Thread: Capacitors for single phase motor

Any place that sells and services electric motors. Being a woodworker, in Perth I would ask at the local Carbatec as they keep spares and switches, including capacitors for all their gear. Got one for the motor on an MC900 from them a few years ago when I still owned one.

Could you use two 30uf's in series?
jaycar have them, STOCK-CODE: RU6608 for $12.95

Got a 50 uf on Thursday from Carbatec Perth - $19.90
Must be the time of the year for capacitors to die.

Try Farnell. They stock a wide range of electrical components and you can buy online. I have experienced good service and fast delivery time when ordering from them. Search for capacitors on their site then select Aluminium Electrolytic and narrow it down through their selection table.

http://au.farnell.com

Originally Posted by Harry72

Could you use two 30uf's in series?
jaycar have them, STOCK-CODE: RU6608 for $12.95

Thinking back on my physics years, they don't add up in series, but in parallel..........thats only if my memory serves me right though.

Try Farnell, or Butler & Smith at Hornsby.

Originally Posted by Harry72

Could you use two 30uf's in series?
jaycar have them, STOCK-CODE: RU6608 for $12.95

No, series will reduce the capacitance, parallel will give you 60uF.

Originally Posted by zathras

No, series will reduce the capacitance, parallel will give you 60uF.

And double the voltage rating, for memory.

Originally Posted by Iain

And double the voltage rating, for memory.

I thought about mentioning volts but left that out.

The voltage rating will double in series, remain the same in parallel.
The capacitance will halve in series, double in parallel.

It was a long time ago.........................1968 just after I qualified as a rad tech RAAF then moved on.

Originally Posted by rodm

Got a 50 uf on Thursday from Carbatec Perth - $19.90
Must be the time of the year for capacitors to die.

Thank you for the replies.

Can I use a 50uf to replace a 60uf.

I know the capacitor is to store a charge so the motor can get going, but is there anybody who can explain the relationship between the size of the capacitor and the motor. Do different brands require different sizes.

I apologise in advance if this gets a bit techo. (I have to live with it so bad luck )

In reality, your single phase motor is really a 2 phase motor, with two (magnetic) field windings at 90 degress to each other.

The idea of the capacitor is that it provides a phase shift of, ideally, 90 degrees to the second field winding of the motor from the normal mains applied to the first winding.

With only a single phase winding active (no capacitor), the motor will be prone to simply sitting there doing nothing, or even worse starting in the wrong direction. Smoke is a bad possibility

With 2 windings spaced 90 degrees apart, and fed with AC mains also 90 degrees apart, a rotating magnetic field is set up in the direction the motor will spin.
The exact value of capacitance required comes down to the inductance of the motor, which works with the capacitance to create the required phase shift.

Once the motor gets above a certain speed, the capacitor is switched out by a centrifugal switch (hear the click) and the motor, due to momentum, is able to keep following the magnetic field, albeit now only using a single winding.

Hence the term starting capacitor.

The 50uF is possibly OK, as long as the motor is encouraged to start spinning, since once up to enough speed it's no longer needed anyway.

With understanding of how the field windings work, it is possible to make the motor spin in the reverse direction, but we won't go there right now
(popular method with some garage door openers BTW)

Three phase motors don't need a capacitor as each field winding (3) is fed by AC mains already separated by 120 degrees. This 120 degrees is perfect, and each winding produces equal power. Thats what makes 3 phase preferred once you get BIG motors. Also, you just have to swap a pair of windings and it will spin the other way. Easy Peesy.

Thanks ray. But I dont understand what you mean by seperated by 90 or 120 degrees apart. To me degree, minutes and seconds are a unit of measurement of angles as opposed to distance.

While you at it, the capacitor blew while the motor was running. I found this puzzling because I knew that after the motor started the capacitor wasn't required. Any thoughts? I just find this stuff interesting..

Could the centrifugal switch be cactus as well? Would this cause the cap to die Zathras?