3-phase pump, run on single phase supply??

lazza

HI.

I've been looking at some the well pumps installed here, which are giving problems (unfortunately i'm a novice when it comes to understanding pumps). I see recurring theme- I see what appears to be 3-phase pumps, with a capacitor between two of the motor's phases (not starter caps), which appears to be a method to run the 3-phase pump of a single phase supply.

Could this be what pump installers are doing? if so, what are the problems associated?

mike95490

I'm not very sure you can "just add a cap or two" and that will allow a 3ph pump to run,  3 phase motors have no "starting" winding, because of the rotation induced from the 3 phases, it's not needed.   A couple of caps may allow a motor to "run" once it was spun up, but it's much better to use a VFD controller to run 3ph from 1ph.
resources:
http://www.danfoss.com/nr/rdonlyres/8c303cb7-23a2-4b8b-aa92-1efe1f83f927/0/waterintrolesson.pdf
 &
http://www.southlandautomation.com/t-VFD.aspx

After about 2HP, the controllers start getting real expensive

BB.

We talked about this some in one of your threads from last year:

http://forum.solar-electric.com/discussion/23995/single-phase-to-3-phase-converters

Basically, you can have a "motor run" capacitor on the three phase motor--Low starting torque and possibly somewhat less efficient.

Or you can have a "motor start" capacitor--Need a separate manual/automatic switch. Larger capacitor that can provide higher starting torque. Once the three phase motor is running--No capacitor needed. Motor cannot run at rated ~2/3 rated HP (i.e., reduced torque).

http://www.phase-a-matic.com/StaticDescription.htm

-Bill

inetdog

A true three phase pump will have three identical sets of coils and will have three hot terminals not counting any switched terminals for starting.
You can run a true three phase motor off a Variable Frequency Drive (VFD) which outputs three phase. Some, but not all VFDs will operate at a reduced maximum power level when fed from a single phase AC input.
A true three phase motor, once you get it spinning, can continue to run off single phase input, but at a very reduced power capacity, not good for pumping.
An induction motor designed for single phase but with a permanently energized secondary RUN winding with a capacitor is most likely what you have.

lazza

Here's a photo of the control box- it's not great as I didnt think much about it at the time- but maybe you can see from the picture what system is operating?

(The two wires on the left are the L-N 230V input, the next 3 (blue, green/yellow, brown) go to the pump and the following two are the capacitor wires)

BB.

I believe the bottom "block" may be a voltage sensitive relay.

Basically, the relay puts a motor start capacitor in the circuit until the motor is near full speed--The high voltage on the "third leg" rises and turns off the motor start capacitor.

-Bill

inetdog

I think the presence of what appears to be a three phase contactor is a red herring and this is just a single phase capacitor start motor.

lazza

Yes, having looked more into it, it does seem that i was incorrectly informed that these are single phase pumps with a start cap.

I took a picture of another well pump setup yesterday, could someone explain to me what's going on here? It has a 20mfd capacitor, which seems a  little low for a start capacitor ??

BB.

If the motor is only a 0.1 kWatt motor (fractional horse power)--That would be what a 20uF capacitor would be used on... (from what I know).

-Bill

inetdog

BB. said:

If the motor is only a 0.1 kWatt motor (fractional horse power)--That would be what a 20uF capacitor would be used on... (from what I know).

-Bill

Also for a given motor horsepower the start capacitor of a 240V motor will be 1/4 the size (in microfarads) of the capacitor for a 120V motor.

Solar2

Wow guys, a little motor knowledge needed here.

Speaking to the original post, Lazza;  what you observed is called a PSC (Permanently Split Capacitor) motor.
It is considered to be a single phase motor since it runs on single phase.
These are called three wire pumps to separate them from two wire pumps that work differently.
This is not three phase.
They have a main winding connected across the line and a second winding that is smaller running at 30 to 50% the current.
The second, smaller winding has a capacitor in series and they are connected across the same line.
The smaller motor winding is wound 90 degrees from the main winding.
The capacitor shifts the phase of the current going to the smaller winding 90 degrees.
Ninety degrees leading is the only phase shift a capacitor can produce.
These motors, even though called single phase, actually are two phase motors with the motor windings separated 90 degrees.
The capacitor stays in the circuit all the time.
A run type capacitor is necessary.
Run capacitors are usually oil filled and are smaller capacity wise (and physically bigger) from a start capacitor.
PSC motors have low starting torque but run higher efficiency.
They have been taking over the field for the last thirty years.
Practically all refrigeration and most fan motors are now PSC.

For reference consider the Capacitor Motor:
The capacitor motor is the same basic 90 degree two phase motor
but has a switch that disconnects the second, smaller winding with capacitor once the motor gets up to speed.
A so called "start" capacitor is used.
The start capacitor is used only for a fraction of a second upon start up.
An bipolar electrolytic capacitor with more capacity can be used which gives the motor much higher starting torque.
A start capacitor works good for the capacitor motor but will burn up when used as a run capacitor for a PSC motor.

Three phase motors have three windings spaced 120 degrees.
Power flow into a three phase motor is steady.
Power flow into a single phase motor goes from 0 to two times average 120 times a second (60 cycle).
This puts more stress on the motor and windings.
Three phase is significantly more efficient.
Three phase motors use less iron and coper for the same Hp, every thing else being the same.

We are starting to see dedicated purpose 90 degree two phase VFDs come into the market.
They are very expensive.
Low cost, general purpose two phase VFDs are not here yet.