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# Brake rectifier2

## Brake rectifier

(OP)
If i measure the voltage accross the magnetic break coil it is 95volts dc, but if i disconnect the magnetic brake coil, it gives 270v dc. If i remove the wire connection and measure directlt from the rectifier, i measure only 140vdc...why it happens like this
Replies continue below

### RE: Brake rectifier

Have you tried with a different voltmeter?
What is the nameplate voltage.
Half wave or full wave rectification?
Have you checked the rectifiers?
Did you take some measurements across the leads and some to ground?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Brake rectifier

I recall playing with a rectifier circuit in a 4160V MCC. I recall that 120V rectifies to around 96 V or so (average of a full wave rectified AC sine wave) but if there is no load the voltage can rise to the peak of the ripple voltage, which can be as high as 1.414*120(169V). Not sure about the 270 though, unless youâ€™re measuring to ground and the DC system is floating.

### RE: Brake rectifier

Please measure the impedance of the brake. Then we can calculate.

### RE: Brake rectifier

Perhaps, the capacitor is inside the rectifier?
ACW

### RE: Brake rectifier

Mr. Gudz (Marine/Ocean)(OP)25 Jun 23 11:46
"....If i measure the voltage accross the magnetic break coil it is 95volts dc, but if i disconnect the magnetic brake coil, it gives 270v dc. If i remove the wire connection and measure directlt from the rectifier, i measure only 140vdc...why it happens like this".
1. We may be able to answer better if you can furnish the brake nameplate and the control circuitry, if available.
2. It would be helpful if you can furnish the following:
2.1 With the brake in off state,
a) remove both coil leads from the terminals and measure the coil only (dc) resistance...(R) in ohm.
2.2 With both coil leads removed from the terminals; switch on the brake.
a) measure the AC power source ac voltage .....(Vac) in Vac.
b) measure the Open-circuit [= without load] DC voltage at the coil terminals...(Vdco) in Vdc.
2.3 With the coil leads connected on the terminals; switch on the brake.
a) measure the DC voltage across the [energized] coil....(Vdc) in Vdc.
b) measure the DC current flowing through the [energized] coil....(Idc) in Idc.
3. Visually check whether is there any " large" capacitor in the circuit.
Che Kuan Yau (Singapore)

### RE: Brake rectifier

A rectifier connected to a big enough inductor will measure around 0.89 times the applied RMS voltage and have a constant current.

A rectifier connected to a capacitor will measure around 1.4 times the applied RMS voltage.

None of your voltages work out exactly correctly, but 95V and 140V could be expected with and without the magnet connected. I have no idea where 270V comes from except being a measurement error.

### RE: Brake rectifier

Is this a brake coil inside of a motor-brake combination where the power to the brake coil is tapped off of the motor leads? Or is is a stand-alone external brake? If stand-alone is the power feeding it?

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

### RE: Brake rectifier

The rectifier output has ripples and is used to smoothen the DC inductor or capacitor. When we use a capacitor, the output voltage will be more than the applied RMS ac voltage. This happens when the rectifier output is not connected to the load.

When the current flows through the diodes, the voltage drop in the circuit cause drop in the output voltage. The brake coil is an inductive type of load and draws a large current at energization and after pick up the current reduces to normal value.

That is why the rectifier's output voltage is less than the voltage at no load.

### RE: Brake rectifier

Quite an interesting interchanging of cause and effect, followed by the application of an AC effect to a DC circuit.
On the one hand;
"A little knowledge is a dangerous thing."
On the other hand;
Keep studying, your not quite dangerous yet.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Brake rectifier

Gudz ,
In older installations, there is often a resistor in the series with DC coil circuit that reduces the current when the brake is applied .Maybe it's giving you a headache .
Good luck

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