Single Phasing in Electric Motors
I once had a customer ask me for some advice to help them come up with some ideas as to why their pump would trip as soon as it was lowered into slurry.
I asked questions such as:
- Have you checked for proper rotation?
- Did you check for pluggage in the discharge line?
After verifying these things, the pump still did not perform. It would trip on overloads as soon as slurry was introduced. I then asked for amp draws but unfortunately got no response.
By chance, I was due onsite at that particular plant the next week so I decided to investigate for myself what may be causing this particular pump to trip. After investigating, we found that the pump was single phasing. This would trip the pump once it is put under a load. This is a common problem so let’s take a look at what single phasing is and some of the causes associated.
Single phasing simply means that one of the line connections of a motor is not connected, resulting in the motor running on a single phase. A single-phase condition subjects the motor to an excessive voltage imbalance, often meaning high currents and motor heating. Induction motors designed to run from a single phase use a start or run capacitor to assure that the motor starts and runs in the correct direction. 3-phase motors rely on the phase sequence of the power supply to assure they start and determine their rotation, so with only two lines connected, it is uncertain if a motor will start and what direction it will rotate if it does.
If a single-phasing motor does start, it may run under light loading conditions without tripping simple overload breakers or fuses. When the motor’s load is increased, the current may increase well beyond the motors full load current or the rotor may lock altogether. Both will trip correctly sized motor protection well before the pump achieves the performance you would expect.
Some of the causes of single phasing are:
- Damaged motor starter contacts
- Open fuse or open pole in circuit breaker
- Open cable caused by overheated or damaged connecting lug.
- Open connection in wiring.
- Open winding in motor
If you feel that you may be operating under single phase conditions make sure to start by checking the amperage across all 3 phases. You will notice a significant difference in amperage on one of the 3 legs if this is the case. Always have a qualified electrician investigate all conditions and connections.
Motors can be expensive. It is best to provide proper safety precautions to protect your motor against single phase conditions.