Mass Confusion

[lccllc 0]

Ok today at work we were testing an overhaul system. We have four motors with electrical brakes, the brakes are 120 VDC. They are wired form 120VAC through a full bridge rectifier, to a relay on the + side, and the – side to a terminal in the MCC. From there they go to the brakes. When the power is on and the brake is wired up and the relay is not energized the voltage is 208 VDC when the – side of the brake is unwired and the relay is de-energized the voltage is 120 VDC. Further more when the relay is energized and the brake is connected the voltage drops from 208 VDC to 120 VDC and the brake turns on. It works but my nerd Brain cannot figure out the theory behind this and I have never seen this be for. I would appreciate the answer to this.

[Ken Moore 23]

quick quess, you are using one leg of a 208V 3 phase system and you grabbed the "high" leg.

[lccllc 0]

No 120 phase to ground to the rectifier. then to a relay then to the brake, the three phase is 208 Y, no high leg. Edited 10 Jan 2009 by lccllc

[PdL 71]

Make a electrical diagram of your situation. It will not only perhaps help you understand but also other users who can't follow your description.

[BobLfoot 302]

My interpretation not the original posters. mrplc_15734_001.pdf Couple of observations. 1. Is either leg + or - of the bridge grounded? 2. "Terminal in the MCC" - what type just a floating junction point or some other i/o card or electronic device? 3. What distances are involved Bridge to MCC to Brake? Possilbe stray induced voltage? 4. What meter are you using, some meters are more susceptible to noise than others.

[lccllc 0]

The meter is a fluke 110, the bridge is not grounded, and now that you say it, the voltage may be floating due to not being grounded. I will ground it on Monday and see if that does it. problem.pdf

[edpuente 0]

Can you confirm us the voltage after the transformer and before the bridge? It would seem it is not a 120/120 V

[JeffKiper 9]

It has been a long time but here we go. If you full Wave Bridge Rectify 120VAC you get 170VDC, so if you had any float on the AC side you could easily see a spike in the DC. This is with NO load on the system. What is the resistance of the brake coil at ambient temperature? What is the ambient temp? What is the incoming voltage 110, 115, 120, 125, 130?

[paulengr 44]

How do you figure 170 VDC off 120VAC? 0.707 * 120 = 84 VDC. Granted it depends on how well you filter it but still, I can't see any way short of an active device (you need a charge pump) to get 170 VDC out of 120 VAC.

[BobLfoot 302]

Per Wikipedia in this article. Note: Where 'RMS' (root mean square) is stated above, the peak voltage is ?2 times greater than the RMS voltage for a sinusoidal signal centered around zero voltage. Sqrt 2 = 1.4142135 so 110 VAC RMS = 155.56348 VAC PK 115 VAC RMS = 162.63455 VAC PK 120 VAC RMS = 169.70562 VAC PK 125 VAC RMS = 176.77668 VAC PK 130 VAC RMS = 183.84775 VAC PK A full wave bridge reached the Peak (PK) not the RMS value when enough holding capacitance is added or little load is involved.

[JeffKiper 9]

120 VAC * 1.414 =169VDC peak voltage

[lccllc 0]

The transformer is 380 to 120 VAC then to the bridge, I do not see the peak of the voltage to 170, after the brake is removed, the voltage drops from 208 to 120VAC. I did not have time to bond the - side today due to some miss termination of my DC drive RJ45 terminations by one of my guys, who would of guess that when you do one end ok and the other end are crossed it will not work. So I spent all day on that. Edpuente the voltage after the transformer is 120VAC. My understanding is the voltage would drop a little after you passed it through the bridge, or am I wrong Edited 12 Jan 2009 by lccllc

[sbaum 0]

This seems like a meter issue to me. Standard True-RMS VOM will read an average reading on DC. 120VAC ( RMS ) x 1.414 = 169.68 VAC ( Peak ) 169.68VAC peak Full wave rectified ( single phase ) X 0.637 will equal a 108.08 reading on your meter. Could you give us both AC and DC reading from the load side of the bridge. That might clear the issues. Find yourself one of those old analog meters and take a reading. I bet it will give you a different reading.

[BobLfoot 302]

If you can do us all a favor and take the following readings. Your 110 should work, but a good analog Simpson would provide more accurate data. Test Set "A". Disconnect the Bridge from ALL DC Loads. 1. Ground the "Black" Lead of your meter to earth ground. 2. Read the AC voltage on Transformer Primary Leg 1 with the "Red" lead. 3. Read the AC voltage on Transformer Primary Leg 2 with the "Red" lead. 4. Read the AC voltage on Transformer Secondary Leg 1 with the "Red" lead. 5. Read the AC voltage on Transformer Secondary Leg 2 with the "Red" lead. 6. Read the AC voltage on Bridge "-" Terminal with the "Red" lead. 7. Read the AC voltage on Bridge "+" Terminal with the "Red" lead. 8. Read the DC voltage on Bridge "-" Terminal with the "Red" lead. 9. Read the DC voltage on Bridge "+" Terminal with the "Red" lead. 10. Read the AC Voltage on the bridge terminals Red = "+" and Black = "-" 11. Read the DC Voltage on the bridge terminals Red = "+" and Black = "-" Test Set "B". Connect the Bridge Leave the Relay Open. 1. Ground the "Black" Lead of your meter to earth ground. 2. Read the AC voltage on Transformer Primary Leg 1 with the "Red" lead. 3. Read the AC voltage on Transformer Primary Leg 2 with the "Red" lead. 4. Read the AC voltage on Transformer Secondary Leg 1 with the "Red" lead. 5. Read the AC voltage on Transformer Secondary Leg 2 with the "Red" lead. 6. Read the AC voltage on Bridge "-" Terminal with the "Red" lead. 7. Read the AC voltage on Bridge "+" Terminal with the "Red" lead. 8. Read the DC voltage on Bridge "-" Terminal with the "Red" lead. 9. Read the DC voltage on Bridge "+" Terminal with the "Red" lead. 10. Read the AC Voltage on the bridge terminals Red = "+" and Black = "-" 11. Read the DC Voltage on the bridge terminals Red = "+" and Black = "-" Test Set "C". Connect the Bridge and close the Relay. 1. Ground the "Black" Lead of your meter to earth ground. 2. Read the AC voltage on Transformer Primary Leg 1 with the "Red" lead. 3. Read the AC voltage on Transformer Primary Leg 2 with the "Red" lead. 4. Read the AC voltage on Transformer Secondary Leg 1 with the "Red" lead. 5. Read the AC voltage on Transformer Secondary Leg 2 with the "Red" lead. 6. Read the AC voltage on Bridge "-" Terminal with the "Red" lead. 7. Read the AC voltage on Bridge "+" Terminal with the "Red" lead. 8. Read the DC voltage on Bridge "-" Terminal with the "Red" lead. 9. Read the DC voltage on Bridge "+" Terminal with the "Red" lead. 10. Read the AC Voltage on the bridge terminals Red = "+" and Black = "-" 11. Read the DC Voltage on the bridge terminals Red = "+" and Black = "-"