SSR / SCR use with brushed AC motor - Page 1
Forum section: electronics / beginners

author: BLACKPUDDING
author_rank: Newbie
timestamp: September 07, 2021, 09:26:02 am
content: Hi all. I've joined this forum in the hope I can get some advice. Please forgive my ignorance as I have just a little electronics / electrics experience although I am very keen to learn lots more. (I'm a retired Chief Engr. from an aircraft engineering world).Hopefully someone can assist.I've a set of paddock rollers for my race bikes. They're powered by a new 2.2kW Makita angle grinder motor and gearbox.I would like to change the foot switch to a lower voltage as I don't think having 240VAC mains power in a simple plastic footswitch under my foot, often outdoors, is a smart thing.I looked at AC solid state relays and tried using one (a Crydom HD4850). I triggered it with a small transformer with 12VDC output that was controlled by the foot switch. To my surprise it worked although without any load on the rollers.I then put a bike on the rollers and due to a minor hiccup with the rollers they jammed - this blew a fuse in the extension lead - which in a way is a good sign methinks. I fixed the mechanical issue but when I checked the SSR, albeit only to see if the AC side was 'open', I found it wasn't - I guess its now toast.So, to let me figure out what's the right SSR (if that's the best way to do this) is the in-rush current increase for a brushed AC motor generally lower than a typical mains AC induction motor?2) the angle grinder had a large wire-wound resistor in the grip which was wired across the open side of the trigger in parallel with the supply to the motor - I didn't fit this in my application as there is an identical angle grinder without it also available - both types are designed to run off a 13A 240VAC household power supply, so I reckoned there was no need... does that resistor limit in-rush current?Sorry for the longwinded diatribe and jumping straight in with a bunch of questions but having trawled the 'net for days and I'm struggling to find the answers.I should say I'm UK based and of course thanks in advance for any guidance / help / jokes whatever... best regards to all!

author: Benta
author_rank: Super Contributor
timestamp: September 07, 2021, 09:42:06 am
content: You can call it brushed AC motor if you like, but the more usual term is Universal Motor, and your search will probably be more succesful using that.Inrush/start current is theoretically unlimited, but resistive losses will help on this point.

author: BLACKPUDDING
author_rank: Newbie
timestamp: September 07, 2021, 09:58:06 am
content: ok, I've learned something! - a universal motor...I did find a Crydom datasheet just now that suggests I should have used a D2475-10 which is for inductive / motor loads, and rated for 2.2kW... I'll maybe try that, if i can get one for a half sensible price.thanks

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2021, 12:47:39 pm
content: Beware: Most cheap SSRs purchased from suppliers other than major distributors and authorized resellers are fake with at best a small fraction of the nominal current rating. Either buy the real deal from a supplier that offers traceability back to Crydom or figure out some other way of doing the job.As Benta has pointed out, the stall current of a universal motor can be extremely high, only limited by winding and circuit resistance, and as the stall current is the current with the rotor locked, i.e. not moving, it draws the same peak inrush current at startup. An order of magnitude higher than the loaded running current is not unusual. You need to be certain the SSR is rated for a high enough surge current for long enough to get your rollers up to speed, and if you want it to survive in the event of a fault, you need either rapid over-current detection to cut the drive signal within a cycle (complicated by the startup surge, so it needs some sort of variable threshold with a decay curve matched to the SSR's surge rating curve) or a massively overrated SSR that can handle the full fault current till the fuse or breaker trips.Then there's the safety aspect - A SSR is *NOT* an isolator and cannot be trusted to break the circuit in any application that needs (or rather: that should have) an E-stop button, so you'd still need a real electromechanical contactor in series with it. The only advantage of the SSR in that case is (with a smart enough control system) to allow the contactor to only be switched no load, allowing a significantly lower rating contactor to be used without reliability concerns.Also, an angle grinder motor is not designed for continuous duty, and wont last in such an application. I wouldn't expect more than 100 hours out of a consumer grade angle grinder in such an application, and considerably less if there is any side loading on its gearbox output shaft. This makes it undesirable to spend a lot on a control system for an essentially disposable motor.It may be worth looking at salvaged treadmill motors and controllers. Provided the treadmill originally used a chain or belt drive, you can easily change the gear ratio for whatever maximum speed you need, the motor is rated for sustained duty, there's soft start to avoid surges, and all the power control electronics you need is already present.

author: BLACKPUDDING
author_rank: Newbie
timestamp: September 08, 2021, 08:14:13 am
content: Hi, and thanks for your input - its very much appreciated.Yep I noted that many SSRs are fake, but the 50A zero-crossing item I bought was the genuine article from a Crydom approved supplier.The working time/duty will only see 10-15 secs. running each time it is used to start a bike, and that no more than 4 times in any one race meeting.I've analysed the set up and side loads are acceptable and less than when used as a grinder and the loads imparted via a 9" cutting disc being twisted / turned / pushed into a piece of material. I've attached a screengrab of the kit for ref.I get the comment on E-stop buttons and the need or otherwise of it to survive in the event of a jam / locked rotor fault, I will address that.When I look at the Crydom info. (copy attached) it suggests that a 2.2kW motor FLA is around 17A and the LRA is 156A meaning I need a 100A SSR.Where I think I went wrong was not selecting a random switching SSR with a high enough rating. I think that's the main change to address my concerns.I still keen to understand the purpose of the large resistor built into the angle grinder. The particular grinder version is advertised as 'soft start' and has absolutely nothing different to the non-soft start version except the resistor, so I can only surmise that its providing the soft start and maybe protects the 13A supply in some manner during a lock up or very high load case.Simply put, an angle grinder disc can get stuck in a worst case scenario, so how does it not trip a std. household 13A supply at that point, or even just during start-up?regards and thanks.