DC Noise
DC Noise
(OP)
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.





RE: DC Noise
RE: DC Noise
Can you tell the formula to calculate the capacitor. And is it wired in series or parallel with the brake.
RE: DC Noise
RE: DC Noise
Big:
Is the noise accoustic, electric or? What does it effect
or what are its characteristics?
<nbucska@pcperipherals.com>
RE: DC Noise
RE: DC Noise
-jXc=-j/(2xpixfxC)~small in ohms or per unit
RE: DC Noise
RE: DC Noise
RE: DC Noise
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.
RE: DC Noise
Skogsgurra (Note 2): Brake coil is disconnected from 24V power supply for operation - for that need a catch diode!
RE: DC Noise
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.
RE: DC Noise
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?\\\