Editors Note: This post is one of the most frequently viewed on our website. This can be attributed to the high frequency of worn and damaged wiring. With proper electrical maintenance and pump startup procedures, this problem can be 100% eliminated.
3-Phase Motors used on Molten Metal Pumps
Nearly all the electric motors we ship with our pumps are 3-phase induction type motors. These motors provide:
- A wide range of power options from 0.5 hp (0.37kw) to 30 hp (22kw.)
- Easily controlled by an variable frequency drive (VFD) often called an inverter.
- Excellent torque and power efficiency ratings.
These motors must be wired correctly to ensure proper pump operation. Incorrect wiring may cause the pump to turn backwards resulting in one or more of the following outcomes:
- Unthreading the Impeller – This is a minor issue provided the condition is noticed immediately and the pump is de-energized.
- Broken Shaft – The impeller provides bearing supports at the end of the shaft. Once the impeller unthreads, in the absence of bearings the shaft may spin out of balance and break.
- Broken OR Cracked Pump Base – The out of balance shaft can break the base of the pump as well.
WARNING! ALWAYS VERIFY THE MOTOR TURNS THE CORRECT DIRECTION ACCORDING TO THE MANUAL BEFORE INSTALLING SHAFTS, IMPELLERS, AND INTRODUCING THE SYSTEM INTO THE PUMP WELL. NEVER START THE PUMP DRY!
Common Wiring Problems
Recently, a customer experienced a pump failure. Upon investigation, it was determined that one of the three phases of the VFD output connection block had burned. This was most likely due to a loose connection that resulted in resistive heating and ultimately burning of the connector block. With only two phases connected, the motor eventually turned in the wrong direction. In this case, a motor that was properly wired at installation, reversed direction with the connection block failure.
Important Note: When a 3-phase motor with only two (2) connected phases is energized, the direction for rotation is indeterminate unless an outside force is acting upon the motor. In other words, you could spin the motor in either direction and when energized it will continue to spin in that direction. In the case of a molten metal pump, the back-flow through the pump can cause the impeller to turn in the “backwards” direction setting up a condition for failure.
VFD Status Can Help Diagnose Motor Problems
As an example, one common VFD panel produces an OC1 error for a number of causes. Below is the table included with the VFD manual detailing the possible causes of the error. All of these errors can be diagnosed by the facilities electrical engineer with the use of a multimeter.
Recommendation to Reduce Motor Related Problem
- Regularly inspect the pump wiring and VFD connection. Replace any worn or weakened components.
- Check the VFD for error codes. Many VFDs display error codes when a motor failure or motor wiring failure is detected.
- When installing a new pump, motor, or VFD; ensure the motor turns the correct direction (according to the pump manual) before installing shafts and impellers.
- Verify that the motor cooling system is functional and that the intake filter is clean and installed.