Brake rectifier operation 2

[Dracula7]

Hi, I'm working as industrial electrician tech support and I'm kind of new in this type of work so here is my question:
The machinery I was troubleshooting today is PLC operated with a VFD controlled motor. Is a 480V AC, 5 HP with a brake rectifier that has 4 terminals. The motor just quit running after the Sandy hurricane passed thru ( the equipment is outside located). I checked all the outputs at the PLC and the motor gets his 480V. While checking the brake terminals I was kind of amazed to see that, even when the motor is in stop (not energized) position, the brake wire has 120V DC at the rectifier (while I tried to run the motor I had 277V on the same wire). The rectifier is wired this way: Phase A goes thru the brake coil at terminal 4 on the rectifier and to the terminal 2 also. Phase "C" ( the one that had the 120V) goes thru a NO contactor (with MOV across terminals) to terminal 3 at the rectifier.
Can someone explain to me how the rectifier actually works and what are the normal voltages that I should read?
Hoping I was clear enough, I'll Thank you all for any thoughts!

 

[rhatcher]

In a general sense, most motor brakes are normally engaged. Specifically, when no power is applied the brake is engaged. When power is applied the brake is released.

The normally open contactor is used to apply power to the brake solenoid coil when the motor is running to release the brake. The rectifier changes AC input power to DC output power for the brake solenoid coil.

A drawing or schematic will be helpful in sorting this out. Note that in many cases the terminals of a rectifier will be marked with "~" for the AC terminals and "+" and "-" for the DC terminals. The terminal numbers are meaningless since there is no standard for numbering that I know of that will identify which terminals are for AC input and which are for DC output. If you can post a drawing or schematic then I am sure that you can receive a specific answer to your questions about the expected voltages at each of the terminals.

 

[Dracula7]

Thanks rhatcher,
I'll try to upload a schematic.

 

[Dracula7]

Ok, here is the diagram for the motor.

 

[rhatcher]

It would appear that a full wave rectifier is being used for half-wave rectification. If so, terminals 1 and 2 would be the AC inputs, terminal 3 would be the "-" DC output, and terminal 4 would be the "+" DC output.

However, note that for half-wave rectification that the connection of line 73 to terminal 2 would not be necessary. Also note that the predicted voltages are different than the values you gave. You had 120VDC with the motor stopped and 277VAC with it running. Perhaps you got it backwards? Or, perhaps 18CR is acting the opposite of what the schematic shows. Or...perhaps my suggestion is wrong.

For the scheme that I am suggesting, the predicted voltages would be as follows:

Motor not running / contact 18CR open
Terminal 4 - ground: 277VAC

The 'circuit' is open so there is no current flow through the brake coil, hence no voltage drop across the coil. You are reading the line - ground voltage of wire 73 (L3) = 277VAC. The rectifier is not having an effect at this time.

Motor running / contact 18CR closed

Terminal 4 - ground: 124.7VDC = half wave rectified 277VAC
Terminal 4 - wire 73 (voltage across the brake coil): 216VDC = half wave rectified 480VAC

The circuit is closed through contact 18CR. AC voltage from terminal 1 (L1) crosses one diode in the rectifier and exits as half wave rectified DC voltage through terminal 4. The voltage from terminal 1 - ground is 277VAC, hence the half wave rectified DC voltage from terminal 4 - ground is 124.7VDC. When you reference the voltage to wire 73 (L3), the total AC voltage in the circuit is 480VAC, hence the half wave rectified voltage from terminal 4 to wire 73 (voltage across the coil) is 216VDC.

Let me know if this helps. If this does not match what you are seeing (ie. I am wrong), please let me know that as well.

 

[Dracula7]

Thanks again rhatcher for the help.
I'm confuse now about the brake coil voltage - while the motor is running there are 2 voltages across the coil? DC and AC also?
Anyway I won't be able to test anything until tomorrow, and another big issue is that I do the troubleshooting over the phone with the help of a field maintenance crew and they many times don't even have the electrical knowledge. So it's hard to say they did the right measurements but at least now I know more about those rectifiers and I hope I'll know how to guide them through this process.
I'll let you know how it worked. And I hope is just the rectifier bad not the motor.
Thanks again!

 

[rhatcher]

I apologize if my post was confusing. I can see where my terminology "216VDC = half wave rectified 480VAC" could be confusing.

Let's try again;
When the motor is running and 18CR is closed, if you measure the voltage between rectifier terminal 1 and wire 73 you will see 480VAC. Between terminal 1 and terminal 4, within the rectifier, is a diode that forms the half wave rectifier. Thus, the voltage from terminal 4 to line 73, across the coil, reads 216VDC. This DC voltage is the result of the the 480VAC between terminal 1 and line 73 passing through the diode between terminal 1 and terminal 4 and being rectified into DC. (480VAC after passing through a half wave rectifier diode = 216VDC)

Try this link for a description of a half wave rectifier with a circuit diagram and equations:

http://en.wikipedia.org/wiki/Rectifier

Check the link, read my posts again, and then let me know if you still need help. hooting.

 

[Skogsgurra]

Good "detective" work!
I would like to add one thing, the 73 wire and terminal 2 in the rectifier create a free-wheeling diode. That diode keeps current up in the brake coil during negative half-waves and avoids chatter.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rhatcher]

Thanks for the excellent (lps) contribution Scogsgurra!

I failed to recognize that the terminal 2 diode would act as a free-wheeling diode that allows continuous current flow and avoids the inductive spikes that would be created by making and breaking the DC brake coil circuit. Also, thanks for the compliment.

I had been wondering about the terminal 2 connection since my earlier posting. I had concluded that since the rectifier appears to be located at the motor, the brake coil is probably connected to terminal 4 and terminal 2 and is not 'separately' connected to line 73 as shown in the schematic. Thus, the connection of line 73 to the rectifier at terminal 2 is necessary as a termination point. Once again, I was failing to see the benefit of the free wheeling diode made available by this connection but, I do still believe that the coil is probably connected to terminal 2, not 'separately' connected to line 73.

Beyond that, I had also concluded that this system may be designed to be adaptable to both 480VAC and 240VAC voltages by utilizing dual voltage motors with 220VDC brakes. This would explain the use of a full wave rectifier when it is not necessary for a design dedicated to 480VAC useage.

For a 480VAC line voltage, the schematic shows the brake coil connected to terminals 3 and 2 (wire 73), thus using half wave rectification to result in a brake coil voltage of 216VDC.

For a 240VAC line voltage, the schematic would show the brake coil connected to terminals 3 and 4, thus using the full wave rectification to also result in a brake coil voltage of 216VDC.

In either case, thanks again Skogsgurra. That was an excellent contribution!

 

[rhatcher]

Correction:

"For a 480VAC line voltage, the schematic shows the brake coil connected to terminals 4 and 2 (wire 73), thus using half wave rectification to result in a brake coil voltage of 216VDC.

 

[Dracula7]

Hello again, and I apologize for my late answer but I'v been bad sick lately. I just managed to send a team in the field to check the motor and I found that you were right (both of you) and the issue was actually different than what I was looking for. The motor was actually running ( I still don't get it how they did not see that from the beginning) but the brake was seized believe it or not but the key stock was completely sheared inside the shaft so the motor was turning but could not move the machine because of the sheared key. Interesting isn't.
Thanks again to both of you for your help. At least I'm more knowledgeably regarding the rectifiers.

 

[rhatcher]

Thanks for the feedback. We're glad that you figured it out.