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

Derating factor of electric motor as a function of voltage unbalance

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
Efficiency of electric motors

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.