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When I see some teardowns of single phase induction motors on the internet, I see their capacitors look like:

enter image description here

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Why are they that big in size compared to other caps, even though their value is in the µF range?

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    \$\begingroup\$ It's all in the permittivity. \$\endgroup\$ – Ignacio Vazquez-Abrams May 16 '16 at 17:43
  • \$\begingroup\$ what do u mean? \$\endgroup\$ – user16307 May 16 '16 at 18:04
  • \$\begingroup\$ they seem pretty the same, there is no ruller on the pics \$\endgroup\$ – Marko Buršič May 16 '16 at 18:06
  • \$\begingroup\$ u dont need a ruler look at the bulge next to the motor: encrypted-tbn2.gstatic.com/… \$\endgroup\$ – user16307 May 16 '16 at 18:12
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    \$\begingroup\$ @Arkoudinos Maybe I couldnt express myself enough. Let me tell you this way: please check a 100uf electrolytic capacitor and a 100uf motor run capacitor. You will see a HUGE difference in size. \$\endgroup\$ – user16307 May 16 '16 at 19:17
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Motor capacitors are not plain electrolytics, because the voltage across them reverses at the mains frequency.

The larger value ones are start capacitors - usually bipolar electrolytic, and rated to the peak mains voltage - but not for continuous operation. They are usually disconnected by a centrifugal switch in the motor, which operates when the motor is up to speed. Consider that not only is the voltage high, and continually reversing, but the ripple current is high too (especially when the motor is still at low speed) and you'll see the apacitor has to handle unusually high power.

Smaller value ones (10s of uF) are used as run capacitors in smaller (less than 1hp) motors. These phase shift the current to the start winding, but remain connected during operation, so must be rated for continuous operation. These are usually film capacitors (or, if old enough, paper in oil) which are much larger per microfarad than an electrolytic.

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  • \$\begingroup\$ Thanks for the comprehensive answer. Actually I was planning to simulate a phase controller circuit where the load is a single phase induction motor in LTspice. Then I came across with some huge cap photos coincidentally. I would like to have your opinion if you would be interested in my previous question as well: electronics.stackexchange.com/questions/234290/… \$\endgroup\$ – user16307 May 16 '16 at 21:55
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In my experience the physical size of a capacitor is proportional to the capacitance times its voltage. Double the voltage, double the size.

In practical terms, voltage is higher still.

  • AC voltage is RMS (sinewave peaks are somewhat higher than the stated voltage).
  • When a motor is disconnected from load, its windings will see an inductive "kick" of potentially even higher voltage. That means a motor capacitor has to put up with a great deal more than 220V for instance.
  • Some motors can be cross-wired to be 120V or 240V, so the capacitor has to be rated for 240 even if you are using it 120.
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There are several reasons, not all of them strictly technical.

  1. Voltage rating is very important. Since motor returns current (regenerates) they (i) take margins on voltage.
  2. Current pumped from bulk caps is in pulses, with high frequency. If you will look at cap datasheet, you will see it is rated for certain ripple current- not enough for a serious motor.
  3. Heat is generated on capacitor due to ESR, so in bigger capacitor ESR may be lower. Or in several smaller caps...
  4. Not least important: you always have to take spare for bigger motors and prepare space for even bigger capacitors in case you miscalculated.
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  • \$\begingroup\$ This answer is largely irrelevant to the original question. Point 1 is about DC. The motor in question is running from AC. Point 2 - There is no pumping in induction motors. Point 3 runs counter to the need for larger size. Point 4 is for motor designers who make mistakes. Doesn't happen often these days, unless you are starting in elec eng as a 1st year student. \$\endgroup\$ – Brian May 31 '16 at 10:37
  • \$\begingroup\$ Wow, i believe it's your third year. Good job so far. \$\endgroup\$ – Gregory Kornblum May 31 '16 at 11:20
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The size of a capacitor depends on a number of things.

  1. Voltage rating. The higher the voltage rating the thicker the dielectric needs to be to avoid breakdown. Worse a thicker dielectric means that you need more plate area for a given capacitance. So double the voltage rating and you quadruple the volume of the capacitor.
  2. Capacitance, to double the capacitance means you need to double the plate area which means (for a given voltage rating) double the volume.
  3. Type of capacitor. There is a tradeoff between capacitance density and how close to ideal the capacitor is. Film capacitors have close to ideal capacitor behaviour but are bulky. Electrolytics give you much better capacitance density but they are far from ideal, in their basic form they only work in one polarity, you can work around that by putting two in series but still they will have high losses in an AC system.

Why electrolytic caps would have problem in 230V AC

It fundamentally comes down to how electrolytic capacitors work. Electrolytic capacitors use an electrolyte as one of the plates and an oxide layer as the dielectric. The clever bit is that the dielectric layer is generated electrochemically by the capacitor itself, so damage to the layer self-heals. This allows a much thinner insulating layer for a given working voltage than a conventional capacitor construction. Also the dielectric can uniformly cover a rough plate surface further increasing the effective area.

However this comes at a price, firstly the electrolyte is a relatively poor conductor leading to a high equvilent series resistance (ESR). This generates heat depending on how much current is flowing in and out of the capacitor. A system where the capacitor is fully discharged every cycle will have far more current flowing in and out of the capacitor than a system where the capacitor is used to smooth a DC bus.

Secondly electrolytic capacitors stop behaving as capacitors at all if the voltage goes significantly negative. This is because the electrochemical process that creates the dielectric is reversed by applying a reverse voltage. You can work arround this by putting two in inverse series but then you have even worse ESR.

why bigger size helps

The film capacitors use metal plates and plastic films. This gives good linearity, low ESR and biopolar operation but it can't benefit from self-healing or microscopic roughness.

You can't just think of them as a "bigger version of the same thing". They are a totally different construction with different tradeoffs.

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  • \$\begingroup\$ How can we describe the reason in mathematical way? Why electrolytic caps would have problem in 230V AC and why bigger size helps in help of some formulas? Such as X =2 pi f L so that so forth and so on.. \$\endgroup\$ – user16307 May 17 '16 at 7:44
  • \$\begingroup\$ Film capacitor size tends to scale as the square of the voltage rating. Double the voltage rating the volume of the capacitor will increase by 4. This is a bit of a simplification, but you can see how higher voltage parts get kind of large. \$\endgroup\$ – Robert Endl Sep 12 '16 at 15:14
  • \$\begingroup\$ Electrolytics are often used as motor start capacitors but have too high an ESR to be motor run capacitors. Continuous operation would overheat them. Polypropylene film is used for motor run. \$\endgroup\$ – Robert Endl Sep 12 '16 at 15:20

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