Controlling Motor Surge

[shedbang]

Guys - you're really doing me proud with this one. Thanks a million.

To try and answer all of the questions...

RF:-

Garage Feed in the house is 40A Type B MCB. In garage, we have 32A for the ramp, 32A for sockets and 6A for lights. They're all Type B.

A very small run of T&E is buried in the wall. It is in a safe zone and metal capped. I THINK this means we're okay per regs.

It's the approach I would initially discuss with my electrician.

Ericmark:-

Cost is not a massive issue, but I shelled out a lot to get it wired this way and I'd rather try and make what I have work rather than starting again. I'm a DIY mechanic, so no it's not 'mission critical'. I probably only use the lift about 3-4 times a month nowadays.

Interesting idea about running a load at the same time. Could try the compressor, or the grinder. Might have a go at that later, but ultimately I need a more permanent solution. Certainly the compressor and ramp together would be putting quite a load on things.

I see your point about the hydraulics (yes, it is), but it's surely just a millisecond surge that we need to squash. You must love it when idiots come up with ideas, but I take it that something like this would absolutely not work if wired between control and motor...

http://www.conrad-uk.com/ce/en/prod...SHOP_AREA_17453&promotionareaSearchDetail=005

 

[RF Lighting]

Unfortunately capping does not exempt the use of an RCD. How much of a ball ache would it be to access this cable now?

As we are really struggling, and it's going to depend on the motor type, but you might be able to modify the ramp to be inverter driven. These can be programmed for soft start, speed control, direction etc, but it will also depend on if the manufacturers allow you to modify there product.

 

[bernardgreen]

I see your point about the hydraulics (yes, it is), but it's surely just a millisecond surge that we need to squash.

The overcurrent can last for a couple of seconds if the motor cannot reach speed any quicker than that.

Hydraulics. Some large hydraulic systems have a manual full bore bypass valve that has to be opened for starting and then closed once the motor is running.

Is there a bypass or relief valve that allows oil to circulate back to the pump when there is no actuator being fed ?

If there is and it is adjustable then setting it to its lowest pressure setting before starting the motor may reduce the mechanical load and allow the motor to accelerate to speed in a shorter time.

BUT adjusting what is a safety device may create a hazard if it is not accurately reset back to normal pressure after starting.

 

[shedbang]

Unfortunately capping does not exempt the use of an RCD. How much of a ball ache would it be to access this cable now?

Not too much stress I suppose. The sensible thing to do would be to access the T&E where it goes into the wall (which can be done), put a box there and convert it to SWA instead. Run that in the wall (made easier as presumably you don't need to run it via safe zones do you?) and reconnect it with the box outside. I'dleave the old T&E in the wall doing nothing.

Biggest pain is that I'd have to get rid of the wife as she went a touch mental when we did this last time (with the T&E) and the Sparky would have to do it on Friday so I can make good on Saturday. That's TWO nights in Champney's.

Bernard - not sure if there is a bypass valve on the ramp, but there is obviously a release valve, which causes it to descend. I wonder if I were to depress release for a few seconds before and after powering the motor it would lower the stress on the motor. Might give that a try later!

 

[ban-all-sheds]

Biggest pain is that I'd have to get rid of the wife as she went a touch mental when we did this last time

Have you got one of those wives who thinks that all building work can be done without making any mess or noise, you just need to know how to do it properly?

 

[SimonH2]

The problem with starting a pump, and I would guess the ramp is hydraulic? is as it starts the load increases so unless started within a set time is can stall because of the hydraulic pressure.

Not just that, but you are effectively starting under load since there is no compressibility.

Interesting idea about running a load at the same time. Could try the compressor, or the grinder.

Compressor is no good - it'll add a lot of load to the supply and make things worse. Small hand grinder will probably have no effect - I suspect ericmark is referring to the big industrial grinders with a 2+ foot diameter x several inches wide lump of rock spinning.

A while ago I was experimenting with options to start a pump for a 4 poster lift I'd acquired with a friend. This was a 3 phase, and "from stock" dad produced a big transformer, manual start-delta starter, taperlock boss to mount an old car flywheel on - and motor to mount it on, and selection of large caps to synthesise a 3 phase supply.
We had some success doing a star-delta start of the motor-flywheel, and direct online starting the pump.
The idea, which did seem to work, was to get the flywheel going with a slow start - and use the energy stored in that to assist in a) starting the pump motor, and b) making the 3 phase more balanced (once running, the motor effectively becomes a rotary transformer).

Alas, my mate fell out with his missus and got divorced before we got the lift installed There's been a few times I could just have done with it - it was a 3 1/2 ton, 16 foot long job out of a garage

EDIT: Seeing the comments about your missus ... perhaps I shouldn't mention divorce.

I see your point about the hydraulics (yes, it is), but it's surely just a millisecond surge that we need to squash.

No, it's much longer than that.
Your air compressor will have some sort of unloader (usually part of the pressure switch) that dumps the pressure out of the compressor to tank pipework. When it starts, this pipework takes a short time to pressurise - during which the motor has a fighting chance to get the whirly bits up to speed.
Your lift will be much worse. Oil isn't compressible (which is why it's used) and so the motor will be starting under load from the moment the pump puts any oil out - hence the startup current*time product will be much higher. For the same power it will take longer to get to speed, or it can get to the speed in the same time with a higher current.

Analogy time.
You have a steep hill, and a car.
If you start on the flat and accelerate up the increasing gradient until you already have some speed before you reach the steep bit - that's like starting your compressor. Even with a small engine, you can get a decent speed up by taking advantage of not having a full load to start with.
If you stop on the steep bit and then do a hill start - that's like starting the pump in your lift. If your engine is small, then you'll take ages to get up to full speed. If you have a bigger engine, then you'll get up to speed quicker, but will use more power to do so.
Compared to starting on the level and having a run up, starting under load will either need more power (= more current, 'bigger' motor) or take longer to get up to full speed. More current or longer time will both increase the chances of the MCB tripping.

EDIT: As I don't think a longer startup time would work, I suspect the motor in your lift is designed for a much higher starting current to get it going quickly.

You must love it when idiots come up with ideas, but I take it that something like this would absolutely not work if wired between control and motor...

http://www.conrad-uk.com/ce/en/prod...A_17453&promotionareaSearchDetail=005[/QUOTE]
Sorry, it's designed for things like power supplies and battery chargers. I think all it would do is give starting problems with the sort of load you have.

Hydraulics. Some large hydraulic systems have a manual full bore bypass valve that has to be opened for starting and then closed once the motor is running.

Is there a bypass or relief valve that allows oil to circulate back to the pump when there is no actuator being fed ?

That is a good point. Probably worth asking the lift manufacturer about it options. It may be that just adding an accumulator would help - by providing somewhere for the oil to go and making the start more like that of the compressor. Or an unloading valve.

Most likely they'll just say your supply isn't big enough.

 

[shedbang]

Bernard - not sure if there is a bypass valve on the ramp, but there is obviously a release valve, which causes it to descend. I wonder if I were to depress release for a few seconds before and after powering the motor it would lower the stress on the motor. Might give that a try later!

Been thinking about this more and more. The ramp rarely, if ever trips on the descent cycle. The way it descends is to lift the safety locks via pneumatic pressure, run the motor for around 3-4 seconds to take the load off them and then slowly releases the pressure in the hydraulics.

I'm thinking that in the first phase of this cycle the ramp releases pressure at the same time as it runs the motor, lessening the startup load (ie some oil simply bypasses if that makes any sense). Hence it doesn't trip. If the ramp allows me to start the motor at the same time as releasing pressure (basically by pressing 2 buttons at once) I may be able to trick it into lightening the load. Will try tomorrow!

 

[SimonH2]

More likely that the 3-4 seconds isn't enough to trip the breaker.

Back to my earlier analogy, if your car has a bad cooling system, it may well boil before you get to the top of the hill. If you only go a few yards and stop then it won't.

It sounds like the lift is similar to the new one at my MoT station.

 

[shedbang]

More likely that the 3-4 seconds isn't enough to trip the breaker

No - when the ramp trips the breaker it does it instantly, literally within a second of pressing the button. Barely enough time for the motor to grunt. It's never had the decency to wait longer before it trips!

 

[mysteryman]

Which is more important to you, resolving the tripping problem, or ticking all the boxes on a checksheet?

I would suggest that you need to find a way of starting the motor off-load. Single phase motors of this size can have a large inrush current.

 

[Bolty]

Afternoon,
A quick check of me handy fuse links chart/mcb chart for motors shows a 2.2kw single phase motor has a starting current of approx 108 amps for a duration of 3.3 ish secs. (assuming direct on line starting).

The chart shows a 32a mcb will be suitable (just) but it will need to be type c or d mcb assuming your installation ticks all the boxes (earth loop impedance values etc etc.

You should consider the use of a proper overload unit to protect the motor also - standard industry practice for motors above 0.5kw regardless of single phase or 3 phase.

Bolty.

 

[matt1e]

Since you don't have a three phase supply can't really use a soft start it would need to be a full inverter.

Not so eric, you can get single phase soft start modules,
Rs do them for one,this may suffice and Datasheet here

Matt

 

[SimonH2]

Not so eric, you can get single phase soft start modules,
Rs do them for one,this may suffice and Datasheet here

Pity they've got errors on the datasheet - the drawings for Figs 2 & 3 are swapped round, and the terminal markings on the one photographed (looks like a 3 phase version) don't match the diagrams.

But with a load like the OPs hydraulic pump, I think it would need an unloading facility or there'd be a risk that it would be unable to start. Once the motor is turning, it's only going to get as little as a fraction of a turn before the pressure is up to full - and then you have a risk of the reduced torque being insufficient, the motor would stall, and only restart when the supply voltage increases sufficiently.

Again, I think the OP needs to talk to the manufacturer - he's unlikely to be the first, or last, with this problem.

 

[matt1e]

Not so eric, you can get single phase soft start modules,
Rs do them for one,this may suffice and Datasheet here

Pity they've got errors on the datasheet - the drawings for Figs 2 & 3 are swapped round, and the terminal markings on the one photographed (looks like a 3 phase version) don't match the diagrams.
.

So they are!well spotted Simon, and as well as the photo on the data sheet being 3ph its rating is also only 1.1kw

The photo on the rs page is correct though

 

[shedbang]

Guys

Thanks for all the help.

So to summarise...

- in theory I could try the RS device to suppress the surge...
- but it may restrict the motor and cause it to stall...
- and I'd need some help wiring it in anyway.
- so must get around to calling the suppliers and seeing what they say...

Contingent on this...

- we're probably back to getting the power supply right to start with...
- the existing physical wiring is probably just sufficient...
- but it needs a switch fuse at the CU end
- and a type C MCB or fuse at the garage with maybe a 100mA RCD on the ramp
- sockets and lights on separate circuits with 30mA RCD and appropriate MCB's in garage.
- a new TT earth in the garage with the lead pipe bonded.
- and a run of T&E buried in the house wall replaced with SWA.

Thanks for all the help. I'll let you know how we get on!