Goods Lift Brake Solenoid Not Operating Correctly

[Intoelectrics]

Hi all,

Just looking for some advice,

I've been asked to take a look at a brake system on a goods lift. The lift is very old and unreliable with many parts now discontinued.
Its a cable and pulley type OTIS lift.

The shaft between the motor and gear box has a simple solenoid operated braking device. The lift engineers have been out to investigate the brake as its binding on. They have taken the whole device away had it re-furbished and fitted it back in situ. The problem still persists, they have explained that the coil in the solenoid was re-wound and tested prior to re-instalment.
The I.d plate on the solenoid is a bit worn and very unclear but it appears to me to be rated at 185v dc. I tested the applied voltage and found it to be 170v ac. I'm now in a bit of dispute with the lift engineer's technical guy over this.
They tell me that they have exchanged a bridge rectifier at some point, I'm arguing that the problem lies with the wrong voltage.
Speaking to the employees who use the lift daily they say that you can hear the squeal from the brake and that its been like that for a while.

Have any of you guys come across this type of thing?

Thanks

 

[westward10]

If the solenoid has definitely been rewound for dc but the applied voltage is ac then it is highly possible the solenoid isn't pulling in enough.

 

[Pete999]

Hi all,

Just looking for some advice,

I've been asked to take a look at a brake system on a goods lift. The lift is very old and unreliable with many parts now discontinued.
Its a cable and pulley type OTIS lift.

The shaft between the motor and gear box has a simple solenoid operated braking device. The lift engineers have been out to investigate the brake as its binding on. They have taken the whole device away had it re-furbished and fitted it back in situ. The problem still persists, they have explained that the coil in the solenoid was re-wound and tested prior to re-instalment.
The I.d plate on the solenoid is a bit worn and very unclear but it appears to me to be rated at 185v dc. I tested the applied voltage and found it to be 170v ac. I'm now in a bit of dispute with the lift engineer's technical guy over this.
They tell me that they have exchanged a bridge rectifier at some point, I'm arguing that the problem lies with the wrong voltage.
Speaking to the employees who use the lift daily they say that you can hear the squeal from the brake and that its been like that for a while.

Have any of you guys come across this type of thing?

Thanks

What voltage have you got across the solenoid?

 

[brianmoooore]

What voltage have you got across the solenoid?

And is it definitely AC?
Where is the bridge rectifier mentioned connected? A 170 volt AC supply fed through a bridge rectifier, with an electrolytic capacitor connected across it's DC output will produce considerably more than 185 volts DC.

 

[Intoelectrics]

What voltage have you got across the solenoid?

169v AC at my last check.

 

[James]

I would change the rectifier and see if it improves

 

[Intoelectrics]

And is it definitely AC?
Where is the bridge rectifier mentioned connected? A 170 volt AC supply fed through a bridge rectifier, with an electrolytic capacitor connected across it's DC output will produce considerably more than 185 volts DC.

Well this is what is confusing me. The job is being done by the lift engineers, I was on site at the time and the company asked me to take a quick look as the lift engineers were scratching their heads. They (understandably) did not want me around and it was uncomfortable. So I checked the ID plate and tested across the solenoid terminals as the brake was dis-engaged. I got 169V AC, the I.D tag (though worn) looks to read 185V DC. I concluded that this was the problem, mentioned it to their tech guy and he began to waffle on about it, very annoyed at my presence there. He mentioned the bridge rectifier but did not give any details. I have now since left site and apparently the lift is now working. I'm not privy to what was done to sort it.

 

[Intoelectrics]

If the solenoid has definitely been rewound for dc but the applied voltage is ac then it is highly possible the solenoid isn't pulling in enough.

This was my conclusion.

 

[Intoelectrics]

I would change the rectifier and see if it improves

The tech guy mentioned replacing the rectifier but did not go into detail.

 

[Pete999]

169v AC at my last check.

What is the operating voltage of the solenoid



















'

 

[marconi]

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...

 

[Intoelectrics]

What is the operating voltage of the solenoid



















'

As I mentioned in other posts, the I.D tag appears to state 185V DC though it is worn and not 100% clear But the DC bit is clear enough.

 

[Intoelectrics]

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]

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]

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.