PWM Surge Problem on VDC side of an Inverter

Thread Starter

TripleStack

Joined Apr 22, 2022
4
Hello all and I hope someone can point me in the right direction. We are powering the following from dual 220A truck alternators through a 5,000W 120VAC AIMS pure sign wave inverter.

Analyzer: VAC 3A
AC-to-DC Power Supply: VAC 2.8A (This powers the PLC+IO Modules, Switch, NUC, and 9 solenoid valves.)
Heated Load #1: VAC 3.25A
Heated Load #2: VAC 3.25A
Heated Load #3: VAC 0.5A
Heated Load #4: VAC 1.3A
Heated Load #5: VAC 14A

Our actual running load has been about 18A.

We connected the alternators positive terminals together with 4/0 SGX copper cable. The OEM setup has each alternator connected to a battery with OEM 2 gauge cable and the batteries connected to each other with 2 gauge OEM cable as well. We ran 4/0 cable from one alternator positive post to the rear compartment of a four door pickup where we installed the inverter. We ran the negative cable from the inverter back to the mounting bolt of the alternator to ensure a good circuit. One-way distance is 16'.

Once the heated loads reach a 400F operating temperature, the PLC uses PWM to maintain the temperatures at +/- 1F. The system works perfectly when in full power heating mode. Once the PWM begins, we noticed that the alternators start surging slightly. We also noticed that the truck interior lights flicker slightly and the blower motor surges. Voltage on the VDC side of the inverter fluctuates between 12.9 and 14.2VDC constantly.

Other similar installs have worked perfectly for years, but they did not include the PWM requirement. If you turn off the truck, the system continues to function so we know the batteries are in the circuit. Appreciate any help!

Scott
 

LowQCab

Joined Nov 6, 2012
2,045
Just a few guesses ...........

When You say "PWM" Temperature-Regulation, exactly what is the Frequency being used ?
Or is this a relatively slow Switching-Thermostatic-Control ?

The 2 Alternators might be having a "back-and-forth" battle between their individual Voltage-Regulators.
This may be aggravated, or even caused by, some odd Electrical-Noise, created by the Inverter,
that is somehow making it's way back past the Batteries, and to the Alternators.

You're also maxing-out the Alternators on start-up-Load, ( which the Batteries "should" handle ).
This is a pretty highly stressed setup, you're lucky it works as well as it does.

The PWM Circuit, and the Alternators could have their Frequencies "beating" against each other,
( Current-Spikes from the various devices going in, and out of, Phase with each-other ).
Changing the RPM of the Alternators could possibly eliminate the problem.
This could also be caused by one of the Alternators having more Belt-Slippage than the other,
allowing one of them to run at a slightly slower RPM.

Ideally, You should have a single Voltage-Regulator for both Alternators.
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Thread Starter

TripleStack

Joined Apr 22, 2022
4
After talking to two EEs, we ordered a 50mF capacitor that we intend to connect with 2 gauge wire directly to the DC terminals on the inverter. The 2 gauge run is about 12", so we shouldn't get any line loss.
 

Thread Starter

TripleStack

Joined Apr 22, 2022
4
Capacitor should arrive tomorrow, yes Sunday! we made a few changes to our PWM frequency, but we are still seeing 14.4-12.6VDC fluctuation. That said, we did figure out that some of the "surge" was our hearing the auxiliary fans kicking on and off when we had the AC running. That does not explain the the light flickering, but at least one symptom has been resolved.
 

Thread Starter

TripleStack

Joined Apr 22, 2022
4
The capacitor is 50F not 50mF! I had to order a 50mF run capacitor to fix our lathe a couple of weeks ago and just wrote that in my last message. We talked to a car audio installer who recommended a 30F, but for $20 more we got the 50F one. The PID controller on the PLC is auto-tuning based on temperatures and % allowed variance, so I don't know the timing, but the on/off PWM pulses are in milliseconds. When everything is on and warming up, we still see a drop from 14.4VDC to 13.6VDC and it holds steady there, even at low idle. Once everything is at operating temperature and the PWM starts maintaining the temperature, the voltage now starts fluctuating from 13.2VDC to 14.4VDC. It constantly jumps all over the place! Still hoping the capacitor helps. The truck runs fine and there is plenty of power to run everything, it is just the voltage fluctuation continues. The capacitor arrives tomorrow, so I will let you know.
 
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