Motor run capacitor theroy with added PFC fun

[Mongrel Shark]

I blew out an old wax paper cap in my old bench grinder. Already have another more modern 4uf cap on the way.

Wondering how to find out if its the best value to use.

Its really old. Maybe it got home repaired with the wrong cap? Maybe they compromised on the original design? Maybe bigger caps where really expensive or unreliable at the time? Maybe its small to keep the power bill down? What about Power Factor Correction? I'm interested in more output power over less consumption, as I hardly use it. Although more out for less in is always nice.

My current plan is to stick a low ohm (1 or less) high wattage resistor (possibly a nail or something) between the grinder and neutral then put it on the oscope.
One channel watching current lag via the shunt and the another channel on the live input, maybe a third channel on the start/direction coil.
I have a stack of suitable caps to play with. Microwave Capacitors, a couple of motor start/run caps from 6uf to 25uf.

I should be able to have a bit of a play around to reinforce book knowledge. Hopefully find the ideal cap value without frying the windings. Test runs will be short and there will be lots of cool down time while I change caps around anyway.

Obviously the wiring is looking a bit sad, I might as well replace all that while its half naked.

If I'm going to go that far I may as well learn how to find the right cap on paper too.

I imagine I could measure the inductance and resistance of the coils and do some math the find the right value.
Google wasn't very helpful here. Can someone point me to the theory? (this bits bold and underlined as its the main question I came here with)

Also if I find a power factor (worth bothering with), would a cap in parallel with the motor be the best way to correct it? Seems like that would waste some power? But if I go in series it seems I'm gong to need a massive cap to pass enough current? And whats that going to do to the start/direction coil phase?? Same if I PFC the run coil alone, Wont it change the phase relationship with the start/direction coil??

Also any other suggestions for making my grinder better/safer or improving my learning experience would be greatly appreciated.

 

[Minder]

The ideal is a capacitor value that will produce a 90° phase shift WRT the run winding.
It is best to use Motor rated AC run capacitors, usually oil filled paper.
M.

 

[Mongrel Shark]

The ideal is a capacitor value that will produce a 90° phase shift WRT the run winding.
It is best to use Motor rated AC run capacitors, usually oil filled paper.
M.

Assuming the start coil is mechanically mounted at 90deg IRT the run coil and the start coil has the same phase shift as the run coil. and assuming 90deg is the best phase offset for the job. But why would 90deg be ideal? Seems to me that would be the most problematic number you could pick for a direction/run winding.... Seems to me that 75deg would be a more favorable secondary field location...

 

[Minder]

If you shift the start winding phase angle 90° look at where it peaks WRT the run winding!.
The idea is to create an efficient vector angle WRT to 180° of the primary/run winding field in order for the motor to be able to rotate.
M'.

 

[Mongrel Shark]

The idea is to create an efficient vector angle WRT to 180° of the primary/run winding field in order for the motor to be able to rotate.
M'.

Thats exactly why I would think 90 deg would be the worst option. Its the one point you could create a stall or direction change buy loading the motor a bit too much....
Perhaps my logic is flawed, but petrol motor and cranks start needing huge flywheels when you do stuff like that. Otherwise they like to try to change direction more suddenly than they actually can and stuff breaks. 180 and 90 are the 2 numbers you never want to use...

 

[duke37]

The motor will be wound to have the starter winding at 90deg to get the pole pieces between the main pole pieces. Thus a phase shift of 90deg would be best but I do not think that this can be fully achieved with a capacitor.

If the capacitor is start only then look at similar motors to get an idea of its value. A grinder should not need a lot of torque to start up
If the capacitor is start/run, then the value will be more critical since it is in circuit all the time.

I would not be worried about power factor correction for a small motor. If you do decide to go for it then the capacitor has to be connected across the supply. The electricity company should charge for the power consumed, not the VA.

 

[Minder]

Without a start winding/cap a 1ph motor current will oscillate at 0-180° hence the rotor does not move, the optimum vector leverage angle from here is 90°
VFD's with phase vector operation attempt to keep the current vector angle to 90° throughout the operation for optimum force.
It can be demonstrated with levers very easily.
The start winding and cap are tuned for a series pass filter for 50hz/60hz for this angle..
M.

 

[Mongrel Shark]

Ok Guys. I was wrong.
A. I pulled the whole thing apart (mainly because the wires looked a bit old and perished). The windings are at 90 deg. So no room to play there.
B. My thinking was flawed, comparing induction motors to piston engine cranks only works to a point (if at all). Once I looked at the winding pattern and visualized the mag fields this would make. It all made a lot more sense.

Here is the scope shot with my Microwave caps in parallel for the start coil
Timebase is 5ms per div.
Yellow is at 100v per div and is connected to the main winding/mains power in junction.
Purple is at 200v per div and is connected to the secondary/start/direction winding and cap junction.
Blue is set to 2v per div and is connected to my 1 ohm (10% face value) shunt resistor and a bit of extra wire.
I should have predicted the safety switch wouldn't like the ref clamp on neutral... So ref is from ground which means a bit of extra noise and uncalibrated shunt multiplier. I did get a reading across the resistor with my trms DMM. current is about 2.17A


Here is a shot of the line noise for those that care. At 500mv per div.
I'm happy to ignore this.


So we can see there is a power factor. I agree that its probably not going to make a drastic improvement to my life if I clean it up. I would like to anyway for the practice.

It looks to me like the not so sinusoidal shape to the blue current trace is because the start coil is PF corrected but the main coil is phase shifting slightly more than the start coil.


Here is a simplified motor schematic
R1 is my shunt.
C1 is the start cap.
C2 is my proposed PFC cap.

I'm thinking the result of creating 90 deg voltage lag this way would push the start coil out to 180 deg but it will still be 90 deg from the run coil so now I will have a PF problem on the start coil, but its using heaps less current so likely an improvement.
My theoretical problem/lack of understanding is that it looks like C2 would be shorting the mains, not unlike a 3 inch nail across the lines...
Does it not do that because its part of a tank with the coils?

 

[duke37]

I do not understand why you are so concerned about power factor. Are you taking your supply from a small alternator?
C1 as shown is a run/start capacitor as shown, not a start capacitor.
A power factor correction capacitor is placed across the mains input to cancel the lagging inductive current. It does not give a short.
The reactance Xc is 1/(2*pi*f*C) so you can calculate the capacitive current. Ic = V/Xc.

 

[Mongrel Shark]

Thanks for clearing that up for me.

I'm not especially concerned about the PF on the grinder, I don't even pay for its power. I just want to have a play for the learning/practice. Been waiting for a motor with a PF to present itself to me for ages.... So I was fairly excited when the cap popped....

After getting it going again on a 4uf MOC (4P) bank I fully assembled and bolted back on its stand. Now its not running again. Sigh... Looks like the second hand MOC's might be giving me trouble, or the bleed resistors are causing problems now I have the grinding wheels on. Dont think its the extra load, I did load it in the first test by pressing a block of wood to the spinning shaft. It had a ton of power. Now it wont start and the amps stay up at the starting peak. The start/run coil is showing reduced voltage. The caps test good, 4.3uf, 0.1% loss (@1.25v) and 2ohms esr (seems a little high).
The shaft spins freely by hand. Its got to be the cap bank... Or I knocked a wire loose, but they all sow conductivity in the right places and no shorts I can find.In the process of making another cap bank. This time from MKT caps...