Stored energy Induction motor - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: March 20, 2023, 05:53:55 pm
content: I'm not sure the following calculations and assumptions are sound, please comment if you see a mistake or if you think it's a close enough approximation?1/2HP single phase induction motor:Inom =6A (measured)Vnom = 120VLRA = 30AR = 120/30 = 4 OhmsZ = 120/6 = 20 OhmsXl = 16 OhmsL = 16/377 = 0.04 H, round up to 0.05Energy = L * I^2 = 0.05 * 30^2 = 45 J

author: Terry Bites
author_rank: Super Contributor
timestamp: March 20, 2023, 06:16:20 pm
content: The average electrical energy stored in the magnetic circuit(s) is zero.Rotational kinetic energy is stored in the rotor. If you know the speed and rotor dimensions you can calculate it. labman.phys.utk.edu/phys135core/modules/m8/energy.htmlIt will be released as the motor decelerates.

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2023, 07:44:21 pm
content: If you cut the power to a small induction motor it will typically coast to a complete stop within a second or two, there's not much energy stored there.

author: floobydust
author_rank: Super Contributor
timestamp: March 20, 2023, 08:11:03 pm
content: The mechanical load i.e. compressor can kick back and, combined with rotor (decay)remanencedoes generate a huge impulse. It also depends if the motor has a run cap.I've had small motors dish out 4kV impulses and rip apart contactors and arc across connectors. So a mathematical approach is interesting but just measure it on the bench, when tuning a snubber.

author: johansen
author_rank: Super Contributor
timestamp: March 20, 2023, 08:21:27 pm
content: Your locked rotor amps are approximately half inductance and half resistance, but not always.So you need to calculate 4 ohms of impedance at locked rotor conditions from the dc resistance of the windings, lets presume its 3 ohms. this leaves about 2.6 ohms for the inductance, yielding only 7mH, which makes only 0.12 joules at 6 amps, or 3.1 joules at locked rotor conditions.the inductance and dc resistance add at 90 degree angles, so 3 squared plus 2.6 squared =16, = 4 ohms impedance.Now lets presume you actually measured 16 ohms of inductive impedance at small signal voltages. that would be .9 joules at 6 amps. then factor in the motor is running at saturated conditions at nominal voltage and I bet you will find the energy stored in the inductance during operation to be less than half of .9 joules.

author: Jester
author_rank: Frequent Contributor
timestamp: March 20, 2023, 09:38:54 pm
content: I'm looking at this from the perspective of a MOV Joule rating when used as a snubber across the relay contacts when the motor is deenergized both under normal circumstances (6A) and when locked (LRA = 30A).

author: moffy
author_rank: Super Contributor
timestamp: March 20, 2023, 10:12:00 pm
content: MOV's have a tendency to explode.

author: johansen
author_rank: Super Contributor
timestamp: March 20, 2023, 11:20:03 pm
content: Quote from: Jester on March 20, 2023, 09:38:54 pmdont over think it.19 cents will protect your relay for a long time.https://us.rs-online.com/product/epcos/b72214s131k101/70102372/

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2023, 12:31:30 am
content: I'm not a fan of MOVs. Each time they are used they sustain some amount of damage, and eventually they either fail silently, or they heat up and sometimes catch fire. RC snubber is probably a better option for preventing contact arcing.

author: johansen
author_rank: Super Contributor
timestamp: March 21, 2023, 03:15:04 am
content: Rc snubber across the relay or across the motor?To critically dampen the motor at locked rotor current at turn off you need a 4 ohm resistor and a capacitor large enough to keep thr voltage down so that most of the energy goes in the resistor rather than the 30 volts across the relay (typical arc voltage)The problem being thats a lot of loses to continuously eat.If all you care about is the 6amp full load current then it matters not. Typical 4 ton ac heat pump can last 20 years on a 20$ ac contactor. 4 to 5 ton heat pump is 100 to 150 amp LRA and 20 to 25 amp FLA.Another way to snubber the turn off losses is to put a capacitor and a full wave rectifier across the relay. A 1 watt resistor will discharge the capacitor pretty quick after the motor is turned on. Resistor is sized such that when the motor is off and the cap is charged up to 170vdc, the cap is dissipating 1 watt.But its always dissipating 1 watt...all the time the motor is off.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2023, 03:48:54 am
content: Maybe a spark gap across the contactor? I thought this was a solved problem, there are millions of motors controlled by contactors and they're generally pretty reliable.

author: floobydust
author_rank: Super Contributor
timestamp: March 21, 2023, 04:39:05 am
content: I consider contactors different than relays due to their much larger contact spacing.I though OP is using a optotriac?edit: that was for the coilOn 1/2HP motor 120VAC I find 47nF 400V or X2 film (i.e. B32921X2473K000 10mm LS through-hole part) and 33Ω 1/4W carbon is OK. Assuming presence of a MOV to limit mains transients say SxxK150 for <500V to prevent the cap from getting damaged. Flameproof snubber resistor if it burning due to a cap short would cause drama.The cap physical size is the limiting factor, everybody hates that and using SMT the pad spacing is another limitation, so 1206 is good for 2mm but your board's pollution/humidity and conformal coating etc. all affect the part choice as far as achieving safe spacings because the cap can see more than mains across it briefly.I don't view motors as a simple inductor, take a scope out to the field and observe and measure the spikes and be surprised. If you want a long life product.Otherwise, do as appliance manufacturers do, use no snubber at all and the product ends up in landfill too soon.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2023, 06:53:41 am
content: I didn't see where the OP said what they were using. A contactor is the way to go for larger motors, but under about 1HP there are cube style relays that seem to hold up fine. I've replaced forced air furnaces that were 30+ years old and still running the original relay that controls the blower motor.

author: Jester
author_rank: Frequent Contributor
timestamp: March 21, 2023, 11:30:21 am
content: I'm using a relay to control the motor. The relay is rated for 40A and 1/2HP @120Vac. I would think that no snubber is required for a normal off condition. This relay will also open if locked rotor current is detected, locked rotor current is 30A. My thought was that a snubber would be cheap insurance for long contact life.Board space is limited so if I add a snubber it has to be SMT. SMT MOVs tend to be small Joule wise, hence my desire to determine Joules.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 12:02:29 pm
content: Context, for those who missed it:https://www.eevblog.com/forum/projects/snubber-circuit-capacitor-question/Quote from: moffy on March 20, 2023, 10:12:00 pmQuote from: james_s on March 21, 2023, 12:31:30 amGuys, please. MOVs have a derating chart. At the energies applied here, lifetime is indefinite, even for a small (chip) part.MOVs are only unreliable when undersized, such as using a 7 or 10mm part for mains induced lightning surge. Even then, it'll last surprisingly long, perhaps hundreds or thousands of events. It's the few that push it over the edge that give MOVs a bad reputation.In other words, it's confirmation bias. You don't see the thousands of successful protection events, only the one that killed it. Rough lot indeed.A MOV connected across the motor, sized suitably for mains surge, will limit arcing energy quite effectively.A MOV could also be used in series with a fusible rated pulse resistor, so it's somewhat isolated from mains surge, but still absorbs the (much more limited) current from switching. On the order of 10 ohms would be fine. A wirewound resistor will probably do.Snubbing (with RC or MOV) has the knock-on benefit of reducing emissions.Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: March 21, 2023, 04:50:24 pm
content: MOVS are used to soak up the occasional transient not the every day ones.As suggested suitably rated RC snubber will last a lot longer. You can put a MOV across that.Buy a bigger contactor!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 05:05:41 pm
content: "AC arc suppression requires the use of a metal-oxide varistor (MOV) [or...]"Good reference, it supports my pointTim

author: floobydust
author_rank: Super Contributor
timestamp: March 21, 2023, 06:06:33 pm
content: Quote from: Jester on March 21, 2023, 11:30:21 amSMT capacitors 10-100nF rated 250-400V are expensive, huge and very few offerings. Vishay's biggest SMTX2X7R cap is 12nF, 2220 case. So 5.6x5mmPower electronics are necessarily big just for the HV spacings and heat. I think you're boxed in with making this Steve Jobs small.Why is the relay so big? 40A resistive is typically good for 1-2HP motors (UL 873/508). Coil power will be around 1W. If you already have this big through-hole part on the board? so what's wrong with a snubber cap or MOV being through-hole?If you use a smaller relay with a decent snubber it will last longer than a larger relay with wimpy arc protection.Furnace and heater (i.e. hot tub, water tank) control boards in the IEC/UL safety standard have an endurance test. It's 100,000 cycles and usually specialty relays are used, you can see some datasheets in the same class are 30K or 100K cycle-rated, different contact alloys.Other appliance relays, I have replaced zillions of them, there is no snubber at all. I have never seen a MOV used across contacts, out in the wild.Pic of recently replaced cube relays on a 1/2HP motor, does this look like 5nF was enough? Manufacturer upgraded to 47nF.

author: Jester
author_rank: Frequent Contributor
timestamp: March 21, 2023, 09:21:54 pm
content: Quote from: floobydust on March 21, 2023, 06:06:33 pmAgreed on the SMT capacitors. Small and affordable SMT MOVs are available so if I can find one that is "big enough" it makes for a simplified solution.The relay is "big" because the primary purpose of this circuit is to trip the motor out (quickly) when it jams and when it jams the LRA is 30A.The prototype works well from a functional perspective trips in about 8ms) and fits in a common 1110 utility box with the required creepage and clearance. I might make more of these so I'm exploring cost and size reduction so if a 50c part will work adequately why use a $2 part? The relay is on the bottom of the board and a SMT snubber fits nicely on the top near the contact pins. A through hole snubber will require a larger board, and hence a larger enclosure I would prefer not to do that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 11:58:24 pm
content: Not badI'm not even very impressed by SMT chip MOVs, simply because 7mm discs are much cheaper, and about the same layout area as equivalent chips. The single-layer footprint does help sometimes though.You can also get chips in smaller sizes, assuming that's adequate for voltage and energy ratings; an 0603 chip affords extremely robust ESD protection for example. (Not that a 0603 TVS doesn't either, heh!)Tim