Drive freq.

Alan Case

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Apr 2002
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Wagga Wagga
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A quick question. What effects does changing the carrier or switching freq. (I think that is the right terminology) on a drive have. ie if I go from 2kHz to 1kHz are there any performance effects.
Regards Alan
 
Hopefully DickDV will give the detailed background, but the general answer is that increasing the carrier frequency decreases lamination noise at the expense of increasing stress on the insulation from dV/dt transients. For many drives very high carrier frequencies also mean derating the drive output current.

I've never had to change the manufacturer's default carrier frequency on my applications. May I ask what prompted the question?
 
Tom's answer is right on. The only thing I would add is that, at very low frequencies, say less than 1kHz, you can get shaft cogging at very low motor speeds. With modern drives and higher carrier frequencies, you almost never see this condition. It used to be a big deal but, then, I've been doing this too long. 'Nuf said!
 
Drive on a pump varies between 30 and 50 HZ. Ran OK for 1 week then when pump runs the thermistor and level sensor wires are having something induced in them even when there is a 200mm separation of cables. (Unsheilded) The induced ***** tells the pump controller (multitrode) the wet well is full so the pump runs flat out and never stops. Dropping the switch freq to 0.5hz with a cable separation of 1 metre stopped this. Putting it back at default 2kHz started faulting the system again. Am going to check today if the inbuilt filters failed after 1 week. Regards Alan Case
 
It sounds to me like you have a grounding problem. Changing the crrier frequency is just masking the basic EMI/RFI problem. The analog signal shields, the VFD, your panel power, etc. should be looked at. Include the internal VFD components - I once had a Siemens VFD drive my communications nuts until they properly grounded the control card on the VFD.

I'm surprised the multitrode is giving you a problem. The ones I'm thinking of are a series of rods, switching at discrete levels based on condictivity between the rods. Some of them are set up to work with metal tanks, and if you are pumping out of a concrete tank you may have to look at the grounding on this as well.
 
This is a nasty problem and becoming all too common as carrier frequencies go up.

Maybe a little primer on drive noise types is in order.

The first type is high frequency components in the current on the incoming power leads. These result from the drive input rectifiers using the current non-linearly to keep the DC bus charged up. Reactors in the input power leads or, in some cases, a DC bus choke reduce this source of noise. In extreme cases, a matrix filter may be used or alternatively, a more complex input rectifier with twelve or eighteen diodes (pulses) and a phase shifting input transformer.

The second source is radiated RFI/EMI mostly from the motor leads. This comes from the high frequency components in the sharp pulses that the drive sends to the motor (carrier pulses). The solution here is to put the motor in conduit or, as is often done in Europe, use shielded motor leads.

The third source is a really bad actor---common mode noise. It is caused by the capacitance between the motor leads and ground. The high frequency components couple thru the capacitance to ground and then must find a way back to the DC bus of the drive. The most likely way is to travel thru ground all the way back to the substation transformer secondary and then thru the AC supply back to the drive. This clearly puts high frequency noise in the ground system and also in the AC power leads. Noise problems can surface long distances from the drive and in a completely random arrangement. Any instrumentation depending on this ground system or AC power can be affected. Note also that the attempts to deal with noise source number two above make this third noise source worse. The only drive solution that I have seen work is a drive isolation transformer with a grounded wye secondary. This allows the high frequency ground currents to travel back to the DC bus via a shortcut and stay out of the rest of the network. Unfortunately, at higher horsepowers, an isolation transformer can be a big expense.

I have had some success in situations where only one instrumentation device is having a problem by dealing with the instrument rather than the drive. This is particularly desireable if the drive hp is high.
The first general rule is not to ground to earth anything on the instrumentation if that is possible. Bring all shields etc back to instrument common but don't tie the common to earth. Another thing that works sometimes is to use a high performance noise filter on the power supply to the instrument. Often this is 120VAC and the filter can be quite inexpensive. I've even put a small UPS with good input/output isolation in the power supply to the instrument with good results.

The bad news is that there seems to be no systematic way to deal with noise problems. I just hate the stab-in-the-dark method but haven't found anything else that does any better. Drive noise problems keep many consultants employed, some with good track records and some not so good.

Good luck. I'm glad I'm not there!
 
Tom, this multitrode is one pole about 2 metres long which is divided into 10 insulated 10 sections of 200mm. Each section has a wire going to it and I think the earth is used as a return. ie as the water rises more sections conduct back through earth. This is a sewerage pump that is 24/7 so I had to keep it going till I could figure out what the problem is hence the change of carriere freq. The work was someone elses retrofit of a VFD so there is no screening of any cables.
If I drop the multitrode in the well with a 200mm separation from the motor cable (8M deep) the cables pick up enough **** to give indication of conduction on all 10 wires. Move it out to 1M separation it is OK. Even at 1M if I up the carrier freq to 2kHz then all 10 wires give indication of conduction. Also at the 2kHz if I leave the thermistor wires connected to the multitrode controller with the multitrode probe unplugged the unit gives indication that all 10 wires are conducting. Yet with the motor not running everything works perfect. The has system run OK up till yesterday, but they did tell me they got unexplained high level alarms every now and again so I suspect it was always prone to this problem, just something has exascerbated it. Thanks to you all incuding DickDV.
 
I am not an expert on VFD's but..

Does the VFD have some kind of EMC filter built-in or external.

If not, then I think that attaching an EMC filter to the output of the VFD would help your situation.

Would it be possible to attach an additional EMC filter to an existing one for "severe cases" (I dont know, just brain-storming) ?

Can't you exchange the existing cables for screened ones ?
 
VFD has an inbuilt filter. We will be upgrading cables shortly. The original reason for my post was too make sure no damage can be done by lowering the carrier freq to a real low level to keep them going in the meantime. Regards Alan
 
Another shot in the dark

You might try using an RC network across the multitrode and at the instrument to effectively shunt the noise to ground. The values should have a low impedance at the carrier freq. I don't know what values you would need but a QuenchArc might work.

Sid
 

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