What Is The Difference Between Motor With 1 Capacitor, 2 Capacitors And No Capacitors

John_Dennis

John_Dennis

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what is the difference between a motor with 1 capacitor, 2 capacitors, and no capacitors?
 
That’s not an easy question to answer, because I can think of about two dozen different AC motor designs of the top of my head most of which do not use capacitors. Since you are asking on this forum I’m guessing you want to know about motors found on most lathes, mills, band saws etc.…

Here goes:

No Cap motors are usually Split-Phase motors, these use a secondary winding that has a different size winding to create rotation. Split-Phase motors take longer to get up to speed and have less torque than capacitor motors. Usually found on small appliances that require low cost reliable motors. Please note that some older equipment has flat capacitors hidden under the motor frame. These can be dangerous because people don’t see the capacitor housing. They think the motor won’t shock them only to get a jolt and getting sliced up from an end mill or lathe tool.


Single Cap motors can be Start Capacitor motors or Run Capacitor motors. First off the Start Cap motor, these are essentially Split-Phase motors that use a capacitor to help the motor get up to speed faster. Once the motor reaches approx. 75-80% of its full rpm a centrifugal mechanical switch takes the capacitor out of the circuit. The Run Capacitor motor uses its capacitor to create a rotating magnetic field to create rotation. The capacitor on the Run Capacitor motor is energized as long as the motor is powered up.


Two Cap motors are almost always Start-Run Capacitor motors. These motors use a Start capacitor and a Run capacitor. Many times you will see two different size capacitor housings on the motor body. The Starter capacitor is used to increase the motors initial torque while the Run capacitor is used to increase the motors efficiency at rpm. They are more expensive to build but have higher torque needed for most single phase equipment. It’s extremely rare to see these motor sized greater than 7.5hp.
 
The other type motor with no capacitors can be a 3 phase motor. It will be labeled as such on the tag and it will have a larger number of wires in the connector box. Most 3 phase motors will have a connection box and not just a hole to pass a cord through as well. A typical 3 phase motor will have 12 wires and a tag that will show how those wires are connected to one another and the line source. They will range in voltage by the way the wires are connected from 208 3 phase to 480 or possibly 575 3 phase power.
 
Another single phase motor I ran across recently is a repulsive start / induction run motor. Haven't seen any new ones but have found a couple of older motors. They gave high starting torque with low starting current. On start they use brushes at 90 degrees to short the armature windings, a centrifugal switch shorts all the armature windings when it comes up to speed and it runs as a regular induction motor. Some lift the brushes off the commutator when they reach speed some let them drag.

Greg
 
I picked up this motor at an Auction with some other stuff and checked it out by wiring up to 110v plug. It powered on and runs quiet. I noticed it can spin in either direction. I've never seen one like this and it has 4 wires coming out. I just used white, Black, and green wires to test it. I was wanting to know what it can be used for and what the Red wire is for.
 
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Just a single speed. 1720 RPM.
I was spinning the shaft by hand clockwise and before it stopped spinning I plugged it in and it spun clockwise.
Spin shaft counter-clockwise and plug in before it stops and it spins that direction with no problem also.
 
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