Thread: Something Strange

I went to use my abbott & Ashby bench grinder today and the thing is running backwards, what would cause this? it's not very old and hardly had any use, I also noted that it struggled to get the wheels spinning from startup.

Ben

OK, I just flicked the switch again, and nothing, she hums but she doesn't spin.

Are these type of motors a capacitor start, could that be the problem?

Cheers.

yeah capacitor start, usually located on the bottom of the grinder. Seems unusual seeing it is not very old, mine is over 30 + years old and never had any issues. Maybe a loose wire?

I once had a pool pump that the capacitor went on it and it too ran in reverse.

Hi Ben, Mine did the same and it was the capacitor that cacked itself, any good electrical repair outlet should have them.
Can't remember what the capacity was, as mine had burnt the details off.
Kryn

OK thanks fellas, I'll remove the bottom plate and have a look, hopefully it is just a faulty connection to the cap.

Cheers.

I think the capacitors must all have a use by date, like about now, as I have had 2 of my A&A grinders fail in the last week as well.
Rgds,
Crocy.

A&A 8in grinder motors don't have a start cap as such. There is a cap mounted in the base on the grinder, but it is permanently connected in series with a second winding set in the motor (i.e. no centrifugal switch to disconnect a start winding once the motor is almost to speed.) Therefore the cap is working all the time that the motor is powered, where a start cap and start winding are powered for about 10 revolutions of the armature each time a cap start motor starts. This explains why the caps do not last forever in the A&A motors, and partly explains why they are slow to start compared to a similar powered cap start motor.

In use the second winding and cap combine to create a second magnetic field with poles almost spaced almost midway between the poles created by the main winding, and thus establishing rotation in the correct direction. As the cap fails, the capacitance changes and the phase shift produced can vary, allowing the motor to run backwards for one or two starts. The cap then fails completely leaving the motor with only the poles of the main winding and no clear indication of which way to rotate, hence the humming and hunting. If the machine does not have end guards fitted, it should be possible to flick start the machine, but the motor should not be operated in this manner as it is designed to have the artificial poles operating permanently, whereas a cap start motor only uses the artificial pole for a few revolutions each start cycle.

I replaced a capacitor in my Total Tools Saber ( house brand) 200mm 750 watt bench grinder. Mine went bang rather loudly and emitted blue smoke.

It was wired as MalB described above.

A 13uf 450V AC capacitor was fitted and it has run without problems ever since.

Grahame

I took a look behind the bottom plate and all the connections seemed fine with no obvious signs of the cap failing, I removed the capacitor and checked it for continuity across the terminal posts and got nothing (assuming caps can be tested this way?)

I don't know much about electronics or how to use the ohm feature on a multimeter but I'm pretty confident now that it's the cap so I'll track one down Monday.

Thanks for all the help

You can't really test a cap with a continuity test. A continuity tester uses a voltage at one probe and then looks to see if that same (or similar) voltage appears at the other prob. This also means a small current will flow from one prob to the other through whatever you are testing.

Capacitors will initially allow current to flow (and show continuity on the meter) but as they charge up, the voltage drop across them will increase and the current will decrease therefore cease to show continuity. A large capacitor will take longer to charge up, showing "continuity" for possibly several seconds but much smaller caps will charge up much quicker and you may not "see" any continuity.

The best way to check a cap is to use a capacitance meter. It places and AC voltage across the cap at a known frequency. Capacitors will show a equivalent resistance at a particular frequency. The multimeter can calculate this capacitance by measuring this resistance (or impedance). It's an inverse relationship, as in as frequency increases, impedance decreases. Opposit to inductors.

Simon

Originally Posted by simonl

The best way to check a cap is to use a capacitance meter. It places and AC voltage across the cap at a known frequency. Capacitors will show a equivalent resistance at a particular frequency. The multimeter can calculate this capacitance by measuring this resistance (or impedance). It's an inverse relationship, as in as frequency increases, impedance decreases. Opposit to inductors.

Simon

Or stick a meggar on it..or even 240v and short out (after taking the voltage off it) with an insulated screwdriver ..or toss it to some someone and say CATCH..lol