Use of capacitor start/run with small fractional HP motors

[GreensandsParticipant @greensands]

I am running a bench mounted Hauser Jig Borer from a mains powered fractional HP motor of uncertain age and make (AEI?) which some may be able to help identify. Starting is achieved by momentarily switching in a starting capacitor and the arrangement has given good service over the years. Should the current motor fail, what might be considered to be a suitable (mains) alternative?

I am also somewhat confused about the use of a start capacitor. On a larger fractional HP motor used elsewhere in the workshop the starting capacitor is fitted with a centrifugal switch which cuts the capacitor out of circuit above a certain speed but the smaller unit appears to be reliant upon manual switching to avoid the capacitor from being permanently connected in circuit.

Are there alternative methods of achieving the starting torque without being dependent upon manual switching?

[GreensandsParticipant @greensands]

[Martin ConnellyParticipant @martinconnelly55370]

The capacitor may only be there to kick the rotor round in the correct direction on start up, it may not actually increase starting torque significantly. If so, any suitably sized and rated motor with the same RPM will be a suitable substitute.

Martin C

[GreensandsParticipant @greensands]

Martin – Yes I understand that the capacitor is there to give the motor a 'kick' in the right direction to assist starting but what would be the consequences of not switching it out of circuit?.

[Martin Johnson 1Participant @martinjohnson1]

There are various ways to configure a single phase motor. The basic offering gives reasonable starting torque and simplicity of a centrif switch and capacitor. If you need more starting torque there are options like cap start cap run or induction start which are more complex and expensive.

I suspect your motor had a centrif switch once upon a time, but that broke and someone lashed up the arrangement you have. I doubt starting torque on a wee mill is stupidly high, so a fairly basic cap start machine should do. To be sure, go to the next higher power rating.

If you leave the switch connected on the existing unit, expect smell of fried insulation followed by expensive smoke.

Martin

[Phil WhitleyParticipant @philwhitley94135]

Switching a capacitor in and out manually is not a normal fitment, this has been added to repair a motor with a faulty centrifugal switch! The single phase capacitor start motor is so configured to get over the problem of starting a 1 phase motor in the correct direction. If you disconnect the start winding and capacitor from a single phase motor, and apply power to it, it will remain stationary and just buzz! If you manually give it a spin it will run up to full speed, but will do so in whatever direction you give it the initial spin in. In reality if the motor comes to rest with (depending on design) the poles on the rotor exactly half way between the stator segments they will occasionally start, but with very low initial torque. A circuit consisting of a capacitor, a start winding and a centrifugal switch are connected in series, and the ends of the circuit are connecded in parralell with the run winding. the centrifugal sw is normally closed so that when power is applied to the motor the charging time of the capacitor delays the phase cycle through the start winding, and then discharges the capacitor through the the start winding, giving the rotor a high torque push in one direction and as the motor speeds up, the centrifugal switch opens and the motor continues on the run winding alone. This is why that to reverse a 1 phase motor you swap over the ends of the start winding, which reverses its polarity and gives the rotor the push in the opposite direction. In both cases it is effectively starting as a 2 phase motor, the difference in phase angle being provided by the delay time whilst the capacitor is charging. Not a simple explanation unless you have a nodding aquaintance with electics! That does look like an AEI motor, and if it is functioning adwquately in its present configuration I would leave it as is! Fractional HP motors are fairly easy to come by and not expensive, but a used British made motor is far better than a new one from China, unless you buy one of the more expensive(TEC are V Good) of the oriental ones. The cheap ones are VERY noisy and inneficient! a single phase 1/2hp TEC motor is about £80 new!

[John HaineParticipant @johnhaine32865]

It may be a cap start and run motor, in fact if it doesn't have a centrifugal switch it probably is. With a changeover switch you can easily reverse these motors. My Quorn has one though made by Parvalux.  You can leave the cap connected in that case and indeed the motor would give more torque.

Edited By John Haine on 22/11/2022 13:17:06

[Clive FosterParticipant @clivefoster55965]

Capacitor start motors without internal centrifugal switches are made but, if the ones I've seen are typical, its generally on big motors. I had a 4 hp one on a compressor for some years. Presumably due to the large contacts and fast action needed to handle big motor current.

I used a contactor to do the current switching controlled by a simple momentary action push button switch. It was pretty trivial to hear when the motor was up to speed. At the time I didn't need tank pressure control. Had I such a need I'd have used a time relay instead.

Assuming Greensands has contactor or relay to control his motor I'd simply use a decently husky two pole momentary action push button as the start control. One set of contacts going to the contactor coil and the other connecting the start capacitor. Just hold the button down long enough for the motor to come up to speed. That worked well for me when I used the "capacitor trick" to make a simple static converter for the 1/3 rd hp 3 phase motor on a big Pollard drill.

Clive

[AdrianRParticipant @adrianr18614]

I have seen motors where the start switch doesn't open overheat as they don't spin up to sync speed.

Your motor may have had a centrifugal start switch or a timed start relay, it could not even be a single-phase motor that has been modified.

[Author]

Posts