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  1. #1
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    Default Adding a start capacitor do-able??

    The new belt grinder attachment must present a fair bit more starting load on the 600 watt bench grinder motor, than the std wheels did, so a rolling start is now required to get it rolling.
    Is it possible and feasible to add a start capacitor (or other device) to increase the start torque? Cheers.


    The beast

    Belt grinder 1.jpgBelt grinder 2.jpg

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    Phil, those grinder usually have a capacitor inside the base - maybe it just needs replacing?

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    Quote Originally Posted by BobL View Post
    Phil, those grinder usually have a capacitor inside the base - maybe it just needs replacing?
    Really?? That would be very handy.....I'll check it out, Cheers Bob.

  4. #4
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    I tried to id the grinder from the pics and suspect that the label says Abbott & Ashby, but could be wrong on that aspect. Have worked in metal shops over the years where there are A&A grinders, and the cap in the base is only a run cap, not a start cap. They don't have either a dedicated start winding or the associated switchgear, and basically struggle to spin up the two wheels that they come fitted with, typical time to running speed in the order of 30 seconds, and very little torque once they get there, so a very light load against a wheel will slow them. Due to a lack of dedicated winding and switch gear, they cannot be upgraded to cap start, it would be easier and cheaper to get a more powerful 3 phase motor and VFD to drive it.

    We also used to run a batch of belt grinders with 2400 long belts, motors were 3HP to 12HP dedicated polishing lathes (double ended motor with long shaft either side to keep work well clear of motor to allow work to be swung around at all angles to polish all over). The little 3HP unit was basically configured as a linisher for deburing the end of material once it was cut to length, but was occassionally also used for touch up grinding or surface finishing, but was lacking power for that as the prolonged contact periods slowed it down.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  5. #5
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    Hi Phill, Guys,

    I hear what Mal says, but in my opinion it would be worth checking as Bob says and trying a new capacitor.
    Best Regards:
    Baron J.

  6. #6
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    Quote Originally Posted by BaronJ View Post
    Hi Phill, Guys,

    I hear what Mal says, but in my opinion it would be worth checking as Bob says and trying a new capacitor.
    Indeed. The project was to re-purpose the A.& A. grinder. Buying a new motor would be nice, but it isn't going to happen.

    There is indeed a capacitor in the motor Mal, and I think it might be a start, or start/run capacitor because without it, I cannot spin-start the thing, but with it I can.

    The question now is, will fitting a bigger one make any useful improvement?
    The one in there is 16mf.

    EDIT:
    Looking at the formula for calculating start caps and run caps, this is definitely a run cap, so I have no idea why the motor won't start without it.
    Last edited by ptrott; 16th Jan 2020 at 08:16 PM. Reason: More information:

  7. #7
    BobL is offline Member: Blue and white apron brigade
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    A run capacitor also acts a bit like a start capacitor or at least contributes albeit slightly to the starting capacitance of a motor circuit.
    The run (Cr) and start (Cs) caps are usual in parallel which has the opposite effect to resistor addition (resistors in series increases resistance, resistors in parallel decreases resistance) so when the caps are in parallel the net capacitance is increased. This allows more current to pass through the auxiliary motor coil (RA/XA) which is what helps the motors start especially on motors under significant starting loads, and keeps it spinning. In theory you could make the start cap bigger but then the auxiliary coil would always have more current going through it which will (sooner or later) over heat the auxiliary coil.

    Screen Shot 2020-01-16 at 7.02.46 pm.png

    The first thing I would do is check the capacitance of the original start cap - have you got a meter for this? If not someone local to you might have a meter for this maybe ask your local sparky?
    If not just get a 16µF (same as mf) replacement.

  8. #8
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    Hi Guys,

    Does this motor have a centrifugal switch ?

    If not then the capacitor is a start one and the motor will behave in a similar manner to a shaded pole motor.

    The capacitor value is not critical, plus or minus 10% would be fine, though my inclination would be to go larger rather than smaller ! On the basis that motor capacitors loose value with age.
    Best Regards:
    Baron J.

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    Quote Originally Posted by BobL View Post
    A run capacitor also acts a bit like a start capacitor or at least contributes albeit slightly to the starting capacitance of a motor circuit.
    The run (Cr) and start (Cs) caps are usual in parallel which has the opposite effect to resistor addition (resistors in series increases resistance, resistors in parallel decreases resistance) so when the caps are in parallel the net capacitance is increased. This allows more current to pass through the auxiliary motor coil (RA/XA) which is what helps the motors start especially on motors under significant starting loads, and keeps it spinning. In theory you could make the start cap bigger but then the auxiliary coil would always have more current going through it which will (sooner or later) over heat the auxiliary coil.

    Screen Shot 2020-01-16 at 7.02.46 pm.png

    The first thing I would do is check the capacitance of the original start cap - have you got a meter for this? If not someone local to you might have a meter for this maybe ask your local sparky?
    If not just get a 16µF (same as mf) replacement.
    The capacitor checks out as O.K. Bob.
    I am thinking that a momentary switch with a 100u/f 450V start cap might be the go.
    I can hold the button for a couple of seconds while it spins up, then release the button,...any reason why that wouldn't work, much like as though it had a centrifugal switch?

  10. #10
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by ptrott View Post
    The capacitor checks out as O.K. Bob.
    I am thinking that a momentary switch with a 100u/f 450V start cap might be the go.
    I can hold the button for a couple of seconds while it spins up, then release the button,...any reason why that wouldn't work, much like as though it had a centrifugal switch?
    Sure, worth a go and if you have something like a 20µF cap even that might be all that's needed

  11. #11
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    By opening the circuit at the run cap and trying to start the motor, you have totally removed the direction information from the motor, and there is no way you will flick start the motor fast enough to get it to self sustaining speed without the run cap in circuit.

    A single phase motor needs something to tell it which direction to run, and generally to give it a kick start in that direction. At minimum, this involves the auxiliary winding and run cap, or for a more complex higher starting torque solution, a start cap, switching and start or auxilliary winding.

    These are not required for a 3 phase motor as it uses three main windings energised by separate phases and that inherently provides the direction information and starting torque.

    The combination of a capacitor and the inductance of the aux winding induces a phase shift for the current flowing through the aux winding relative to that flowing through the main winding. The combination of the two currents gives the motor its direction information and allows it to start to spin abet with poor torque, and ultimately gain rated running speed if not overloaded.

    The use of a start cap and speed sensitive switching with a start winding does something similar, except that because the start cap is significantly larger in capacity than a run cap, it allows a much greater current to flow, and produces a greater phase shift relative to the main winding current, hence more starting torque and a more rapid acceleration to rated speed.

    If a motor is designed to be cap run, the auxiliary winding is generally a similar number of turns to that of the main winding, but a finer wire as it is carrying a smaller current continuously. If a motor is designed to be cap start, the start winding is generally wound with the same size wire as the main winding, but is carrying up to 5 times the current, so is prone to overheating quickly. For that reason, speed or time sensitive switching is added to the start winding circuit to switch the circuit out once the motor approaches rated running speed. For some applications a cap start cap run motor is used to provide enhanced torque while running. This is achieved with a combination of a normal cap start configuration, plus a small run cap in parallel with the start cap and switching, providing a small current through the start winding permanently. Because this current is small relative to that of the main winding, it does not generate the same heat in the winding that the current flowing through a start cap would, and so does not risk a motor burnout.

    You could, in theory, add a momentary switch and an extra cap in parallel with the run cap to make the motor function as a cap start cap run unit. However, you run the real risk of a burnout of the aux winding if the start cap current is excessive or the switch is energised for too long, as the aux winding is designed for continuous low currents, rather than high intermittent starting currents. Personally I would not be looking at more than matching the run cap value if trying this, but doubt that it would give you a real boost in starting torque to start a belt properly. I certainly would not be looking to add a 100uF start cap.

    A&A grinders have a reputation for good long term reliability, and are well built and balanced, but really they are not what you would call a powerhouse for heavy duty duty work like belt grinding or buffing, although people do use them for that somehow.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  12. #12
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    Quote Originally Posted by BaronJ View Post
    Hi Guys,

    Does this motor have a centrifugal switch ?

    If not then the capacitor is a start one and the motor will behave in a similar manner to a shaded pole motor.

    The capacitor value is not critical, plus or minus 10% would be fine, though my inclination would be to go larger rather than smaller ! On the basis that motor capacitors loose value with age.
    Not quite right, Baron,

    1. Although a centrifugal switch is the most common form of start winding control, there are other approaches as well, such as timer relays and current sensing relays as well. A timer relay will energise the start winding and cap for a fixed period when the motor is energised, then isolate them, regardless of motor speed after that time. A current sensing relay senses the motor current and will energise when it detects the inrush current to the motor, and release once the inrush current decays significantly, indicating that the motor is spinning and getting toward operating speed. Dewalt radial arm saws and some of their other larger SP gear used the current sensing relay approach.

    2. If there is not something to switch the cap and winding in and out of circuit, i.e. the cap is permanently in circuit, then it is a run cap rather than a start cap.

    Abbott and Ashby grinders don't have anything to switch the cap and aux winding in and out of circuit and have quite long start up times when fitted with a couple of grinding wheels, and slow down as soom as a moderate load is applied to the wheels. All characteristics of a cap run motor.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  13. #13
    BobL is offline Member: Blue and white apron brigade
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    Well described Mal.

    Quote Originally Posted by malb View Post
    Personally I would not be looking at more than matching the run cap value if trying this, but doubt that it would give you a real boost in starting torque to start a belt properly. I certainly would not be looking to add a 100uF start cap.
    That's more or less what I was trying to say . . . .

    FWIW, like many others have, I had a Multitool (50 mm belt, 150 mm disc) on a single phase Abbot and Ashby. It worked OK once it got up to speed but I do remember it was very easy to slow down and very easy to prevent starting so I transferred the Multitool to one of my 3P grinders. I still have the A&A and run a wire wheel on one side and Scotchbrite on the other. To remind myself of the low starting and running torque I just went down to the shed and using a piece of wood even lightly held it against the hub of the wire wheel was enough to prevent it starting..

    Phil, just wondering if your belt is just too tight, maybe before you burn anything out try reducing the belt tightness?
    Rather than playing with the start electrics it might be safer adding some sort of adjustable cam type tensioning system that reduces belt tightness during start up and applies a bit more tension once it's going?

  14. #14
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    Thanks for the detailed explanation MalB.
    I have ordered a cap. online, and the local Jaycar have a suitable momentary switch.
    Given that all I need is the get this thing rolling from stationary, it wont need to be "ON" for more than 1 or 2 seconds, so the chance of burnout in that time would be unlikely, would it not? It starts easily with a flick on the belt, so not much help needed.
    I guess time will tell. It isn't a machine that is used daily, or even weekly, so it should last a while, which is all I need at my age.
    If it does go kaput in the short term, then a 3ph & vfd might be the next move.
    It won't be used for heavy work either, drill sharpening, other tool sharpening, cleaning up cut edges etc. As it stands, it is useless as a bench grinder, so it either does the new job or it sits unused, so worth a try from my p.o.v.

  15. #15
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    Quote Originally Posted by BobL View Post
    Well described Mal.

    Phil, just wondering if your belt is just too tight, maybe before you burn anything out try reducing the belt tightness?
    Rather than playing with the start electrics it might be safer adding some sort of adjustable cam type tensioning system that reduces belt tightness during start up and applies a bit more tension once it's going?
    No Bob, the belt is not too tight, not enough to make any difference. I thought that yesterday so tried a loose start, but still wouldn't.
    The belt is tensioned by a spring which is not very strong, just enough to keep the belt running smoothly. Any less tension and it start to flap a little.

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