# Only Running Capacitor

Discussion in 'General Electronics Chat' started by LHBTC, Feb 12, 2015.

1. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
I know about motors with a starting capacitor, with start/run capacitors and without a capacitor.

But is there any motor that comes with ONLY a running capacitor? If there is, how does its circuit look like.

Nov 30, 2010
16,248
6,744

File size:
27.3 KB
Views:
153
3. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
Thanks #12, Does that mean that the running capacitor here do both purposes (the electrical "push" to let the motor start rotating + running capacitor purpose [Which I believe is regulating the current as per the torque needed to keep rotating])? Am I right?

4. ### #12 Expert

Nov 30, 2010
16,248
6,744
I'm not all that good with motors, but I have seen a PSC motor that would not start until I replaced the capacitor. That tells me the capacitor is necessary to get it started.

That's all I have.

5. ### AnalogKid Distinguished Member

Aug 1, 2013
4,515
1,246
With single-phase AC, there are points in the energy wave when the energy is zero. The idea behind three-phase AC is that there is no point in time when all three waves are zero, so the motor is getting a more constant stream of energy with which to work. The schematic in post #2 is a low-cost approximation of this. The capacitor introduces phase shift in the energy wave to the 2nd winding, so while there is a point in time when the total energy in both windings is low, it never is zero. This gives you a flatter torque curve, but torque or speed regulation is not an explicit characteristic of this motor type.

The switched capacitor motor uses the greater torque of two windings for starting and getting up to speed quickly, then drops the second winding for greater efficiency with light to medium loads.

ak

6. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
So are you saying that the running capacitor's purpose is to eliminate the zero energy point? But I think the running capacitor could be set in a three phase setup:

I am not an Electrical or Electronic engineer. But I basically want to know that purpose of the running capacitor, and is it possible to use it alone without the starting capacitor (Perhaps the first question will answer the second, hopefully)

Feb 11, 2015
9
0
bump

Aug 1, 2013
4,515
1,246
9. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
Hi Ak,

The links explains well the start cap, run cap motors. What I want is the running cap without the start cap at all (if that is a thing), and how it works bla bla bla.

10. ### debe Well-Known Member

Sep 21, 2010
945
184
For a single phase motor with a Run capacitor the circuit you asked for is the one #12 posted. these types of motors are usualy used in aplications that don't require a lot of torque at start up. & no they wont start without the capacitor.

11. ### KLillie Member

May 31, 2014
126
14
Maybe you want a
Permanent-split capacitor motor?

Jul 18, 2013
10,500
2,364
The two basic cap start types are combined permanent cap start/run where there is only one cap used for both starting and remain in for run.
The second has both a start and a run cap, the start cap is to provide sufficient energy to the start winding for the initial start and switch over to a smaller run cap for continuous run.
The reason for the cap is to provide a 90° phase shift in the start winding WRT the run winding, if no capacitor is used the field oscillates across 180° and results in zero rpm.
For your OP question, the single cap start/run cap that is kept in circuit is connected the same way, in series with the start/run winding.
Max.

Last edited: Mar 1, 2015
13. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
I think you nailed what I need. Wiki doesn't explain it simply, so I searched somewhere else and I got this:

The permanent split-capacitor (PSC) motor uses only a run capacitor to provide the phase shift required to start the motor.

Thanks!

14. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
Ok, I assume that you mean by 90 phase shift that to provide two forces (from the two windings) that act together as a torque?

like this(?):

===================

Then as a summary I can say that these are the common motor types WRT capacitors (and please correct me if I am wrong):

1- CAPACITOR-START, INDUCTION-RUN: have only a start capacitor to initiate the torque.
2-
CAPACITOR-START, CAPACITOR-RUN: have start cap to initiate the torque (and once achieve that, disconnects from the circuit), and then a run cap to assist with the torque for good.
3- PERMANENT-SPLIT CAPACITOR MOTOR: have only a running cap, that will be able to initiate the torque and it will continue to contribute to that all along the way.

Jul 18, 2013
10,500
2,364
Basically, if some kind of phase shift (~90°) is not produced initially in a second winding then the magnetic field oscillates across 180°, so no rotation results.
Motors with failed caps can be given a swift turn and will usually run once spun.
Another method instead of a cap used in small motors such as fans is the shading ring, this is a shorted copper turn, the high current in it produces a phase shift in that part of the stator.
Max.

16. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
Thanks Man, simple and sweet!

Can you confirm the summary I have written before though?

Jul 18, 2013
10,500
2,364
Your DWG in post and summary in #14 is correct, the one in #6 incorrect, one too many windings.
With the start and run type both caps are in circuit (in parallel) initially, then the start cap is switched out when up to speed.
Max.

18. ### LHBTC Thread Starter New Member

Feb 11, 2015
9
0
Thanks Max; max info!