A fuse blows on one phase, or a wire on the line is cut, and your 3 phase submersible motor is left running on two phases. It hums, it heats up, and if nobody switches it off, it burns. This is called single phasing, and it is one of the most common reasons farm motors burn. Here is how a single phasing preventer stops it, and why every BTCO 3 phase starter already has one built in.
What is single phasing?
A 3 phase motor needs all three phases (R, Y and B) to run. Single phasing is when one of the three phases goes missing while the other two are still there. The motor doesn’t stop by itself. It keeps trying to run on two phases, and that is what burns it.
Why one missing phase burns a 3 phase motor
When one phase is gone, the two phases that are left have to carry the whole load. The motor draws much more current than normal, the winding overheats, and the insulation starts to fail. If single phasing happens while the motor is stopped, the motor hums and can’t start, and it overheats even faster. Down in the borewell, you won’t see or hear any of this until the motor is already burnt.
What causes single phasing on a farm line?
- A blown fuse on one phase, at the transformer or in your panel
- A cut, broken or damaged wire on the HT or LT line, often after wind or rain
- A loose or burnt connection at a switch, a connector or a contactor contact
- A fault at the transformer that feeds your line
Signs of single phasing
- The motor hums but does not start
- The motor runs, but less water comes out than usual
- One of the R, Y or B phase lights on the starter is off
- The volts shown for one phase on the starter’s display look wrong
- A fuse keeps blowing on the same phase
What is a single phasing preventer (SPP)?
A single phasing preventer watches all three phases and switches the motor off as soon as one goes missing, before the winding overheats. Many farmers buy it as a separate box and wire it next to their starter. In a BTCO starter it is built into the MPAU unit that controls the starter, so there is nothing extra to buy or wire.
How single phasing protection works in a BTCO starter
- It watches all three phases all the time, and shows the volts for each phase on the display.
- A phase goes missing: the starter switches the motor off straight away, before it overheats.
- It tells you why: the display shows Mains Problem, so you know to check the fuses and the line, not the motor.
- It starts again when it is safe: if the starter’s auto switch is ON, it starts the motor again by itself once all three phases are back, after the auto start delay you set.
- It alerts your phone: with the BTCO GSM Module fitted, you get the trip alert in the BTCO Starters app or by SMS.
See single phasing protection at work on a BTCO starter:
What to do when your starter shows Mains Problem:
Separate SPP box or a starter with SPP built in?
| Separate SPP box | BTCO starter | |
|---|---|---|
| Stops the motor when a phase is missing | Yes | Yes |
| Extra box and wiring in your panel | Yes | No, built in |
| Dry run, overload, earth fault and cable fault protection | Usually no | Yes |
| Voltage fluctuation protection | Depends on the model | Yes, trips on current |
| Starts the motor when power returns | Only with an auto switch | Yes |
| Shows why it tripped | Usually a light only | Message on the display, such as Mains Problem |
| Phone alerts | No | Yes, with the BTCO GSM Module |
Every extra box in a panel adds wires and joints that can come loose, and a loose joint is itself a cause of single phasing. One unit that does everything means fewer things to go wrong.
Which BTCO starters have single phasing protection
| Your motor | BTCO starter | Single phasing protection |
|---|---|---|
| 3 phase, single cable, 3–10 HP | DOL-03 fully automatic DOL starter | Built in |
| 3 phase, double cable, 7.5–30 HP | FASD-03 fully automatic star delta starter | Built in |
| 3 phase, single cable, 3–10 HP, on juice line, 2 phase or 3 phase | DOL-2/3 Combined juice starter | Built in, on 3 phase supply |
| 3 phase, double cable, 7.5–25 HP, on 2 phase or 3 phase | FASD-2/3 Combined star delta starter | Built in, on 3 phase supply |
| You already have a 3 phase DOL or star delta starter | MPAU auto start unit | Built in, HT and LT line |
Single phase motors don’t need single phasing protection. The BTCO Single Phase starter protects them with low and high voltage protection, dry run and overload protection instead.
Already have a starter? Replace the SPP and auto switch with one MPAU
The BTCO MPAU fits 3 phase DOL and star delta starters for motors up to 30 HP. It does the job of a single phasing preventer and an auto switch together, and adds dry run, overload, voltage fluctuation, earth fault and cable fault protection, with a digital display that shows the volts and amps on all three phases.
Single phasing is only one of the faults that burn motors. See every protection your motor needs in our submersible motor protection guide, and read why submersible motors keep burning, and how to stop it. Also useful: what a fully automatic starter does and dry run protection.
Frequently asked questions
What is single phasing in a submersible motor?
Single phasing is when one of the three phases feeding a 3 phase motor goes missing, for example because a fuse has blown or a wire is cut. The motor then tries to run on the two phases that are left.
What does single phasing do to a 3 phase motor?
The motor draws much more current on the two phases that are left, so its winding overheats. If it is not switched off quickly, it burns. If single phasing happens while the motor is stopped, it may hum and fail to start, which also overheats it.
What is a single phasing preventer (SPP)?
It is a device that watches the three phases and switches the motor off when one goes missing, before the motor burns. It can be a separate box fitted next to the starter, or it can be built into the starter, as in every BTCO 3 phase starter.
Do I need a separate single phasing preventer with a BTCO starter?
No. Every BTCO 3 phase starter, the Combined starters and the MPAU have single phasing protection built in. They stop the motor when a phase goes missing and show Mains Problem on the display.
How do I check if my motor has single phasing?
On a BTCO starter, look at the R, Y and B phase lights and the volts for each phase on the display. If a phase is missing, the starter stops the motor and shows Mains Problem. Then check the fuses and the wires on your line, or call your electrician.
Will a starter with single phasing protection work on juice line or 2 phase supply?
A normal 3 phase starter will not run the motor on juice line or 2 phase supply, because a phase is missing. The BTCO Combined starters are made for this: they run 3 phase motors on juice line (up to 10 HP) or 2 phase supply (up to 25 HP with the Combined star delta), and protect against single phasing when you have 3 phase supply.
How much does a starter with single phasing protection cost?
It depends on your motor’s HP and the model you need. Call or WhatsApp us with your motor details and we’ll tell you the right starter and its price.
Stop single phasing from burning your motor
Call or WhatsApp us with your motor’s HP, single or double cable, and your power supply, and we’ll tell you which BTCO starter protects it. Or visit your nearest BTCO Starters dealer. More answers in our motor starter FAQ.
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