Motor control Selenium Rectifier Bad

[NEW ENGINEER AT OIL REF]

The part is showing below went bad. It is part of pump control circuit(480V as shown below). i searched the internet part number for replacement and i could not find anything? Any suggestion on what to replace it with or what would be a good solution
Thank you

 

[zbang]

Any packaged bridge rectifier with the appropriate voltage/current specs would do the job; there must be thousands (check Mouser or Digikey).

 

[retirede]

Any packaged bridge rectifier with the appropriate voltage/current specs would do the job; there must be thousands (check Mouser or Digikey).

You have to be a little bit careful. Seleniums have internal resistance that a common diode rectifier does not. I expect for the OP, that resistance is needed to limit the current flowing through the zener diode shown. It’s being used as a cheap voltage regulator.

If that zener wasn’t there, it would probably be fine - you’d simply end up with a slightly higher DC voltage.

 

[zbang]

Ah yes, I'd forgotten about that, OTOH without knowing the zener voltage we can't know if it really matters.

 

[garbo]

Have not seen one of these in years. Somebody installed one of them in a NEMA 4 box and when it burnt out produced fumes that coked several people. Being the load is very low I would go with a cube full wave rectifier rated for 600 volts and at least 10 amps. With such higher amp bridge rectifier you should be able to get away without using an aluminum heat sink . Made a speed control for my dads old 1/2" drill that probably pulled 5 or 6 amps. Used a 25 amp 1" square cube ( full wave ) rectifier mounted inside of a deep 1900 box mounted on 1/4" aluminum plate and used the white colored heat transfer paste and have used it with no problems.

 

[synchro]

Have not seen one of these in years. Somebody installed one of them in a NEMA 4 box and when it burnt out produced fumes that coked several people.

Yes, the fumes from those selenium rectifiers when they burn out smell really bad.

 

[NEW ENGINEER AT OIL REF]

You have to be a little bit careful. Seleniums have internal resistance that a common diode rectifier does not. I expect for the OP, that resistance is needed to limit the current flowing through the zener diode shown. It’s being used as a cheap voltage regulator.

If that zener wasn’t there, it would probably be fine - you’d simply end up with a slightly higher DC voltage.

You suggest if I take that Zener diode off and run my circuit without it would work and not going damage my control circuit?? it is a good solution until i find correct replacement
note: the bridge rectifier shown rated for 600V

 

[NEW ENGINEER AT OIL REF]

Have not seen one of these in years. Somebody installed one of them in a NEMA 4 box and when it burnt out produced fumes that coked several people. Being the load is very low I would go with a cube full wave rectifier rated for 600 volts and at least 10 amps. With such higher amp bridge rectifier you should be able to get away without using an aluminum heat sink . Made a speed control for my dads old 1/2" drill that probably pulled 5 or 6 amps. Used a 25 amp 1" square cube ( full wave ) rectifier mounted inside of a deep 1900 box mounted on 1/4" aluminum plate and used the white colored heat transfer paste and have used it with no problems.

The existed bridge rectifier i have is rated for 600V, I think I will just take off the selenuim and run my circuit without it.
Please see attached

 

[retirede]

You suggest if I take that Zener diode off and run my circuit without it would work and not going damage my control circuit?? it is a good solution until i find correct replacement
note: the bridge rectifier shown rated for 600V

It would be interesting to know the specs of the zener before doing that.
What do the motors drive? They are going to try to run faster with a higher DC voltage supply.

 

[NEW ENGINEER AT OIL REF]

It would be interesting to know the specs of the zener before doing that.
What do the motors drive? They are going to try to run faster with a higher DC voltage supply.

It is a crude oil pump( 450Hp, 480V and 486 FlA).

 

[hbiss]

It is a crude oil pump( 450Hp, 480V and 486 FlA).

Ah, no. What are "M" and "M" that are in series off the DC supply? What voltage do they require and at what current? Also, what is the number off the zener so we might determine its voltage and current rating.

My feeling is that with a solid state replacement FWB rectifier you will need a series resistor to complete the voltage regulator. What that value and wattage needs to be depends on the voltage required by the load (M and M) as well as their current draw and the zener used.

-Hal

 

[winnie]

You suggest if I take that Zener diode off and run my circuit without it would work and not going damage my control circuit?? it is a good solution until i find correct replacement
note: the bridge rectifier shown rated for 600V

Emphatic NO.

The suggestion is that if the original circuit didn't have the Zener diode, then simply replacing the Selenium rectifier with a modern Silicon rectifier would be fine.

But the original circuit has a Zener diode to act as a voltage regulator, and the parasitic resistance of the rectifier limits the current flowing through the Zener.

If you simply replace the rectifier with a modern one, then it is quite likely that excessive current will flow through the Zener. If you replace the rectifier and eliminate the Zener, then excessive voltage will be placed across the circuit.

@hbiss has it right. At a minimum you need a new rectifier and a series resistance, and without knowing the rest of the circuit you can't really design the value for the resistor. It is also likely that the Zener is toast if the rectifier died.

-Jon

 

[ptonsparky]

Ah, no. What are "M" and "M" that are in series off the DC supply? What voltage do they require and at what current? Also, what is the number off the zener so we might determine its voltage and current rating.

My feeling is that with a solid state replacement FWB rectifier you will need a series resistor to complete the voltage regulator. What that value and wattage needs to be depends on the voltage required by the load (M and M) as well as their current draw and the zener used.

-Hal

I, didn't know what to make of that circuit. Still don't, but a simple replacement didn't quite sound right.

 

[retirede]

Ah, no. What are "M" and "M" that are in series off the DC supply? What voltage do they require and at what current? Also, what is the number off the zener so we might determine its voltage and current rating.

My feeling is that with a solid state replacement FWB rectifier you will need a series resistor to complete the voltage regulator. What that value and wattage needs to be depends on the voltage required by the load (M and M) as well as their current draw and the zener used.

-Hal

At first glance, I just figured those were some kind of small motor, but now I’m beginning to doubt that.

 

[NEW ENGINEER AT OIL REF]

Ah, no. What are "M" and "M" that are in series off the DC supply? What voltage do they require and at what current? Also, what is the number off the zener so we might determine its voltage and current rating.

My feeling is that with a solid state replacement FWB rectifier you will need a series resistor to complete the voltage regulator. What that value and wattage needs to be depends on the voltage required by the load (M and M) as well as their current draw and the zener used.

-Hal

M & M are two big coils are used to energize the motor, Each M is 240 Ohms and rated for 250VDC ( both coils measured at 477Ohms).
The output of the bridge rectifier after measuring it is 418V DC. Please see attached

 

[NEW ENGINEER AT OIL REF]

I, didn't know what to make of that circuit. Still don't, but a simple replacement didn't quite sound right.

i have attached the full schematic

 

[synchro]

Since the bridge rectifier is being fed by 480Vrms, the peak voltage across it will be 1.414 x 480V or about 680V. Rectifiers and diodes are rated for PIV (Peak Inverse Voltage) and not RMS voltage. And so I would get a bridge rectifier rated for 1000 PIV (which are commonly available), if not even higher. Solid state devices can be destroyed when their ratings are exceed for only very short durations.

I think the device shown in the schematic as a zener diode may be for suppressing excessive transient voltages when the switches or contacts are operated. If so, its avalanche voltage (where it starts conducting) would be higher that the normal peak output voltage of the rectifier, and it would only be conducting on short transients that exceed this avalanche voltage. This would then protect the rectifier diodes from transients higher than its (PIV) Peak Inverse Voltage rating.

M & M are two big coils are used to energize the motor, Each M is 240 Ohms and rated for 250VDC ( both coils measured at 477Ohms).
The output of the bridge rectifier after measuring it is 418V DC. Please see attached

If the zener was being used as a simple shunt regulator with a series resistance, when the coils are switched off the zener would have to be conducting at least as much current as the coils draw when they are turn on. Otherwise, when the coils are turned on the voltage drop across the resistor would increase, the zener would stop conducting, and it would drop out of regulation. And with 400V or more across the zener, it would be dissipating hundreds of watts. I have never seen a zener rated any where near 400V or with a dissipation rating in hundreds of watts. It would be helpful to see a picture of the zener device. If it is relatively small then it might be for suppressing transients rather than for a voltage regulator.

If you really wanted to regulate the voltage, and active regulator with a series pass transistor would be much more efficient. And even so, a voltage regulator would only be "clamping" the rectified single-phase sine waveform to a maximum level. When the voltage waveform falls below this regulated level it will be passed through without regulation. There still can be current flowing through the coils when the voltage waveform drops to zero because of the coil's inductance.

 

[retirede]

If the zener was being used as a simple shunt regulator with a series resistance, when the coils are switched off the zener would have to be conducting at least as much current as the coils draw when they are turn on. Otherwise, when the coils are turned on the voltage drop across the resistor would increase, the zener would stop conducting, and it would drop out of regulation. And with 400V or more across the zener, it would be dissipating hundreds of watts. I have never seen a zener rated any where near 400V or with a dissipation rating in hundreds of watts. It would be helpful to see a picture of the zener device. If it is relatively small then it might be for suppressing transients rather than for a voltage regulator.

It appears that when the control relay (MX) de-energizes the coils, the AC is switched off.

 

[synchro]

Given the vintage of the equipment with its selenium rectifier, I suspect that the symbol in the schematic which looks like a zener diode may actually be an International Rectifier Klip-Sel transient suppressor or its equivalent. These also used selenium technology and consisted of a stack of plates like with selenium rectifiers:

https://www.steampoweredradio.com/p...ransient Suppressors Engineering Brochure.pdf

For a replacement, a more modern technology such as MOVs (Metal Oxide Varistors) would be more appropriate and available.

 

[NEW ENGINEER AT OIL REF]

Given the vintage of the equipment with its selenium rectifier, I suspect that the symbol in the schematic which looks like a zener diode may actually be an International Rectifier Klip-Sel transient suppressor or its equivalent. These also used selenium technology and consisted of a stack of plates like with selenium rectifiers:

https://www.steampoweredradio.com/pdf/international rectifier/International Rectifier Selenium and Transient Suppressors Engineering Brochure.pdf

For a replacement, a more modern technology such as MOVs (Metal Oxide Varistors) would be more appropriate and available.

if my Selenuim Rectifier has 16 plates( this the only information i know about my Selenuim), what would be the right spec for the MOV?