Induction motor transients and where to shunt etc. - Page 1
Forum section: electronics / projects

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: January 25, 2017, 11:22:23 pm
content: I will be using a solid state relay to control a small induction motor. Being inductive, the motor probably generates a transient when the current is halted, and the relay needs protection from transients appearing across it to prevent damage to the SCRs. The documentation for SSR's generally shows the transient protection (e.g. TVS diode) placed in parallel with the relay, which would shunt the transient back through the mains. But usually transients are shunted back through the inductance like with a flyback diode.I'm worried about shunting it back to mains and damaging other things, and the SMPS for the circuit controlling the relay is just upstream from it.Am I overthinking this given the lack of information on the topic? Can the TVS be placed in parallel with the motor instead of the relay?

author: Benta
author_rank: Super Contributor
timestamp: January 25, 2017, 11:29:03 pm
content: Do you really mean induction motor?Those are normally 3-phase. If running on a single-phase supply they are 2-phase, with a capacitor for the second phase.Both are well-behaved when it comes to generated transients.More info is needed.

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: January 25, 2017, 11:44:53 pm
content: Quote from: Benta on January 25, 2017, 11:29:03 pmThis is the motor in question (water pump)http://www.saerelettropompe.com/product/kf-pompe-a-canali-periferici/?lang=enThe nameplate mentions a 10uF capacitor being present.But I just had a thought, given that SCRs switch off when the AC current nears zero, even with a highly inductive load, would there even be much of a transient? The di/dt would be low given the low current at the time of switch off.

author: Benta
author_rank: Super Contributor
timestamp: January 25, 2017, 11:53:47 pm
content: I wouldn't worry. Place protection as your SSR manufacturer recommends and leave it there. Induction motors are friendly.

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 26, 2017, 01:49:10 am
content: Yep induction motors are friendly, an SSR uses a triac so there is no switching transient as the SSR turns off at zero amps.Transients will be coming down the mains, and 2kV transients are to be expected, this won't bother the motor, it will suck it up like a sponge.The SSR won't appreciate the transient, if you are lucky the triac inside will "breakover" and turn on for a cycle, so your motor will just twitch a bit.It's better to fit the TVS, and it would always go across the SSR, this will limit the transient to ~ 500v, whatever's left over gets passed to the motor.Your water pump will be capacitor run motor, with one phase across the mains, and another motor phase powered through the capacitor.Your'e generally better off to upgrade the capacitor to a higher voltage unit if you can get one to fit (440v capacitors might last ~ 4 times longer on 240v than the standard 220v part) . Metal film capacitors have a defined wear mechanism and after 5 to 10years the capacitance will have dropped to half its value at which time the motor will be sluggish to start, ignore the warning signs and the motor will stall at start up and burn out!!!. Best to measure all your motor capacitors every 5years or sooner, a 5minute $10 capacitor replacement is beter than $500 pump replacement and a days worth of messing around. Also expect to replace the mechanical seal and bearings after 5-10years.

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: January 26, 2017, 02:59:28 am
content: Cool, sounds good. What's a good voltage rating for the mains in NZ (same as Aussie, 230v 50Hz -10%/+6%). I'm thinking 1.5KE400CA (346V standoff, 380V min breakdown, 548V clamping)?

author: Circlotron
author_rank: Super Contributor
timestamp: January 26, 2017, 12:15:32 pm
content: Also put a 100R 5 watt and 220nF 250vac X cap in series, and wire this combination across the relay "contacts". That will damp the remaining transient that occurs when the scr or triac switches off at near zero current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2017, 12:46:53 pm
content: Across the switch is the correct place to put the TVS.The switch is the discontinuity, therefore it is where the voltage is being generated. Yes, the bulk of the EMF comes from the inductive load, but that simply acts like a CCS over the time scale in which a switch opens*. A CCS doesn't generate a voltage, it generates a current through the load; the load then converts that current into a voltage, according to its resistance (V = I*R).*This is most true of mechanical switches, which make and break (electrically) in fractions of a nanosecond. SSRs either turn off in tens of microseconds (TRIAC type), or milliseconds (photo-MOS type). The latter are quite slow indeed, and may not need a TVS for that reason (though I would still recommend one, just in case).You frequently see the same confusion in circuit design, and in industrial wiring. Techs will often place the TVS or capacitor or transorb or snubber across the relay/contactor coil, when it's actually the driving contact(s) that generate the transient. The distinction becomes quite important when there's ten meters between the contact and coil! This is the biggest single reason why equipment needs to pass IEC-61000-4-4 (electrical fast transients -- chattering spikes of several kV!).Tim