I am working on an experiment to determine if the use of a DC controlled proportional AC relay has any detrimental effect on the system as a whole and the Quattro Inverter/Charger in particular. The relay has two amperage control modes burst and phase angle, my test uses burst mode. The relay is designed to control resistive loads only.

Test Environment:

Inverter/Charger: Victron Quattro 48/5000/70-100/100 120V (In Inverter Mode)
Battery BMS: Victron Lynx Ion BMS 1000
Batteries: Two Each Victron 24V/200AH HE NMC batteries wired in series.
Proportional Relay: Crydom PMP2425W (25A Rating)
PLC: Automation Direct BR-DM1E-10AR3-D
Load: 1320W 120VAC resistive heating element (This was the only load active)

Link to the Crydom relay datasheet is below:

http://www.crydom.com/en/products/catalog/pmp-nova22-proportional-control.pdf

My problem is an anomaly I discovered when the relay was set at exactly 50% output. Upon further testing I noted that this behavior existed when the relay was set to produce between > 45% and < 55% output power. At these settings the relay was sending exactly one cycle power off and one cycle power on. At all other settings the number of power on/off cycles does not match. Also, in the problem range, the power was firing when the Voltage passed 0 going negative; for all other output percentages, the power fired when the Voltage passed 0 going positive.
What drew my attention to this is a RMS voltage drop of about 4V in the problem range.

QUESTION: I don't have a clue what is causing this, or if there is any way to mitigate it. Any help would be appreciated.

Oscilloscope traces for 25%, 50%, and 75% output power are below:
The scope setup is:
Channel A (blue) is Voltage measured at the inverter. (Active/Differential Voltage Probe)
Channel B (Red) is Amperage measured at the relay output (Load). (Clamp AC/DC Amperage Probe)

25% Trace:


50% Trace:


75% Trace:

 

At these settings the relay was sending exactly one cycle power off and one cycle power on. At all other settings the number of power on/off cycles does not match.

I'm not clear on the problem.
In the burst-mode it varies the number of complete on/off cycles at the output to control the power, so the increment change in power is one complete cycle. Thus at 50% power the number of on and off cycles is equal, and at other power levels the number of on versus off cycles varies to give the percentage power you want.
What, exactly, type of output do you expect?

Also, in the problem range, the power was firing when the Voltage passed 0 going negative; for all other output percentages, the power fired when the Voltage passed 0 going positive.
What drew my attention to this is a RMS voltage drop of about 4V in the problem range.

I don't see how, whether the cycle starts at positive or negative would make a difference in the RMS voltage.

If you want a smoother variation of power then you need to use the phase-angle mode.

 

I'm not clear on the problem.
In the burst-mode it varies the number of complete on/off cycles at the output to control the power, so the increment change in power is one complete cycle. Thus at 50% power the number of on and off cycles is equal, and at other power levels the number of on versus off cycles varies to give the percentage power you want.
What, exactly, type of output do you expect?
I don't see how, whether the cycle starts at positive or negative would make a difference in the RMS voltage.

If you want a smoother variation of power then you need to use the phase-angle mode.

I am mainly concerned with the 4V drop in RMS AC when at 50% power. This only occurs when the on/off cycles are equal. I infer from your answer that this is nothing to worry about. It could be that with other loads on the inverter they will help buffer the effect. Tried the Phase angle mode but it produced an interesting looking voltage trace.

See below.
Thanks,
Pat