MOTORS FOR PUMPS
What is necessary to know about electric motors?
Motor nameplate data
The rated shaft power output at the rated voltage, Rated Power Output:- current and frequency. Units are kW for IEC motors, and HP for NEMA motors.
- Service factor: Service factor is an indication of how much overload a motor can take. Motors should not be operated in the service factor continuously.
- Full load efficiency: This is often given as “nominal” or “guaranteed minimum” and provides an indication of motor efficiency.
Factors that can affect motor performance
- Frequent starts and stops Can cause premature motor failure
- Power Quality (i.e. harmonics)
- Voltage - unbalance or under/over voltage Creates additional heat Increases motor internal losses. Motor is de-rated for high voltage unbalance
- Environmental conditions Poor cooling due to high ambient temperatures Partially clogged motor vents. Dirty/wet application
- Operating in the service factor area. NEMA recommends that motors should be de-rated when operating in the service factor area
For a 100 kW motor with a voltage unbalance of 3.5%, would result in the motor being derated to 85 kW.
How to calculate voltage unbalance?
Maximum number of motor starts and stops
Effect of an oversized motor
The efficiency does not change much between these motor sizes within the recommended load range
The difference in power consumption for oversized motors is minimum
It is a common misconception that you can save energy by downsizing a motor. As can be seen in this diagram that is generally not true at all. Electric motors consume power which is defined by the load applied to the shaft of an electric motor. In our case, this is the pump!
Motor improvements
Determine existing condition of motors
•Electrical measurements
•Motor circuit analysis, infrared thermography
•Efficiency
Plan ahead for repair/replace options
•Use MotorMaster software to evaluate savings by upgrading to a premium efficiency motor
•Application of variable speed drive will impact new motor type
Evaluate how pump upgrades may affect the motor
•Opportunity to resize the motor
•Effect of variable speed drive
•Impact on motor service factor
Determine how motor upgrades will impact pump performance
•The Higher rpm of premium efficiency motors will increase pump capacity and power
•The absorbed power is roughly proportional to the cube of the speed!!!
•A 2% speed increase could lead to 8% higher power usage.