Induction Motors and Inrush Currents Questions

[Mukrakiish]

In my industry I generally work with motors (3/4 hp to 7.5 hp) and we've come to an application where inrush is a problem. Most of the time it's not a big deal but we wanted to experiment with using an inverter coupled to our motors (cheaper than using DC motors). The catch is our motors are either starting under full torque or < 1 sec it is full torque which pushes inrush current + FLA very quickly.

With inverters, the peak currents are usually double their capacity and from what I understand the DC-DC bridge inside is what is most sensitive to voltage/current spikes. So it's not a matter of linear heat generation and more of the DC-DC bridge shutting down the system before the inrush passes.

I haven't really messed with NTC resistors (inrush limiters) but I'd like to. I'm curious as to the effect inrush current has on induction motors and their initial starting torque. Maybe I'm not searching the right terms or what it is, but I haven't found anything concrete of the effects and the like. As a second thing, I was hoping to learn how long does the inrush last and what kind of curve is generated from it in a standard induction motor. Since I don't have differential probes I'm not going to hook my scope up to the motor to see, and google doesn't turn up anything in terms of inrush times or formula (but if you folks know the math for it I'd love to be able to crunch the numbers to be able to spec the NTC to match up with the inverters max specs).

Any of you guys have experience that could fill in a few gaps for me? 

[Benta]

The current through induction motors is coupled to slip, which at operating speed is coupled to load torque.

At DOL start from 0 rpm, slip is of course infinite. Most induction motors I've come across are designed to pull 6...7 times FLA at startup.

If you're using inverters, it's a different story as you have the option to ramp up voltage and frequency. This of course limits slip and you are in a position to do a controlled startup by programming the inverter accordingly and limit the starting current to perhaps 2...3 FLA.

[Mukrakiish]

Our particular motors are a 6.7 rating on inrush making things difficult.

The inverters we are dealing with are not programmable style, which removes the extra options there. That's why I'm looking into NTC's since we aren't really "VFD'ing" these. That puts the cost back into the DC motor level.

[Benta]

NTCs or other soft starters will basically all do the same thing (limiting current/voltage). My worry is starting under full load. It could be that VFDs are the only option at the end.

[oldway]

I suggest you first read some explanations about asynchronous motors.
https://en.wikipedia.org/wiki/Induction_motor#

When starting directly on the mains, the slip is infinite.

Looking at the characteristic curve of the torque as a function of the slip, you can see that the torque remains very low for infinite slips.

There are some special asynchronous motors that have a high starting torque even with a high slip values: these are the wound rotor motors where a resistor is inserted in the rotor at startup.

To reduce the starting current, several solutions can be used:
1) Star / Delta start
2) Multi-tap auto-transformer start
3) Electronic soft start

All these solutions have the disadvantage of reducing the current and thus also the starting torque, thus prolonging the starting time.

The real solution is the frequency converter (VFD) because it solves all the problems:
- starting torque equal to the nominal torque because the motor always operates with a slip close to 1
- no overcurrent at startup

Just adjust the starting ramp not too fast to not exceed the slip value corresponding to the torque maximum of the characteristic curve of the motor

[IconicPCB]

deep bar squirel cage motors  are the other alternative

[T3sl4co1l]

Single or three phase?

If the latter, a VFD.  If the former, you're kinds screwed -- as mentioned, slip starts ~infinite, which really means the rotor is feeling less and less of the forward-rotating field, and more and more of the backwards-rotating field.  At zero, there is no induction torque, and the rotor can be started in either direction.  Instead, a starter winding kicks it moving, at low efficiency (the loss is part of what generates the phase shift necessary to induce rotation), and some amount of torque (usually a modest fraction of maximum torque) in the correct direction.

You can't use a VFD on a single phase motor (at least over much of a useful range), because the start winding kicks in at low speed, its phase shift is frequency dependent, and without two main windings to create a rotating field, the rotor bumps weakly from the pulsating field.

An alternative would be a PM or DC motor, which has a lot of starting torque.  Use an ESC to run it.  (A 3ph PM machine basically runs from a VFD too, so there's that.)

Tim

[Zero999]

If you're confined to single phase, get a three phase motor, suitable for operation, at the single phase voltage in your area and run it from a VFD. Here in the UK and possibly most of Europe, it's common to find three phase motors rated to 400V, when connected in Y and 230V, when connected in Δ.

[Benta]

If you're confined to single phase, get a three phase motor, suitable for operation, at the single phase voltage in your area and run it from a VFD. Here in the UK and possibly most of Europe, it's common to find three phase motors rated to 400V, when connected in Y and 230V, when connected in Δ.

I think that's backwards. Star is 230 V, delta is 400 V.

[SeanB]

No Hero999 is correct. Connect in star for DOL use on 400VAC 3 phase mains, and connect in delta for use in areas with 230VAC 3 phase mains. 400VAC 3 phase mains gives you 230VAC between phase and neutral, and 230VAC 3 phase mains gives you 115VAC between phase and neutral. However most common in the USA is split phase supply, with the single phase distribution transformer having a grounded centre tap and 2 115VAC line phases.

[Benta]

Ain't no such thing as 230 V 3-phase mains, and it would never give 115 V phase-neutral.

400 V 3-phase means 400 V phase-phase and 230 V phase-neutral.

[Zero999]

Ain't no such thing as 230 V 3-phase mains, and it would never give 115 V phase-neutral.

Yes there is. It's what you get from VFD, run off a 230V single phase mains supply. It will have 230V phase-to-phase and no neutral.

400 V 3-phase means 400 V phase-phase and 230 V phase-neutral.

Yes I know that. What I was stating was: a motor designed to work from 230V across each winding, would run from a 400V supply connected as star, or a VFD run off 230V, connected as delta.

EDIT:
See links below:
http://www.vfds.org/single-phase-vfd-with-220v-input-output-924125.html
https://www.amazon.co.uk/1-5KW-Single-Phase-Variable-Inverter/dp/B0151B2S2K

[Gregg]

For single phase motors requiring high starting torque the old school solution was repulsion start induction run motors.  The major drawbacks are the cost of manufacture and the mechanical parts needed to transfer form repulsion to induction that can wear out and seize in poor environments.  However it is a very interesting concept. 
Today it is cheaper and reliable solution to use 3 phase motors with VFD even with single phase input.

[oldway]

With motors up to 7.5HP, the single phase solution is virtualy impossible, that's why I did not asked this question...It are three phases motors for sure.

A single phase 5.5KW 230V motor has a nominal current of 33A and a starting current of 5.5 x 33A = 49.5A

Let's imagine two of those motors start at the same time, the start current would be 99A....far to much for a single phase 230V mains supply.

Edit: Other reason: single phase motor is not a favorite technical solution in industry because single phase motors are less reliable than 3 phase ones.

[Zero999]

With motors up to 7.5HP, the single phase solution is virtualy impossible, that's why I did not asked this question...It are three phases motors for sure.

A single phase 5.5KW 230V motor has a nominal current of 33A and a starting current of 5.5 x 33A = 49.5A

Let's imagine two of those motors start at the same time, the start current would be 99A....far to much for a single phase 230V mains supply.

Edit: Other reason: single phase motor is not a favorite technical solution in industry because single phase motors are less reliable than 3 phase ones.

Normally yes, but it's possible to get a VFD with a built-in boost converter, to generate 380VAC three phase, from a single phase 220VAC input. I've never used one before. I just stumbled on it today.
Link to Aliexpress

With inverters, the peak currents are usually double their capacity and from what I understand the DC-DC bridge inside is what is most sensitive to voltage/current spikes. So it's not a matter of linear heat generation and more of the DC-DC bridge shutting down the system before the inrush passes.

It sounds like the inverter is incorrectly configured or faulty. The variable frequency drive should automatically limit the motor current to the correct level. A modern VFD will also include power factor correction and inrush current limiting.

[C]

As many have stated, a VFD is best.

Going back to old school.

Extreme was a multi-stage starter.
Each stage increased power to motor increasing the RPM.

You had a relay for each stage.
When relay is DE-engerized the contacts are bypassed with a resistor.
The series resistors of each stage limited the Current.

Each stage starts with max current allowed and as motor RPM increases the current drops some before switching to next stage.

Wire was often used as resistors, steel or something not a good conductor.

Huge extreme was using a starter motor to get main motor started.

C

[oldway]

I described in my previous post the different ways to start a three phase asynchronous motor following the old school methods.

Inserting resistors in the stator circuit was not used, it was preferred to use a three-phase autotransformer generally with 3 or 4 taps.

But to start with a low starting current and a maximum torque at the time when the VFDs did not exist, we used a three-phase asynchronous motor with wound rotor and we connected resistors in the rotor circuit ...

We gradually decreased these resistances when the motor accelerated ... When the starting sequence was finished, one short circuited the rotor.

https://electricalbaba.com/difference-between-squirrel-cage-and-slip-ring-induction-motor/

NB: For indication, in Europe, the maximum 230V single phase connection authorized by the electrical energy distributors is 40A.
Exceptionally and with additional costs, this value can be increased up to 100A.
Above this, the connection is three-phase.

[C]

However most common in the USA is split phase supply, with the single phase distribution transformer having a grounded centre tap and 2 115VAC line phases.

Think you will find that it is commonly stated as

"2 115VAC line legs" as both are of the same phase.
 
C

[tautech]

In my industry I generally work with motors (3/4 hp to 7.5 hp) and we've come to an application where inrush is a problem. Most of the time it's not a big deal but we wanted to experiment with using an inverter coupled to our motors (cheaper than using DC motors). The catch is our motors are either starting under full torque or < 1 sec it is full torque which pushes inrush current + FLA very quickly.

Any of you guys have experience that could fill in a few gaps for me? 

The solution is very easy; fit D curve motor rated breakers on the mains supply.
They're rated for substantial overloads for a specified period to overcome the current surges on startup.

[HackedFridgeMagnet]

In my industry I generally work with motors (3/4 hp to 7.5 hp) and we've come to an application where inrush is a problem. Most of the time it's not a big deal but we wanted to experiment with using an inverter coupled to our motors (cheaper than using DC motors). The catch is our motors are either starting under full torque or < 1 sec it is full torque which pushes inrush current + FLA very quickly.

Any of you guys have experience that could fill in a few gaps for me? 

The solution is very easy; fit D curve motor rated breakers on the mains supply.
They're rated for substantial overloads for a specified period to overcome the current surges on startup.

Yes that was my first thought. A VSD would be nice though.

[tautech]

In my industry I generally work with motors (3/4 hp to 7.5 hp) and we've come to an application where inrush is a problem. Most of the time it's not a big deal but we wanted to experiment with using an inverter coupled to our motors (cheaper than using DC motors). The catch is our motors are either starting under full torque or < 1 sec it is full torque which pushes inrush current + FLA very quickly.

Any of you guys have experience that could fill in a few gaps for me? 

The solution is very easy; fit D curve motor rated breakers on the mains supply.
They're rated for substantial overloads for a specified period to overcome the current surges on startup.

Yes that was my first thought. A VSD would be nice though.

Sure, the the mention of inverters/VFD was only in some effort to manage the startup overload condition when neither are necessary with properly rated motor rated breakers.
They're the standard industry solution for motor start overload management.
However like any MCB their long term performance is influenced by the # of overload trips they endure.

I also find them useful for older simple transformer type welder mains supplies.

[drussell]

Ain't no such thing as 230 V 3-phase mains, and it would never give 115 V phase-neutral.

I think he meant 208 volt.  The vast majority of 230 volt equipment will obviously run just fine on 208 (wired delta for 3-phase motors, or across two phases for a single phase 230, the motor will just draw a bit more current than if it sees 230), an electric range will just have elements that put out slightly less heat, etc. but it basically just works.

[Gregg]

Ain't no such thing as 230 V 3-phase mains, and it would never give 115 V phase-neutral.

I think he meant 208 volt.  The vast majority of 230 volt equipment will obviously run just fine on 208 (wired delta for 3-phase motors, or across two phases for a single phase 230, the motor will just draw a bit more current than if it sees 230), an electric range will just have elements that put out slightly less heat, etc. but it basically just works.

230 volt delta 3 phase is quite common in the US in older industrial areas.  It is 230 volts between any two legs and one phase has a center tap to give the standard pair of 115 volt legs to the center tap which is the grounded neutral.  The other leg is 208 volts to neutral and seldom is anything wired for 208 volt single phase.  For instance, Bridgeport milling machines run on 230 volt 3 phase only as they are two speed motors.

[SeanB]

Scary motor you get here in south Africa is a 22kW single phase capacitor start capacitor run motor. Very popular to drive irrigation pumps, as all the farmer has to supply is a single line of poles to the pump house, with a single 11kV line running at the top, in aluminium wire, with ground return at the pump station using a reasonably big earth mat. The 11kV line runs a small 30kVA transformer on the pole, and this then gives a 100A feed using aluminium wire to the motor. All lowish cost, and more importantly, the wiring is less valuble to the ever present thieves, as the Eskom cabling they use in rural areas most commonly has 7 strands in it, 5 aluminium cores, one steel centre core and a single one of the outer 6 strands is an aluminium coated high carbon steel wire, which is roughly Rockwell 40C hard. Can be cut with difficulty in the field using a hacksaw or angle grinder, but the scrap yards do not want this cable unless it has the high carbon steel core stripped out, as this destroys the yard power cutters that make the cable an easy to handle length. As this is easy to identify yards do not want to have it on premises when inspected, it makes them become the focus of investigation for dealing.