Hello,
I've got a green Bosch lawnmower ARM32 which seems to be broken. When I press the buttons I get a loud hum from the motor but the blade does not spin. I've noticed if I press the switch on and off, there is a metallic clunk every time I press the switch in to start the mower.
I've looked and there are no blockages and -when unplugged!- the blade spins freely by hand.
Is it time to buy a new one or can I or an expert fix it?
Thanks.
My initial thoughts are you've wound the bottom plate up too tight. If you are winding the helical blade backwads the blades will ride over the bottom "blade" but when the motor tries to power it the leading edge of one of the helical blades will catch on the corner of the bottom blade.
slacken off the adjustment screws untill the blade doesn't jam when rotated in a "forwards" motion i.e. from sky to grass.
You can adjust the centre of the bottom blade if it's not cutting by a couple of subtly "firm" hammer blows.
Then re-adjust both adjustment screws to get the blade to the same "gap" all the way along.
I may be wrong...... but I'll bet a beer on it I'm not. :¬)
HTH Pete
Sounds like it's an induction motor with a dead start/run capacitor. The test for this is to see if it starts if you give it a spin, but this is rather dangerous, obviously, so it's up to you if you think you can do this safely without removing fingers. (Don't spin it with your hand!)
Hello, If I have understood correctly, you seem to be suggesting that there are two blades, one on top of the other and they are catching. is that right? If so, I don't think this is happening because as far as I am aware there is only one blade. There is a diagram at:
Thanks.
Thank you. I think you may be right. When I bought the lawn mower I am sure that the marketing emphasised that it had an induction motor, saying that induction motors were quieter than other types of motor. Is that true? I have had a quick look on wikipedia to find out what an induction motor is but I didn't see anything about noise or power compared to other (synchronous ?) motors. The neighbours' mowers do seem louder, so are induction powered mowers rare?
I don't think that the exploded view I quoted in my other reply shows a capacitor. Is it an integral part of the motor? I hope not because that web site prices the motor at more than thrice the cost of the mower! I presume the start cap. is easy to identify being the largest/only capacitor? If I buy one of the same capacitance and same voltage rating, should everything be ok?
What would cause the cap to fail: is it age? I bought the mower about eight years ago, is that good for a lawn mower/start cap? How long should they last?
Why does the motor get louder when the cap fails, and why is there the clunking noise? Have I understood wikipedia correctly, is it that the cap is broken so it does not supply enough current to get the motor started?
Thanks again.
In message , Stephen writes
Pete thought you had a cylinder mower by the sounds of it.
Thanks for the explanation. Are cylinders better than rotaries, should I need to buy a new one?
I found this:
BTW I notice what I call the blade, that site alls the knife, is looking a bit worse for wear. Can these be sharpened on a bench grinder or is there a better way?
Thanks again.
Nothing cuts grass cleaner than a well sharpened and set cylinder. Nothing gets out of whack faster, that a cylinder and wood, stones or children's toys.
Your choice.
Yup, much quieter than a universal motor - which is the other common alternative.
They are getting much more common - since rules about noise emissions have tightened. They also have a much more suited torque curve for grass cutting - developing maximum at full speed, rather than maximum when stalled.
On many motors they are housed in a bulge on the side of the motor...
They can dry out, or leak... I had a similar problem on my SIP chip collector recently. The cap looked rather knackered - the outer case was leaking and it was a bit soggy!
The cap is there to create a phase shifted supply voltage (in effect creating another phase to create a rotating magnetic field that will drag the armature around). Once it is spinning, its no longer needed (the momentum of the armature and the slight "slip" caused by the mechanical load on it, will keep it moving) and a centrifugal switch will normally switch the cap out of circuit. You normally hear a clunk when the switch activates - normally as it is coming up to speed, or as it spins down. Alternatively you may be hearing a solenoid clicking or something like that.
There is no reason why it would get louder as it fails - however once it is failing to spin and just stalling at startup, you will get a loud hum from the motor as it just sits there with the armature buzzing in the alternating magnetic field.
easy to grind the blade, and it makes a real difference to performance. They do get in a sorry state. I always ground in situ, with an angle grinder. Dont forget the grind should be one sided, so the grass cuttings are whisked upwards.
NT
I think there is also a requirement for the motor/blade to stop within a few seconds (10?) after switching off. The noise you hear may possibly be an automatic brake being released.
Bob
Thank you for all the replies. I removed part of the lawn mower body from above the motor: it was held in place by three clips, one on each side. Looking into the mower I can see the motor and the capacitor is bolted onto the side of the motor, so I am hoping I can change the capacitor without having to change the motor. I'm hoping this way I can make a cheap repair rather than have to buy an expensive new mower.
The side of the capacitor is obscured by the mower body. I need to remove the motor to read what is on the side of the cap. Incidentally, there does seem to be a bulge three quarters of the way up the capacitor. Does this indicate failure?
I tried to spin the blade using a garden cane and standing well back whilst wearing goggles and the motor will run once spun, so I guess this confirms it?
On the motor is a label which says "CB 12uF 450VDB". Is this referring to the start capacitor? I am puzzled by the VDB: 450 Volts but what is DB?
To remove the motor to read the capacitor directly I need to remove four 17mm bolts. Unfortunately these are recessed and I cannot fit a socket or box spanner into the recess. Any idea how I loosen these?
It looks as though the two wires from the cap goes straight into the motor, which presents me with a second problem: how to connect a replacement capacitor?
Will the capacitor still be holding a charge. Do I have to be very careful handling it? If so, what is the safest way to discharge it so that it no longer poses a threat?
Thanks.
Sorry to reply to myself.I had another go. What I thought were nylon nuts were actually covers. Once I flipped these off I found smaller steel nuts that I could fit a socket around. The motor is now loose but it is too big to get out the hole, so I've got to work out how to remove the rest of the mower body! Why aren't these things simple?
Bulge in cap' usually means FKD. Discharge it by shorting the wires together if you're worried, (Use insulated pliers) but if it's bulging already it's had it.
Maplin or R.S components may well have a suitable replacement
DC it should be. Peak voltage of 230vAC is about 400-415v. So 450V DC ~
250VAC.Get a slender box.
Solder or crimp to cut wires, or dismantle motor.
No. Almost certainly not. its generally shorted by the totality of the windings.
It's probably VAC.
Go to
The actual capacitance value isn't very important - the higher the value, the higher the starting torque, but the higher the current draw. The voltage should be 400VAC or higher, but all start capacitors are anyway. Check the physical size will fit, and if it won't, go down to the next smaller capacitance value.
Maybe push-on blade connectors at the capacitor. If not, get a wire ended one, and cut/join the wires.
Probably not, as it's dead. Also, don't power it up for a while before opening it.
We used to grind them off-mower and then take off a bit from whichever back edge needed it to get the balde balanced.
OP: My original comment was about a cylinder mower. Sorry if I confused.
Cheers Pete
I managed to dismantle the mower. It was simpler than I first thought. Just a few well hidden screws. I wasn't sure if I touched the two wires together whether it would go bang. Don't I need a series resistor? Judging from the other posts though, it should discharge through the motor windings?
The capacitor I have has something not unlike long, fault, spade terminals that slide into a connection block on the motor, so I may buy the wired cap you mention and solder the wires to the terminals.
Thanks again. That's saved us a fortune.
Well it's past 9pm, you're still alive and persevering well. Follow the ideas and recommendations from Andrew below and you'll get there
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