Discuss Goods Lift Brake Solenoid Not Operating Correctly in the Industrial Electrician Talk area at ElectriciansForums.net

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The solenoid, being an electromagnetic device, produces a force by the action of the current flowing through its coil. Being a coil wound on a metal former it has both resistance and inductance.

The current through the coil depends then on the waveform of the applied voltage V between A1 and A2. For a continuous unchanging voltage (cc in my attached diagram and in red) the current is only limited by the resistance of the coil R since there is no back emf produced by L.

For a pure alternating sinusoidal waveform (ac and in black) the current throught the coil is limited by the impedance of the coil viz Z = R + jXL or Z= sqrt(Rsq + XLsq); thus if the ac voltage has an rms voltage the same magnitude as the continuous applied voltage the current flowing through the solenoid Iac will be smaller than Icc and decrease further with increasing frequency.

For full wave rectified ac (no smoothing) energising voltage once again the solenoid will present an impedance (not pure resistance) and Ifw will be less than Icc and decrease with frequency.

The addition of a smoothing capacitor will filter out the ripple of the full wave rectified waveform meaning that the applied voltage too becomes more a like continuous unchanging voltage - the current through the solenoid will tend towards the same value as Icc as the ripple reduces since the impedance of the winding tends towards R as the harmonics of the rectified ac are filtered out.

In sum then a coil wound for dc will require a higher ac rms voltage energising voltage to produce the same force since I = V/Z for ac and V/R for pure dc.

As a thought experiment once could imagine an alternating voltage V being full wave rectifed and smoothed perfectly by a great amount of shunt capacitance so that 185V dc was produced as a continuous waveform. Now gradually reduce the shunt capacitance until none is in circuit - the current through the coil will decrease even though the applied ac voltage V of the source remains constant because Z increases. The solenoid become weaker.

(Actually you have to think of the Z of the circuit for each harmonic and Isol is the sum of all the harmonic currents)....

You get the gist...
Thanks, this makes sense
 

brianmoooore

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What we need to know is if this mysterious rectifier is present, is working, and connected so as to supply this solenoid.
I presume that, being a safety component, the solenoid's 'normal' position is that the brakes are engaged, and that energising the solenoid releases them (or should do).
 

Lucien Nunes

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Lift brake solenoids are normally DC, often energised from the control DC supply via contacts on the direction contactors. However the DC voltage does not need to be smooth so the output of a rectifier can be connected directly to the solenoid without a reservoir capacitor. The voltage is not then smooth but the current will be, due to the high inductance of the coil. If you connect a meter on an AC voltage range it might display an AC voltage due to the AC component of the unsmooth DC, the exact reading depending on both the meter and the rectifier circuit.

Energising a DC solenoid with AC will cause reduced operating force, chatter and hum and possibly heating of the magnetic circuit which will not necessarily be laminated. If the solenoid has an anti-parallel suppression diode, connecting AC will either destroy the diode or blow the fuse. Older units have a selenium rec or resistive suppression, which will respectively let out smoke or work fine on AC.

Can you post some pics? I'm passably familiar with older Otis gear.
 

PEG

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Hi,in connection with the topic,might i suggest the OP ensures any possible future liability risks,in testing or altering this braking system.

Having had many involvements in similar devices,any issue or mishap,at any time in the future,will be firmly dropped in the lap of the last-to-touch.

It usually is not the 12 people hurtling via gravity to doom,in a goods lift they should not have entered....
...it will be the 1" misalignment or slip,which catches a toe or finger,followed by the wait in casualty,watching the waiting room TV,with it's blame hunting brief ad's...

Be 100% sure of two things...

1) The squealing you mention,is not a siezed or failing shaft/coupling,resonance on one or more reeving falls,or a sheaving fault.

2) It's not you who gets the curly finger 😗
 

Intoelectrics

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What we need to know is if this mysterious rectifier is present, is working, and connected so as to supply this solenoid.
I presume that, being a safety component, the solenoid's 'normal' position is that the brakes are engaged, and that energising the solenoid releases them (or should do).
Yes, this is exactly how it works. Regarding the rectifier, I have no details on his as the engineer attending site, who I was in discussion, with clearly did not want me involved. The client just asked me to take a quick look because the lift engineers were scratching their heads at the time. I didn't and could not spend a great deal of time investigating. But I'm intrigued on what the problem was and how it was rectified (no pun intended).
 

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Lift brake solenoids are normally DC, often energised from the control DC supply via contacts on the direction contactors. However the DC voltage does not need to be smooth so the output of a rectifier can be connected directly to the solenoid without a reservoir capacitor. The voltage is not then smooth but the current will be, due to the high inductance of the coil. If you connect a meter on an AC voltage range it might display an AC voltage due to the AC component of the unsmooth DC, the exact reading depending on both the meter and the rectifier circuit.

Energising a DC solenoid with AC will cause reduced operating force, chatter and hum and possibly heating of the magnetic circuit which will not necessarily be laminated. If the solenoid has an anti-parallel suppression diode, connecting AC will either destroy the diode or blow the fuse. Older units have a selenium rec or resistive suppression, which will respectively let out smoke or work fine on AC.

Can you post some pics? I'm passably familiar with older Otis gear.
Thanks,
I'll see if I can get back in the lift control room and get some pictures next time I'm on site. It would be good to get and understanding of it, no matter how old I get I still enjoy learning!
 

Intoelectrics

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Hi,in connection with the topic,might i suggest the OP ensures any possible future liability risks,in testing or altering this braking system.

Having had many involvements in similar devices,any issue or mishap,at any time in the future,will be firmly dropped in the lap of the last-to-touch.

It usually is not the 12 people hurtling via gravity to doom,in a goods lift they should not have entered....
...it will be the 1" misalignment or slip,which catches a toe or finger,followed by the wait in casualty,watching the waiting room TV,with it's blame hunting brief ad's...

Be 100% sure of two things...

1) The squealing you mention,is not a siezed or failing shaft/coupling,resonance on one or more reeving falls,or a sheaving fault.

2) It's not you who gets the curly finger 😗
Thanks for the heads up,

Fortunately I'm not responsible for this repair, basically just an interested party. I wouldn't normally get involved with the control side of lifts as I have no experience at all in this field. Like I state, I was asked to have a quick look and you could clearly see the brake not disengaging properly causing the squeal as it bound on. You could see the armature move back to open the brake shoes but there appeared to be not enough clearance. It appeared as though the return spring was too strong and the armature shaft was not moving fully in position.

Since this brake system had been removed and fully re-furbished, my first question was regarding the adjustment, and the engineer told me that it was adjusted, set and tested as per design spec. Obviously I'm in no position to argue this. The cover was off to the terminals of the solenoid so as they tried the lift and the brake attempted to dis-engage I tested across the terminals a couple of times. This is when I got the 169/170V ac readings. The I.D plate though not clear appears to state 185V dc.

So, with no experience in these matters my assumption is/was that the applied voltage was firstly ac not dc and secondly too low to operate the solenoid as it was designed to do, thus causing the brake to bind on.

The lift is now back in operation, so I assume the problem sorted, though I have no details on what was done to achieve this.
 

PEG

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Thanks for the heads up,

Fortunately I'm not responsible for this repair, basically just an interested party. I wouldn't normally get involved with the control side of lifts as I have no experience at all in this field. Like I state, I was asked to have a quick look and you could clearly see the brake not disengaging properly causing the squeal as it bound on. You could see the armature move back to open the brake shoes but there appeared to be not enough clearance. It appeared as though the return spring was too strong and the armature shaft was not moving fully in position.

Since this brake system had been removed and fully re-furbished, my first question was regarding the adjustment, and the engineer told me that it was adjusted, set and tested as per design spec. Obviously I'm in no position to argue this. The cover was off to the terminals of the solenoid so as they tried the lift and the brake attempted to dis-engage I tested across the terminals a couple of times. This is when I got the 169/170V ac readings. The I.D plate though not clear appears to state 185V dc.

So, with no experience in these matters my assumption is/was that the applied voltage was firstly ac not dc and secondly too low to operate the solenoid as it was designed to do, thus causing the brake to bind on.

The lift is now back in operation, so I assume the problem sorted, though I have no details on what was done to achieve this.

Cheers for the reply. If the lift company have re-commissioned the lift,then no worries.

I would have the same interest as yourself,in such a device and it's problems,especially if of a vintage 😊

My concerns were not intended to spoil any interesting discussion on the function of period DC equipment,but to warn against a well intentioned and honest offer of assistance,allowing you to be drawn in to a possible can of worms.

In the past,i have seen and been closely involved,with such incidents,and it is not always the true villians,who the courts,or our safety friends in Sheffield,finally crucify.

I was once asked to "cast an eye",on a similar lifting device,having found myself in the middle of a falling out,between two,rival lift maintenance companies,and a building maintenance outfit.

The many visits and parts fitted,with no lasting fix,resulted in one of the engineers,and myself,inspecting the winding gear,where an old trick with yellow marker,revealed a siezed sheave,which the rope had cut through,almost to the bearing. This was the cause of noise,stretch and mis-timing.

This was an obvious long-standing issue,and had been missed by the engineer or his team,so put his company on the spot,but my involvement with LOLER and PUWER,meant i had a duty of care,regardless of any professional employ. He bit the bullet,and submitted an amended report,which allowed mine to remain in me diary😎

At no point,during any of the above,did i receive payment,or even a "thank you"..... the lad asked me where i bought me yellow marker from,but not even a cup of tea... 😂
 
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Hi,in connection with the topic,might i suggest the OP ensures any possible future liability risks,in testing or altering this braking system.

Having had many involvements in similar devices,any issue or mishap,at any time in the future,will be firmly dropped in the lap of the last-to-touch.

It usually is not the 12 people hurtling via gravity to doom,in a goods lift they should not have entered....
...it will be the 1" misalignment or slip,which catches a toe or finger,followed by the wait in casualty,watching the waiting room TV,with it's blame hunting brief ad's...

Be 100% sure of two things...

1) The squealing you mention,is not a siezed or failing shaft/coupling,resonance on one or more reeving falls,or a sheaving fault.

2) It's not you who gets the curly finger 😗
Good advice

Even worse a brake failure resulting in someone falling down a lift shaft after leaving the door open at a landing

I would leave it to the lift company to sort, rest assured the LOLER examiner will be scrutinising any brake repair/refurbishment documentation on his/her next thorough examination and if in any doubt supplementary tests will be called for at the expense of the lift owner.

If the examiner feels the brake is in any way problematic they will issue an immediate defect and likely inform the HSE or other enforcement authority.

Worst case of any accident involving the lift (even if nothing to do with the braking system) you are likely to be called upon due to your involvement in the lifts recent maintenance/repair history

Its simply not worth getting involved with
 

PEG

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Noticed a news item,regarding a fatality,involving a lift engineer,at Oldham Hospital.

Just to prove my mention of events which can lead to full investigations.....very sad,and my thoughts are with the family,of a fellow engineer,who went to work and did not come home.
 

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