Mike Dorsett avatar image
Mike Dorsett suggested

How to improve the transient current output from a Multiplus?

I need to run a battery spot welder from my solar system, which is based on a Multiplus 2 3kW inverter. However, the strength of the spot weld is limited due to the limited transient current performance of the inverter. Rotating generators have much higher transient currents - i.e. the current & energy you can get if you instantaneously short the output. The spot welder is a transformer based design - a capacitative discharge design would not have this problem- that turns on the transformer for a few mS to do the weld.

One solution I have tried is to run a small motor in parallel with the welder - with a capacitor to correct the power factor. The motor is a 1.5kW old compressor motor, with a quadrature winding. the power factor was 0.75 with no load on the motor. With a 20uF capacitor, this is corrected to 0.97 (still inductive). This has significantly improved the weld quality - the motor instantaneously acts as a generator when the welder 'fires'.

At the moment, I don't have any method of measuring the transient currents, and am relying on a strength test of the weld - however that is a destructive test.

Any one got any better ideas? I might try a larger motor if I can find one....

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yaute avatar image
yaute commented


We are facing the same situation with many customers, and the only way to analyse the transient current is to use a grid analyser, then it's far easier to find a stable work around.

Having a motor synchronized with the spot weld will perturbe the feedback loop of the reactive power internal management of the Multiplus because if they will never being fully synchronized.

Depending of the VDE you must comply, you can tick the reactive power within VE config / tab Grid / subtab grid code settings. The default filter time reactive power is usually to long. You can also set Q on demand.

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Mike Dorsett avatar image Mike Dorsett commented ·

"Depending of the VDE you must comply"

I have a completely off grid installation in Fiji. Not sure what you mean by the above comment, but if this relates to local authority regulations, that would not apply in this case. Grid code is set to none.

"perturbe the feedback loop of the reactive power internal management of the Multiplus"

What information do you have on this? I would assume that in inverter mode, the inverter will handle most reactive loads - though there is no information in the manual. I would assume as the inverter is capable of 4 quadrant operation, the output load power factor will cope with -1 to +1.

Just to clarify, the motor and spot welder are on the AC output of the inverter, I'm not using the inverter in a grid connected ESS mode.

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Phil Gavin avatar image Phil Gavin Mike Dorsett commented ·

This is just a guess, but cheap enough to try.

Instead of getting a bigger motor, attach the existing motor to a flywheel.

Use a car rim, motorbike or wheelbarrow wheel or something lying around as proof of concept.

Let me know if it works, please.

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Mike Dorsett avatar image Mike Dorsett Phil Gavin commented ·
Hi Phil,

Thanks for the thought.

However, the transient current is mainly limited by the resistance and inductance of the motor windings, as well as the decay of the magnetic field. Please note that we are dealing with 10's of ms here - the inertia of the motor rotor is such that I don't hear it try and change speed. The three main contributing factors are directly related to the motor power rating / physical size. I did come across one brand of generator manufacturers that used an 80kW alternator coupled to a 22Hp engine - such that the engine could be way overloaded by the alternator, just to get enough transient current to start larger electric motors. Part of my improvement to the trial version will be rewiring to get the circuit resistance down. it's currently using some 0.75mm^2 wire from the motor to the welder.

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raymiller avatar image
raymiller commented

As you have a MulitPlus 11 and presumably you are welding during the solar day consider adding a grid solar inverter to the load side of the inverter. The manual specifies the maximum size of the grid PV inverter, I think 2kW from memory could be added. This at least would give you an extra 2kW.

You could, of course, run an AC generator on the input to the Mulitplus and use the inverter in the assist mode, that way you get up to the generator + AC rating of the Mulitplus to a maximum of 32A.

It would be worth checking what your DC voltage sage is when you attempt a weld, (a battery-isolated CRO would be ideal), will be needed to you may need to up-rate your battery cables. A useful technique I use to measure all the voltage drops in the circuit depower all the circuits and run a low voltage i.e 3 V 10 constant current source (battery terminal to inverter input) and measure using a multimeter on mV all the voltage drops. 10A is sufficient to measure 100uohm reliably and good for locating connections or equipment you thought were very low resistance.

good luck

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Mike Dorsett avatar image Mike Dorsett commented ·
Thanks for your input. I'll have to check on the availability of a single phase 2 - 3 kw (up to 1:1 is allowed) Grid tie inverter. I have some spare solar capacity, currently used for dc charging. Whilst I have a CRO it's an old Tek 2225 - would prefer a DSO to record the pulse. I have used a camera with the CRO with moderate results. The welder is supposed to give up to 1500A pulse, nominally at 5.5Vac - which translates to ~35A pulse on the mains current.

I like your cable / connection measurement method - similar to 4 terminal method for measuring low resistances - I have a 4A psu that I use for this - however being off grid it's difficult to turn the system off for measuring...

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raymiller avatar image raymiller Mike Dorsett commented ·
yes poor person's 4 wire low ohm meter, but just as effective. I ordered a small 10A SM power supply with CC ability from China and coupled to an 18V battery from a power tool, which runs the SM power supply adjusted to 3V for hours, very portable and isolated.
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