Bench grinder

[Piero]

Hi Guys

One thing I am not overly knowledgeable on is electric motors.

A while ago, I got a load of lathe stuff, and inc in the sale was this large bench grinder, (its big for a bench grinder)

anyways, it does not have a start cap, nor a run cap, never came with one, nor do I think it ever had one,

It normally starts in the right direction, but not always, (so I always spin the wheel first to get it going before hitting the start button)

This aint been a issue (tell me if I am wrong) but what is does do, is not have heaps of power!!!!

when sharpening a tool bit, it does slow down some, far more than I would expect from such a big motor

it takes a while to get to speed, and a while to go to stand still from turning on or off.

what could cause this weakness in the motor ???

 

[Shox Dr]

It a 3ph motor.

 

[Piero]

wont not have guessed so,

The chap I in directly got it off, had it in a little shed at the bottom of his garden, he was a model engineer, this was his bench grinder for the lathe tool bits,
there was not 3 phase at the house, and it came with a 13am plug on it

 

[Shox Dr]

Bodged. It needs a cap or a vfd.

 

[Piero]

Thanks for the pointers guys.

Ill take some photos when I am next down at my workshop (tomorrow) and post them.

 

[Piero]

So it's not as big as I first thought.
1/2 hp for a grinder ain't that bad though.

It's 230 v and single phase.

No start or run capacitor.

Should I fit one then????

 

[roofman]

So it's not as big as I first thought.
1/2 hp for a grinder ain't that bad though.

It's 230 v and single phase.

No start or run capacitor.

Should I fit one then????

the cap will be under the bottom plate

 

[Richard.]

Taking a while to slow down when you turn it off is good - decent bearings and two heavy flywheels (grinding stones)

Mine takes an eternity to slow down once turned off.
At least 3 minutes. Bloody annoying to the point I stop it just before I've finished the job.

 

[Pete.]

the cap will be under the bottom plate

I agree my dad had a similar looking one and the cap was in the base.

 

[Piero]

you would have thought the cap was in the base but its not.

def no cap anywhere

 

[roofman]

you would have thought the cap was in the base but its not.

def no cap anywhere

well my money is on 230V 3 phase then

 

[Piero]

could be.

Ill whip it off from its stand hand have a look at the wiring (take a pic and post )

 

[rtbcomp]

It could have a non-functioning centrifugal switch, they don't always have capacitors. If it's stuck closed having the start & run windings connected at the same time could account for a loss of power, or maybe the run winding is faulty.

 

[Piero]

Best I look closer

Thanks guys

 

[Piero]

Looks a very basic motor.
Tested the ONLY two wires on the motor.

The two wires from the windings have condinuaty.
Tested for ohz it read 8.3

Tested each wire to see if it went to ground and they don't.

ONLY thing I noticed was one of the brushes was sticking.

Now I know very little about electric motors.

Heres a question.
Why is nothing conceded to the brushes.

They are simply conected to the metal body they site in and in turn the are conected together

 

[Seadog]

It's what is known as a repulsion start induction motor. It doesn't need a starting capacitor.
You can read more about them at - https://en.wikipedia.org/wiki/Repulsion_motor

 

[Pete.]

That type of motor is similar to my dc lathe motor in that you can adjust the brushes to tweak the performance. Probably the reason it is so weak is because the brush holder needs rotating. Mark the holder and make a small adjustment by turning it more in the direction of rotation, this should increase the performance. Mind you don't overspeed the wheels.

 

[rtbcomp]

If the 2 brushes are connected together it's a repulsion motor like @Seadog says.

If the brushes are connected to the mains and or the field coil it's a universal motor (AC or DC)

 

[Piero]

Thanks guys.

Yes I did mark the plate with the brushes before stripping it all down.

There is no scale note was there ever any marks to say we're it should be.

What's the does and don'ts about adjusting this plate (with the brushes)

 

[Seadog]

I don't think there are any. Motors have been made where the brush plate can be rotated from outside. This allows the motor speed to be controlled and even reversed. As @pete says, make sure you don't set the speed too high and exceed the rating for the wheel.