DC Noise

[bigdog]

I have a 3 phase induction motor that has a brake. I'm conrolling the motor with Mitsubishi VFD. We seem to be getting some noise from the brake when it is activated. I would like to know what kind of device can I use to filter the DC voltage. The control voltage the brake is 24VDC.

 

[jbartos]

Suggestion: DC is normally separated by a blocking capacitor.

 

[bigdog]

jbartos

Can you tell the formula to calculate the capacitor. And is it wired in series or parallel with the brake.

 

[sreid]

I'll bet that your problem is that you need a catch diode across the brake coil. When the coil is de-energised, there is a huge inductive voltage kick from the coil. The diode is connected cathode to coil plus, anode to coil minus. There is probably some note about this in the motor controller manual.

 

[nbucska]

I'm sure Mitsubishi knows about catch diodes...

Big:
Is the noise accoustic, electric or? What does it effect
or what are its characteristics?


<nbucska@pcperipherals.com>

 

[blacksea]

I filly agree with "streid" reply - but I add a little note: a catch diode must be locate to brake coil as near as possible.

 

[jbartos]

Comment: The blocking capacitor value has to be calculated/selected such that there is very little voltage drop for the AC current flowing through it. DC current blocking is then automatic.
-jXc=-j/(2xpixfxC)~small in ohms or per unit

 

[OperaHouse]

In some cases you need to worry about speed. A diode will slow down the dropout of the brake. Wouldn't expect this to be a problem here. Speed can be increases by putting a 10 or 22 ohm resistor in series with the diode. I would choose a current value for the diode of twice the normal coil current. Theoretically, place as close to the coil as possible. Most times it is OK in the control cabinet. Other possible solutions are the use of a low voltage 10J MOV or a RC network accross the coil of 22 ohms and a 1uf foil capacitor in series. In fact, I have seen high current systems that use all three (talk about shotgun). If the coil is only about 2A, this is a prety easy case.

 

[Farkel]

Have you seperated the brake coil circuit from the motor, so the drive does not effect the brake coil with variable voltage, and frequency?

 

[Skogsgurra]

I think that some confusion is at play here:

1 Bigdog already stated in his first posting that the brake is fed from 24 V DC. So there is no need to tell him to check if the brake is separated from the PWM motor voltage. I really hope that the rectifier is a full wave one. A half wave rectifier would certainly result in noise and bad performance.

2 Bigdog does not say if the 24 V is smoothed or not, but it is plausible to think that it is derived from a full-wave rectifier (see #1) and then you do not need any catch diode, aka free-wheeling diode. They are only needed to catch the overvoltage when you break the DC. The noise is present during operation, so a catch diode is of no importance with respect to that problem. Nor are MOVs and RC circuits. And it is not important at all to place the diode or RC network close to the coil. It works well wherever they are connected to the circuit. Do not confuse them with bypass capacitors.

3 There is some talk about DC blocking. I think that those postings miss the goal by 100 %. The brake coil shall have as much DC component as it can get. Blocking DC and letting the AC component through is doing just the opposite.

If you really need to filter the DC voltage, a parallel capacitor - say 1000+ uF and 50 or 63 V - will do the job. But that is usually not needed since the time constant of a DC coil usually is in the 100 - 1000 ms region and that will smooth the current better than a capacitor will. A capacitor will also increase the voltage by some 40 % and that will problably make the coil run very hot.

I think that you should check the current waveform with a DC clamp and an oscilloscope. That will help you differentiate between an electrical and a mechanical problem. If the current (not voltage!) is reasonably smooth (less than 20 - 30 % ripple) and if it is within rated value +/-10 %, then the problem is not electrical.

 

[blacksea]

Farkel: Brake coil is component of brake and of course isn't removed from motor!

Skogsgurra (Note 2): Brake coil is disconnected from 24V power supply for operation - for that need a catch diode!

 

[Skogsgurra]

Blacksea,

For the sake of correctness; Farkel never said that the brake should be removed from the motor. I never said that a catch diode would not be needed - only that a catch diode does not solve the problem since the noise exists during operation and not during switch off.

 

[jbartos]

Suggestion to bigdog (Electrical) Apr 20, 2004 marked ///\\I have a 3 phase induction motor that has a brake. I'm conrolling the motor with Mitsubishi VFD. We seem to be getting some noise from the brake when it is activated.
///Please, would you elaborate where the noise is located. This is needed in view of the other posting that interprets the original posting 24VDC to be filtered, possibly for some bigger ripple.\\ I would like to know what kind of device can I use to filter the DC voltage.
///Please, would you elaborate on the kind of filtering it is needed. Whether it is large noise superimposed to the dc ripple or anything else. I interpret DC filtering as the dc component removal from AC waveforms. This is a typical interpretation and terminology in electronics.\\ The control voltage the brake is 24VDC.
///What is the power voltage for the brake?\\\