Motor query for 3 phase VFD conversion

[John HaineParticipant @johnhaine32865]

Many modern washing machines use VFDs with 3 phase motors so presumably a mains filter designed for one of these should at least be safe.

[SillyOldDuffer]

On SillyOldDuffer Said

Clear as mud!  Can anyone do better?

Power, in watts, is the work done in unit time, ie, 1 second.

Torque, in N.m, is a force that tends to rotate a body about a chosen point.

For an electric motor power is torque times angular velocity in radians per second.

It may be helpful to do a dimensional analysis. In SI base units the Watt is equivalent to kg.m­­­­­­­²/s­³. Additionally the newton is kg.m/s².

For a motor power is torque times angular velocity which equals N.m.rad/s, and in base units kg.m.m.rad/s³

Simplifying gives kg.m².rad/s³

In the SI system the radian is dimensionless, ie, it has no units and is just a number, so in base units we get the power of the motor as proportional to kg.m²/s³ which is power in watts.

Following on from SoD’s comment that a stepper motor can give torque at zero speed so can an induction motor using vector control. There are two basic types of vector control, open-loop and closed loop. In open loop there is no rotor position sensor. This limits the minimum speed to around 0.8Hz. In closed loop, with knowledge of rotor position, an induction motor can give full torque at zero speed. Although the rotor is not rotating the magnetic field from the poles is, at the appropriate slip frequency.

The ability to have full torque at zero speed is important in electric vehicles as it is the equivalent of slipping the clutch for a hill start.

Julie

[John HaineParticipant @johnhaine32865]

Torque induction motors can give high torque at zero speed or even being driven backwards, used in high quality tape drives or the like for tensioning.  Not so good for supplying power though.

[SillyOldDuffer]

On SillyOldDuffer Said:

<SNIP>

There are significant safety issues with mains filters. Don’t DIY them unless you know what you are doing. If it was easy electronic equipment manufacturers would make their own rather than buying them from Schaffner et al. Even using parts from an existing filter can be dangerous. …
If you have to ask how, don’t even think about doing it.

Robert.

Can you explain in more detail please Robert?  John, who if I remember correctly is a retired electrician, is asking about fitting a sealed manufactured EMC filter from a washing machine in front of his VFD.   How is that dangerous, especially if he does the work?

In terms of performance, not safety, my feeling is a washing machine filter won’t be brilliant at suppressing the muck coming out of a VFD.  That’s why I suggested it might be improved by adding capacitors.  I plead guilty to assuming readers know the capacitors have to be mains rated components, not standard fare.  I also failed to explain what values might be suitable, or how to test whether or not the filter is improved by them.

Though John may not know anything about filter design, I’m confident he could wire one up without coming to grief.  And important to note, no-one is expecting a washing machine filter to be as well engineered as one designed to meet an EMC specification.   It only has to reduce the problem by a useful amount – a mere 50% reduction would be helpful.

Any job can be tackled provided risk is managed.  In this example I’m not aware of anything about fitting a washing machine EMC filter that’s high risk or high impact if it goes wrong.   As always I might have missed something though!  What’s the worst that could happen?

Ta,

Dave

 

Hi Dave,

The safety issue isn’t emc. It’s about earth connections becoming live and to a lesser extent fire.
Using an ex equipment commercial “block” EMC filter is not a problem as long as it has adequate current rating and is correctly earthed including the case if it’s metallic. As  John says many washing machines use electronic motor drives so the filter may be suitable for a small VFD.

The origial post said:
“.. ex washing filter block, capacitors and chokes,”
It’s the capacitors bit where the trouble lies. The capacitors used in mains EMC filters have very specific safety requirements relating to failure modes / probabilities. All of them must withstand the significant voltage spikes and surges that appear on the mains. A normal 300 V AC capacitor is NOT suitable for use on 240V mains. Capacitors connected between Live and Neutral must also be designed so that they do not draw excessive current if they do fail. If a capacitor drew just 5A when failed (a VFD is likely to have at least this fuse rating) it would dissipate over 1kW. That’s a big fire.
Capacitors connected to “earth” must not pass too much current even when workinueg normally. This limits the maximum value you can use. Because of the shock hazard if they fail and there isn’t a good earth they have to be even more reliable. Capacitors used in filters must be safety approved. Those across the mains are “X” rated those to ground are “Y” rated. There are classes within those ratings.

For VFDs to meet emissions requirements they sometimes require filter capacitor values higher than those allowed for “normal” equipment. This is allowed but special precautions are required. Typically that means they are connected to a fixed supply with a dedicated isolator.

Ther worst that could happen with an incorrectly made filter?
1/ Existing poor earth (maybe 1 strand left that passes a basic PAT  but fuses with a fault)
2/ Incorrect capacitor fails short circut making metalwork live.
3/ You grab metal work and get a fatal electric shock.
4/ Your spouse/child/friend tries to rescue you and also suffers a fatal shock.

Yes the probability is low but that high consequence means you HAVE to use the correct components in the righr places. Commercially if you build your own you have to prove it is a safe design. Unless you are making thousands of units in’t not worth the cost, time and risk so you buy a Schaffner / Corcom whatever pre built and approved unit.

Robert.

Robert.

[Author]

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