Electronics > Beginners
Induction Motors and Inrush Currents Questions
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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?  ;D
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
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