Light Dimming

[Crosleey]

I have encountered a problem in working with an HVAC contractor,and as a factory HVAC equipment,I haven't come up with a solution as yet.
Help would be appreciated.

I have a 2200 Sq.Ft, home 10 years old.The house is heated by an attic installed High-Velocity Heat Pump system.Recently the current 10KW resistance-
supplementary heater,located at the supply side of the air handler,experienced
a temperature overload which did not open at 150*,so the thermo fuse,or thermo
cutoffs opened at each heating element,opening the heater circuits.The system
was never designed to have the electric heaters and heat Pump operate at the sametime.Therefore,since the systems was never separated by outdoor temperature..as it normally should be.The heaters cycled on high limit for several
years.All as a result of improper installation.

Currently my contractor,who is asking for tech/Electrical advise,has installed
a new heater for the system.The new recommended heater is 10 KW,but uses
inverter technology.Otherwise.it's design is such,that now the Heat Pump and electric heat can be operated at the same time.The inverter design,modulates-
the 10 KW from 2-10KW or anywhere in between to maintain 110 * supply air
temperature during heating mode.
The system has been checked,and amperage read While operating.the amperage
modulates from 10-38/20-30a and so on modulating the heater voltage and
heating output as necessary.The voltage to the heater is 247Volts I PH

The wire size is #8 Copper 30 feet.The circuit is protected by a double pole
50A breaker.The house has a 200A service,with a sub panel which was set
for the heater and air handler,using #2 copper,from the main...about 3 feet
away.
The Outdoor split system heat Pump is functioning properly,at 247 volts
and 20 amps.supplied by a #6 Copper-U F

The HVAC tech was not able to tell me what size service entrance wire existed,but was told that the house had a heavy up a few years ago ???
The total service operating Amperage with all systems operating is 73 Amps
(Measured ) Nevertheless,we have a constant light dimming problem when the
inverter (Electric Heater is operating...The measured voltage at the lighting circuit is 122 Volts...Yet,when the heater is not operating,the lights return
to maximum output ( Brighter)I ask that they check to see if the lighting circuit was positioned on the same side as the Heater,and Heat pump,which he verified was not and balanced.

It this point,as you can see,I am puzzled.I have suggested that they contact the power provider,and verify what size wire is at the overhead service,and check all lug connections.Any ideas here ???? Thankshttp://forums.mikeholt.com/images/smilies/confused.gif
:-?

 

[wirenut1980]

Check for loose connections definately, and I would also suggest someone needs to monitor the voltage and see if the drop during startup is what should be expected, i.e compare against calculations. That should help tell you whether there are loose connections or not as well. If it is determined that all is well with connections then you either have to reduce current drawn that is sagging the voltage, or reduce the impedance to the load by installing a larger transformer, or larger wire from the service drop to the load.

 

[brian john]

Loose Connection.
Voltage Drop.

Light flicker (dimming and restoration of light to near preflicker level) of lights from motor inrush) is a byproduct of several factors and generally is difficult to over come.

AC motor loads (in particular compressors) but all motor loads have inrush currents. These inrush currents result in voltage drop in the branch circuit, panel bus, service laterals, utility transformer and possibility the utility HV feeders.

Flicker is in addition user dependent; some people are more susceptible to flicker. For some reason flicker seems to bother women more that men (based upon personal experience).

A Voltage Drop (VD) as little as 3 volts on a nominal 120 VAC system is noticeable (by me), 2.5% VD.

We have had cases were the VD was all a byproduct of primarily single phase 120 VAC loads, in these cases we were able to put the lighting on L1 and all the motor loads on L2, the HVAC compressors were something the homeowner had to live with. This minimizes the flicker.


To test for this VD and resulting flicker to determine the source of the inrush, use a min max amp clamp and min max multimeter at the main service watch the lights(it may take two workers) and correlate to the meters.

 

[Lxnxjxhx]

The brightness of an incandescent bulb is proportional to V^(3.5), so small voltage reduction spikes may have a big effect on the perceived brightness. The time constant of the filaments may lessen this somewhat, so 7 watt bulbs should seem to flicker more than 100 watt bulbs.

You may still measure 122 v, but the reading might be different with an average responding meter vs. a meter that actually responds to rms.

I'd use a scope and current sense resistor to trace this.

If it is due to current pulses being drawn by the inverter, you may be able to get a filter somewhere that clips these while leaving the sinusoidal currents untouched.

If this were a DC circuit, the easy solution for current spikes would be just a large capacitor with a ripple-current rating sufficient for the application.

 

[Lxnxjxhx]

Forgot

Forgot

The invertor manufacturer may be able to recommend a fix. He knows the best of all what kind of non-sinusoidal current pulses his invertor pulls.

 

[Lxnxjxhx]

forgot again

forgot again

The invertor probably runs at 10 kHz or so, to keep internal transformer sizes small. This current pulses are so fast you may not see flicker, just the perception of a dim light.

If you put a scope across the line, you may see "dents" in the sine wave voltage, these dents occurring each 100 microseconds (for 10 kHz).
Assuming the source impedance for the invertor supply is 1/4 ohm or so, a 20 amp pulse would show a peak-to-peak dent of ~five volts.
Since incandescent lamps respond to RMS values, this might have a large effect on the brightness.

If you live near Washington, DC, I'd like to take a crack at this. I get to learn a lot on jobs like these.