Pump start box - capacitor failures

[Zenon2cubed]

I have a 1/2 HP Grundfos submersible pump a 3-wires, operating at 230 VAC single phase.
Pump Model: 10S05-9
Pump PN: 91545095
Motor PN: 96465606
Control Box PN: 98821631

Lately I've been burning up capacitors. I was testing the wires with a touch meter and it seems that the start winding is staying energized throughout.
I think these pumps have an internal centrifugal switch that disconnects the start winding and when the pump control box relay sees no voltage at the start winding the relay sends power to the main winding.

If my understanding of how this motor / control box works is correct, it seems like the centrifugal switch is broken and not disconnecting power to the start winding.

I've heard reference to solid state control boxes that sense current to the start winding and based upon sensing the current draw of the start winding they will turn off the start winding and power the main running winding.

Is my understanding correct, and what should I do?

Thanks.

 

[Zenon2cubed]

Also, using the touch meter on the main winding wires I see that one has power and the other does not. With 230 VAC I would expect both to be live.

 

[Sponsor]

 

[Reach4]

1. A touch indicator is not of much use.
2. Normally if the pump is off, the pressure switch interrupts both hots. Is that the case with your pressure switch?
3. I think that there is not a centrifugal switch in your pump. Older boxes would use a relay that sensed the current/voltage to detect when it was time to take the start capacitor out of the circuit. Newer boxes may have an electronic module. I would replace that relay or module in the control box.

 

[Zenon2cubed]

1: Noted, thx
2: Correct, the pressure switch mechanically breaks all power to the "control box"
3: There is a relay, but it doesn't seem to be energizing the main winding. I've gone through three control boxes (Grundfos, Red Lion, Franklin) and all are frying capacitors. This pump has been running for about 5 years without issue, so it seems something has changed....

Regardless, can you recommend a different style control box?

 

[Reach4]

Basing comments on Franklin boxes.
The power for the main winding does go thru the relay but only to get sensed. The main winding is always on.

The relay interrupts power to the start winding once the motor is up to speed.

Can you switch to a Franklin or Red Lion control box? While I expect it would work, I have seen nobody recommend that.

Grundfos sellers often say to contact them to get the service manual. I don't know if one would send you the manual or sell you a replacement relay. They want to limit sales of their 3-wire pumps to only pro pump installers.

 

[Zenon2cubed]

The pump is fixed now, here's the details to close the loop.

Measured resistance from all wires to ground as infinite.
Red-Yellow: 18.4 ohms
Red-Black: 23 ohms
Yellow-Black: 5.3 ohms

Run current measured with a clamp-on ammeter:
Yellow: 10 amps
Red: 2 amps
Black: 6 amps

The Red is not supposed to have any current in operation, so I ran wires into the control box to measure the L1-L2 voltage sag in operation.
While running L1-L2: 112.8 VAC
No load L1-L2: 120.4 VAC

These readings were the smoking gun.

The pump has a 240 VAC motor and L2 was connected to neutral.
The electrician screwed up when the pump was initially installed.

I opened up the main feeder box and moved the L2 wire from neutral to L2 and then measured in operation:
Red: 0 amps
L1-L2: 240 VAC

The Red wire (start winding) blips to some current for an instant and drops back to zero.

The pump always seemed lazy getting up to pressure and now it gets there in a hurry, roughly 1/2 the time as before.
The pump is currently wired with a Grundfos control box.

Thanks for your help Reach4.

 

[Reach4]

Computing as described in https://terrylove.com/forums/index....-3-wire-pump-resistances-with-unknowns.67706/

I get

Rstart Rrun Rwire
17.7 4.6 0.4

 

[Fitter30]

Which capacitor is going bad? The start is in black plastic and run is in metal. The potential relay are volt sensitive that engage and disengage the start winding at a specific voltage. When measuring motor winding the three wires were disconnected? Installation manual towards the bottom of page under links. Your motor
MS 402 page 19 as the resistance of winding

https://www.sheret.com/account/catalog/product/131702?srsltid=AfmBOopM1dx7oBxm4ZwWp8V4bX9qWQRic6bmrEEhrUT5Y_pzVuMh2QPT

 

[Valveman]

Lately I've been burning up capacitors.

On rare occasions lightning will hit the start relay and take out the coil. Then the relay can't turn off the start capacitor and the pump goes off on overload. But 999 times out of 1000, start capacitors are destroyed from starting to much. The only time a start capacitor is even in the circuit is on start up. To keep from burning up starting capacitors, it really is as simple as eliminating pump cycling.

 

[Zenon2cubed]

Thanks for the great info Fitter30 and Valveman!
The plastic start capacitor was releasing the green smoke. There is no run capacitor.

The relay is OK, my problem was that a 240VAC pump was being run on 120VAC! :-o
A professional electrician rewired the whole building specifically to make 240VAC available for the pump, but when he made the final termination he connected one leg of the pump power feed to neutral.

With 120VAC the pump would run and reach full pressure very slowly, I confirmed with a clamp-on ammeter that the start winding was energized whenever the pump was running, and this continuous duty was causing the capacitor to fail.
Once I opened the main junction box and reconnected the pump feed from L1-N to L1-L2 the pump was receiving 240VAC.

With 240VAC fed to a 240VAC motor the start capacitor current blips so instantaneously that I can't even get a good reading on my clamp-on ammeter. The pump also comes up to pressure very quickly now.

I hope this thread can document my observations and save someone aggravation in the future.