Can a battery handle the start-up current of a heavy motor on a weak connection?

Yes, often: a battery can handle the start-up current of a heavy motor on a weak connection. A semi-off-grid system puts it with inverters next to your connection to carry the start and the running load. Tame the start first with a drive; for a machine at full power all day, a cabin stays the answer.

You have one machine that makes the business: a flax line, a stone saw, a crusher, a big compressor or a mixer. Then the connection study says: a bigger connection or a medium-voltage cabin of your own.

A flax-processing company in Wielsbeke faced exactly this and avoided the cabin with a generator and a battery.

Why does one heavy machine push your connection to medium voltage on paper?

A connection is sized on the worst moment, and for one heavy motor that is its start. Switched straight on, it asks several times its running power until it is up to speed. Add the rest of the site, and the number passes what low voltage can give.

You would pay for a cabin, and years of reserved power, for a start of a few seconds. Most of the day the machine runs at a fraction of that power, or stands still. A bigger connection sizes everything on that worst moment.

Owners on forums hit the same wall. A plasterer on a Dutch-language forum wrote that his compressor "cannot go straight into the socket, or the fuse trips at once". A reply said he needed "at least a 50 kVA generator" for the start-up current.

A stone-cutting factory with a 75 kW motor asked the Victron community who could run its motors without trips. A farmer with a pellet plant found no "supplier who can guarantee their system will work with the multiple 3 phase motors".

An illustrative example, not a customer case: a 75 kW motor draws roughly 90 kVA at full load, with typical efficiency and power factor. The start factors are rules of thumb from Cummins Generator Technologies.

In Flanders a connection above 250 kVA is always medium voltage with a customer cabin. Between 25 and 250 kVA, Fluvius decides (Fluvius connection tariffs).

Which grid you are on and what it changes: low voltage or medium voltage for a business. Whether the cabin is worth it: a medium-voltage cabin or a battery.

How high is the start-up current of an electric motor?

At standstill a motor behaves almost like a short circuit, so it draws a very high current until it is up to speed. That start-up current (inrush current) mostly builds the magnetic field, not useful power.

Cummins Generator Technologies, which builds alternators for generators, gives 4 to 9 times the running kVA for a direct-on-line start. Motors built for high starting torque take 10 times, special low-torque pump motors 3.5 times. During a direct start the power factor is only 0.1 to 0.2.

ABB gives 5.5 to 7 times the rated current, with a starting torque of 1.9 to 2.1 times the full-speed torque. That current gives the torque to break loose a loaded machine. Cummins names a loaded conveyor belt or a crusher with a jammed stone as loads that need a hard, direct start.

That is the catch in every trick to lower the start: less current means less torque. When the supply cannot keep up, the main breaker trips or the voltage dips for other machines, contactors and electronics. On a generator you hear the engine drop in speed.

Why a new pump or machine trips a small connection: more power without a grid upgrade.

Direct-on-line start, star-delta starter, soft starter or frequency drive: what does each do?

Each start method trades start current against start torque. Direct on line gives the most torque and current, a frequency drive the lowest current with full control. Star-delta and soft starters sit in between.

ABB writes that star-delta cuts start current and torque to a third of a direct start, with high transient currents at the switch. Its first (star) stage asks about 2.5 times the running kVA (Cummins). A badly set switch-over can give a second peak of 5 times.

On farms, Inagro gives 4 to 6 times the rated current for a classic star-delta starter. With a frequency drive it is about 1.5 to 2 times.

A drive also saves energy on pumps and fans that do not always need full flow. Lowering the average speed from 100% to 80% can save up to about 50% in some applications (Inagro).

For a heavy machine, the drive is often the cheapest part of avoiding a cabin. It does not reduce the power the machine needs at full load.

How does a battery inverter carry a motor start?

A battery inverter is a second supply next to your connection or generator, with a peak rating well above its continuous rating. At the start it takes the extra current from the battery for those seconds.

The datasheet of the Victron Quattro, the inverter in our worked case on the solutions page, shows both numbers. For the Quattro 48/8000:

• Continuous output at 25 °C: 6,400 W (8,000 VA); 5,500 W at 40 °C.
• Peak power: 16,000 W, so about 2.5 times the continuous rating for a moment.
• Maximum output fault current: 100 A for one second.

The Quattro 48/15000 gives 12,000 W continuously and 25,000 W as a peak. Three units form a three-phase supply; up to four sets of three 15 kVA units run in parallel for 180 kVA. With PowerAssist, the battery makes up at once for too little grid or generator power, and recharges when the load drops.

A set that holds 2.5 times its rating for a moment cannot carry a direct start of 7 times, unless it is far oversized. With a drive or soft starter the start falls to about the running power. Then the inverters are sized on the running load plus margin, which is why we look at the start method first.

One owner warned that a central three-phase inverter "is going to get a real kick in the bum every time you push a start button". Measure with a peak-hold meter, he advised, because "surges can be unbelievably high".

Another named three limits that must hold at the same moment: the motor inrush, the inverter overload tolerance and the battery current.

Why does the flax company in Wielsbeke start its machine on a generator?

Because one machine is the heavy consumer, and it runs as long as the work lasts. On grid power it would have needed a large medium-voltage cabin. At Wielsbeke a generator starts the flax machine and runs during the work, as is common at a concrete plant.

When the machine stops, the generator stops and the smaller loads run from the battery. The generator is sized for the machine, the battery for the rest of the day, not for the start.

A generator has its own start problem. Cummins writes that motor starting "continues to create technical problems" for generator builders. It sizes the alternator on the start kVA and the voltage dip the site accepts.

Soft starters and drives bring harmonics that a generator notices, so the start method matters there too. Read how a generator with a battery share the work, and how a transfer switch for a business generator switches between grid and generator.

When does a battery replace the medium-voltage cabin, and when not?

A battery replaces the cabin when the heavy load is short or limited in hours, and the grid or a generator refills it in between. When the machine runs at full power most of the day, every day, the energy comes from a cabin or a generator.

A plant nursery in Jabbeke shows the middle route. Its 3 x 400 V 32 A connection was too small for a new irrigation pump. The proposal was medium voltage, at a high price.

We fitted frequency drives on the heaviest pumps and a semi-off-grid system of 45 kVA and 92.4 kWh. The street connection stayed, only as a charger, for a heavier 63 A supply that drives the pumps.

For a running load well above 63 A, a business battery behind the connection or transformer is the next step. Already on medium voltage with a transformer too small for the new machine? Read a cabin transformer that is too small.