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OT: Sizing a VFD for an Ice Cream Maker with Multiple Motors/compressors


Jan 31, 2011
BFE Nevada/San Marcos Tx
A friend needs help with sizing a vfd for a commercial Ice Cream machine, the local electrician thinks he needs multiple VFD's because it has multiple compressors, and maybe a few misc motors. I have not looked inside it so don't know exact numbers. He sent a pic of data tag, it indicates 208-230V, 60hz, 3ph, 4.1KW, minimum circuit ampacity 15A, max fuse/circuit breaker 20A, total amps 13A.

What size vfd would be required to run this plug and play, no rewiring required. This is a self serve machine with multiple buttons on front for customer to select from.

Edit: Power available to run this unit is 120/240v single phase.
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With multiple motors, stating and stopping independently, you may have to start more than one while running others at the same time. Starting unloaded takes 5 to 7 times the running current of the motor in question. Starting against loads can take more.

So it must supply something over the start current of the worst combination of motors that could start at once, plus the run current of the worst combination of motors that could be already on at the time of the start.

Since most VFDs can do 150% current for a short time, you can use part of that extra to cover the start, but not the run currents. Don't shave it too close, I'd want at least 20% margin.

You might get away with less, but you might not, also.
Unless you can find a VFD that's explicitly rated by the manufacturer for use as a supply for multiple hard-start motors, or explicitly rated as a fixed frequency phase convertor, I think this is one of the worst possible uses for a typical VFD. Modern VFDs are more tolerant than early ones, but they aren't oblivious devices like rotary convertors. They interact actively with the load, and most of them are designed to interact with one motor, not a collection of motors starting and stopping independently.
If the VFD has a configuration setting that basically lobotomizes it, makes it just pump out fixed frequency AC regardless of what the motor(s) is doing, then that's about as good as you're going to get. But that's not what they are designed for. You certainly cannot make use of any soft-start features with multiple independently switched motors hanging off the output.
People here have run their whole shops off of a single higher rated VFD.

Of course you cannot do variable speed, at least in most cases. The VFD has to be in V/Hz mode, nothing that requires the VFD to auto-tune itself to the load. And, you cannot depend on the VFD to provide current limiting, the motors need to have their own protection..

But with a suitably high rating, a VFD can do this. Not really much different from what a Phase Perfect would do... just supply mains frequency power in 3 phase and proper voltage.
Plenty of VFD manuals have instructions on setting up for this kind of application - banks of pumps, fans, or compressors is a common scenario. It's not verboten.

And yeah, total load plus starting current for largest motor, or largest group of motors that could start simultaneously.
Starting unloaded takes 5 to 7 times the running current of the motor in question. Starting against loads can take more.
Locked-rotor current and starting current are the same; peak current. The amount of load doesn't change that. Starting against loads, however, does increase the length of time that the peak current has to be supplied for.
It's an ideal use for a VFD. Set it to permanently run at 60 or 50 (400 for an aviation ice cream machine? :D ) as appropriate, choose the power to suit the rating plate on the back of the ice cream maker and let it rip. Job done.
Must say, all this makes little sense to use an oversized VFD in this application. You have electronics and control boards within the the ice cream machine that would not be too happy running off the PWM output of the VFD, even if that did work you are looking at a 12-15kW VFD if you go on the premise that you need a VFD 5-7X peak output for the load of 4kW. Assuming the VFD set at ~150% overload, you are looking at a 15-20 Hp VFD to use as a power source and that is before derating for running off of single phase. You are going to plug that into a 230V single phase power and electrical code typically requires you to size the input wiring for 125%of the VFD rated input. Does not seem to be very practical for this application with multple sized motors and probably a bunch of control circuits. You could get a Phase Perfect Simple 7.5 Hp (PTS007) with a 3 phase output of 26A, for about the same cost as a 20-25Hp VFD and run it off of a 50-60A 230VAC single phase breaker.
Oddly, rectified loads with SMPS in them like the power supply or computer are pretty tolerant of even square wave power. Of course, the power wiring needs to be routed away from sensitive areas, but that is normally already done for reasons of EMI output reduction.

The VFD MAY need to be up-rated for starting against a load, for one reason: The short term current capability.

The short term current is time-limited. Some have a higher overload rating for 2 seconds or 5 seconds, and a lower one for a 1 minute overload. The start into a load may easily take a high current for several seconds, and you need to be aware of the capability.

Also,relying on the overload capability should not be pushed too hard.... The overloads are pretty accurate, but not perfect. I would still suggest leaving at least 20% margin.
Most sub-systems are on breakers, which do not necessarily play well well VFD outputs unless designed to do so. Also the subsystems, and transformers, etc. Not sure of the rational these days when there is a better option that generates the the 3rd phase in more or less a sinusoidal output. One is purposely built to provide sinusoidal 3 phase the other is not as a plug and play three phase source, if the price level is comparable, I just do not see the rational of using a significantly oversized VFD that is not designed for the purpose. Sure it could work, on the other hand it may cause system problems in a complex machine.
The VFD is running motors. That is what it is designed to do. The "other option", which I assume is a Phase Perfect, is essentially the same thing, but just set up for one output line, not three. The innards of the "PP" which output power are the same basic design as a VFD, and they are "oversized" as the rated load demands, similarly to the VFD which would drive the same set of loads.

Both VFDs and the PP output a synthesized sine wave using PWM with varying pulse width. The PP has a filter on the output, but it is common to install a "line reactor" on the output of a VFD, which will do duty as a sine filter.

No sense arguing about whether an oversized VFD works, that is a given, we know it works. People do that successfully. I have done it.
I have also designed VFDs intended to start and run motors "dropped on the line". The change in design was to use output devices which could handle the starting current. That is what you get when oversizing a VFD, the control section does not need to change, the outputs do.
A commercial ice cream maker will have a number of small motors and one or more relatively small compressors. You aren't DOL starting a rolling mill. Enormously over rating the VFD would be a complete waste of money.
Obviously the VFD only needs to do the job. It does not have to be 3x larger than what does the job. You need to figure out what the "job" means in terms of current requirement.

All you have to do is cover the requirement, plus a bit for margin. That is going to vary depending on the total number of, and current requirements of, the motors that work at once, or may be started at once.

The oversizing is to cover starting currents. It need not do any more than cover that for whatever size motors are involved. . If it does, you are good.
I just noticed that PhasePerfect has a new low cost setup rated at 7.5hp ($1700 or so). Might be worth a call to them

If a device is computer-controlled or sensitive to voltage drop we typically recommend a Phase Perfect Enterprise or Performance, which can handle high inrush. This includes hard starting/high-inertia loads like lathes and hydraulic pumps, they come up to speed a lot faster and with a lot less motor wear when the voltage is balanced during motor start. This isn't that application...

This ice cream machine is the sort of application we intended for the Phase Perfect Simple. The Simple has much smaller IGBT's, and so will fold voltage back on the generated leg under high demand (large motor starts). In practice, this means the voltage balance under load can look a lot like a rotary phase converter. A 7.5 HP Phase Perfect Simple will spin up your 7.5 HP planer, or etc, but it's going to drop that generated leg voltage for part of a second while it's overcoming locked rotor. Devices with a bunch of "dumb" controls typically don't care. If slightly oversized, the Phase Perfect Simple will provide balanced voltage all the time just like the larger units.

There may not be a single motor large enough in this ice cream machine to cause a PTS to fold voltage back. Based on the information posted by OP, our sales team would likely recommend a PTS005 (5 horsepower rated Simple) for this ice cream machine, it runs $1350.