April 2009 Newsletter

Read past issues. April 20, 2009
In This Issue

 

Hubs vs Switches – Using WireShark to sniff network packets

Gotcha #1 : Use a hub not a switch

Why:Switches don’t copy all messages to all ports. They try and optimize traffic so when they learn which port a device is connected to they send all messages intended for that device to that port and stop copying to all ports. (The jargon they use for this function is ‘learning mode’)How do you know it’s a hub: Just because it calls itself a hub doesn’t mean it is one.

    • If it says full-duplex in the product description it’s probably not a hub.
    • A switch that allows you to turn off the learning mode is effectively a hub.
    • A switch with a monitored port copies all messages to the monitored port and thus you can use that port as if it were a hub.
    • If it says ‘switch’ and you cant turn off learning mode and it doesn’t have a monitor port then it is not a hub.
  • A router is never a hub.

 

Gotcha #2 : Mixing 10 and 100 mbits/sec can cause problems.

Not all hubs copy 10mbit messages to 100mbit ports and vice versa.Use a 10mbit/sec hub if you are on a mixed network – almost all other faster devices are speed sensing and will downgrade themselves to 10mbits/sec and thus you will see all the packets. This is not true of some building automation engines where the speed of the port is configured.You can work around this problem by connecting higher speed devices to a self sending switch/hub and then connect that switch/hub to the 10mbit hub.

Recommended Hubs

  • 10Mbit/sec Networks – DX-EHB4 – 4 Port 10 Mbps HUB
  • Netgear – DS104 Dual Speed HUB
  • 10Mbit/sec Networks – D-LINK DE-805TP

Segmentation in BACnet

BACnet messages that don’t fit in a single packet use segmentation. Why would one message need more than one packet ? Well IP packets have a maximum length of 1500 bytes. So if you are sending a BACnet IP message that is longer than 1500 bytes then you need to send more than one Ethernet packet.

Example: Ask your fishing buddy if he wants another beer. The reply is short and fits in s single response packet. Yes. Now ask him to tell you about the one that got away. He will need multiple sentences to tell you the long storey. Your buddy needs you to support segmentation otherwise you will only hear the first sentence of his storey (lucky you.)

Be aware: 1500 is not a hard coded Maximum Transmission Unit (MTU) length in all Ethernet applications. Often the size is set smaller.

Most serial protocols like MS/TP choose a small number for the MTU because am error requires retransmission and the data is slow so it better to catch an error in smaller packet.

 

Practical / Field Issues

How does Segmentation affect you as a user ?If a device has a large number of objects and a message is sent to read the object list then it is possible that the response wont fit in a single packet. If both the device and the requestor support segmentation then there is no problem. If either side doesn’t support segmentation then 1) You are out of luck or 2) The requestor must use a different method to read the object list – for example, reading each object using its index until it reaches an index number with no object.The CAS BACNet explorer works like that – first it tries the most efficient method and then it slowly downgrades itself to try and ensure the response will fit in a single packet.How do you know if a device supports segmentation ?You can read the vendor’s PIC (Protocol Implementation and Conformance) Statement or you can look at the device object’s properties.
segmented-both (0)
segmented-transmit (1)
segmented-receive (2)
no-segmentation (3)

How can you work around the segmentation issue ?If you can configure the services your system uses to read data you have a fighting chance. Ensure you use read-property for a single property – avoiding read-property for all properties. If you have to use read-property-multiple then limit the list of properties to be read and avoid reading all using this service.

Get to know your Power Meter
What is Real, Apparent and Reactive Power

First – a simple question

Right or Wrong? Power = Voltage x CurrentThat statement is correct for DC systems but there are two major complications for AC systems.

  • The value of current and voltage keeps changing. Which value do you use ?
  • The voltage and the current may not be in phase. Multiplying the current and the voltage when they are not in phase requires and adjustment to compensate for the phase. It is this phase shift that forces us to define Real, Apparent and Reactive Power. This phase shift occurs when a power source feeds an inductive or capacitve load”. Most loads are either inductive (motors) or resistive (heaters) and therefore the phase shift is typically in one direction.
    • A motor has a winding. A wound conductor essentially defines an inductor. Thus the winding presents the resistance of the wound wire and the inductance resulting from the winding.

RMS or Effective Value

Peak values in the alternating voltage or current curves only lasts a short instant. They are not really representative of the ability of the voltage and current to do work and thus they are not used in Power Calculations.Scientists use a statistical method to define the effective values. It is called the RMS or Root-Mean-Square values. The result of the definition is that:Veff / rms = 0.707 x VpeakThe same applies to current too.

TIP: You can reasonably assume that all the Voltages and Currents reported by a Power Meter are reported as RMS or Effective values unless otherwise indicated.

TIP: Most multi meters report RMS values

Apparent Power

Apparent Power is the power delivered by a power source to a load like a motor. In almost all real world situations that use AC, you need to supply more power (Apparent Power) to a device than it will do work (Real Power).The (vector) difference between the two represents the work done to overcome the inductive and capacitive effects of the load.Apparent Power is measured in units of VA – Volt-amperes. These are actually Watts but we use the new unit name to reduce confusion. Thus when you see VA on a data sheet you can reasonably conclude that they are talking about Apparent Power.

Apparent Power is calculated:

S(Common symbol for Apparent Power) = Veff / rms x Ieff / rms – Single Phase Calc

Power Factor and Phase

Power Factor is calculated: PF = Cosine ( phase angle in radians)Power factor has no engineering units.The value of PF ranges from -1 to 0 to 1 (lagging – none – leading)

Loads that only present a resistive load (no capacitance or inductance) have a PF of 1.

Inductive Loads

Current phase lags the Voltage
Typical – Transformers and motors (wound conductors)

Capacitive Loads

Current phase leads the Voltage
Typical – Buried Cables, capacitor banks
There is nothing ‘wrong’ with having a power factor that isn’t 1.0.

Real Power and Reactive Power

Think of Real Power as useful power – a measure of how much work is being done.The units of Real Power are Watts.Real Power is calculated : P(Real) = S(Apparent Power) x pf

Reactive Power is the (vector) difference between Apparent Power and Real Power. The energy used to produce the Reactive Power is stored in the magnetic/electrical field of the Inductive Load. In the case of the capacitive load the magnetic/electrical field of the Inductive Load produces the Reactive Power. Reactive Power cannot be harnessed to do useful work.

Reactive Power is identified by the symbol : Q

The engineering units of Reactive Power are VAR – Volt-amperes Reactive. These are also Watts but we use VAR so that we know we are talking about Reactive Power.

Diagram: RMS measurement

Diagram: Leading / Lagging

THD – Total harmonic Distortion (also called Distortion)

In simple terms THD is a measure of distortion reported as %. If a device (any active device but think of rectifiers, variable speed drives … as practical examples) is given a sine wave as in input the output is never a faithful 100% reproduction of the input. A series of harmonics of the original wave distort the original wave form. The THD % is an attempt to ‘numberize’ the degree of distortion to allow for comparison. The % number is somewhat controversial because some harmonics are more important than others and there is no weighting.THD(%) = 100 * SQRT[(V22 + V32 + V42 + … + Vn2)] / VtWhere V2, V3 are the RMS values of each voltage harmonic and Vt is the total RMS output voltage.

Sag / Swell or Dip / Surge

Duration is 0.5 cycle and greater. Voltage sags are the most common power disturbance. Voltage sags can arrive from the utility. In most cases, sags are generated inside a building. For example, in residential wiring, the most common cause of voltage sags is the starting current drawn by refrigerator and air conditioning motors.Sags do not generally disturb incandescent or fluorescent lighting. motors, or heaters. However, some electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able to ride through longer but shallower sags.

Under / Over Voltage

Over Voltage is an increase in effective voltage to more than 110% for longer than one minute. Under Voltage is a decrease in effective voltage to less than 90% for longer than one minute. Take care with this definition because it tends to change from vendor to vendor.

Transient Voltages / Spikes / Surges

Refers to short duration (less than 1 cycle) events. Low frequency transients are often called “capacitor switching transients”. High frequency transients are often called impulses, spikes, or surges. They can be caused when a discharged power-factor-correction capacitor is switched on across the line.High frequency transients are caused by lightning, and by inductive loads turning off. Typical rise times are on the order of a microsecond; typical decay times are on the order of a tens to hundreds of microseconds. Often, the decay will be an exponential damped ringing waveform, with a frequency of approximately 100 kHz.Extremely fast transients, or EFT’s, have rise and fall times in the nanosecond region. They are caused by arcing faults, such as bad brushes in motors, and are rapidly damped out by even a few meters of distribution wiring. Standard line filters, included on almost all electronic equipment, remove EFT’s.

Power Meter Solutions Using FieldServer

From $1,350.00 to convert Power Meters to BACnet.

  • Includes configuration for a single meter
  • Includes Free CAS BACnet Explorer license.
  • Offer applies for certain meters only. Small extra fees apply for configurations which connect to multiple meters. Contact us for more info.

Metasys N2 by JCI Open solutions too. Contact us for more info.

Please send us your feedback about our newsletter to newsletter@chipkinautomation.com. Thank you.

Copyright © Chipkin Automation Systems 2009If you wish to cancel your subscription to this newsletter please e-mail unsubscribe@chipkinautomation.com with the word unsubscribe in the subject line.
Metasys® as used in this document is a trademark of Johnson Controls, Inc.2009 Chipkin Automation Systems – bacnet@chipkin.com
BACnet is a registered trademark of ASHRAE.
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