Can I make an AC inductor (reactor) rated 75mH 40A from a 3-phase induction motor?
For my test bench I need an inductor (aka "reactor") that meets the requirements below. I have tried to purchase it outright as a made-up item, or the parts to make it (core and windings), but have had no luck finding what I need "off the shelf" at an affordable price and reasonable delivery time. Yes, I am aware of off-the-shelf items such as:
https://www.digikey.com.au/en/products/detail/hammond-manufacturing/195R20/455524
So I wondered: could this be done with an inexpensive standard three-phase induction motor?
Why? Because these are available at low-cost from used-parts dealers.
Why three-phase not single-phase? Because a simple VA rating for this inductor gives 37kVA (see under requirements section), which is rather large hence rare for a single-phase motor.
I searched several catalogs of three-phase induction motors, but could not find the data presented in a way that translates easily to the requirements for (a) inductance and (b) current rating. Just as an example, here is a typical motor datasheet (5.5kW 415V 3-phase):-
My questions:
Question 1:
I seek to understand how to interpret the typical motor datasheet to determine its characteristics when it is used as an inductor, in particular, its inductance value, and its AC current rating (at 50 or 60Hz).
Question 2:
One of the differences between an inductor and a motor is that an inductor only has two terminals, while a 3-phase induction motor has six terminals. To connect the motor as an inductor, I can think of at least three ways as listed below - perhaps there are more - and again I seek advice on this.
(a) Connect the two inductor leads to just one motor winding.
(b) First connect two or even three motor windings in series, then connect the inductor leads to the remaining unconnected outer two terminals;
(c) First connect two or three motor windings in parallel, then connect the inductor leads.
Question 3:
I would prefer to leave the motor intact rather than modify it (eg: removing its rotor, or changing the air-gap between rotor and stator). Of course, I may have to consider either loading the rotating shaft with a suitable load (perhaps a fan?), or locking it in place, and again I seek advice on this.
Inductor Requirements
The ideal inductor for my bench will have the following characteristics:-
- Inductance value required: L=75mH (+/-10%)
- Impedance at 50Hz: 23.56ohms (calculated from X=2πfL, with L=75mH)
- Maximum current: 40A (RMS) at a frequency of 20Hz. Note that the current will be limited by the maximum voltage applied (from a VSD, hence the 20Hz).
- VA rating@50Hz: 37.7kVA (calculated from V=23.56Ω.40A, I=40A, VA=V.I). The actual VA for the use-case at 20Hz will be much less (2/5 of this, or 15.1kVA).
Thanks in advance.
2 answers
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The 70.9/40.8 A rating for L.R. is rated only for ; Locked rotor time 19s (cold) 11s (hot)
I doubt this meets your requirements unless you modify from a forced air cooling motor to a pumped liquid cooling system.
I'd contact a local oil-filled transformer supplier and get an autotransformer.
They can estimate heat loss W/kg and $/kg for the core and Cu. The taps allow you choose closest value.
e.g. https://beaverelectrical.com/content/EER7KH8C.pdf
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An induction motor with its shaft held still will mostly look like an inductor with a series resistance electrically. However, there will be some issues:
- It will be difficult to guess the inductance from the motor datasheet.
- There will probably be higher effective series resistance than for a deliberate inductor.
- It will be more lossy. The rotor is now just the secondary of a transformer that is effectively shorted. It doesn't add any DC inductance, but will dissipate power when AC is applied.
- The upper frequency limit where this thing still largely acts like an inductor will probably be quite low. If you only care about behavior at 50 Hz, then this probably won't matter.
All around, this does not sound like a great idea. By the time you get a motor big enough to act like the inductor you want, you will have probably paid for a proper inductor.
Price and availability
Since you are very price-sensitive and this seems to be a one-off, look into getting a core and winding your own inductor.
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