.co.uk

 

[aolalde]

The impedance of the motor is low due to the connection change from wye to delta, a factor of 1/3 affects the motor impedance, but the high slip reduces the impedance too, the more time with no voltage the more the speed drops and slip increases.

The induction motor spinning will perform as a generator for a few cycles 4 to 10 (66 to 170 milliseconds at 60 HZ)after disconnection from the line. If the contactors reconnect before the residual voltage is dissipated the current will be increased because two different systems of voltage and frequency are interconnected.