Motor - Power Flow Tab

DC_Motor_Power_Flow

Figure 1: Power Flow Tab of the Motor Data Dialog Box

Option Descriptions
Option Description

Load Model

Enables you to select the motor kVA from the Specified data or from SCADA (Supervisory Control and Data Acquisition) data. SCADA data can be read in by clicking File > Import.
Motor Specifications

Motor kVA

Calculated from motor specifications. This is for reference only and cannot be changed except by new specifications.

Load Type

Motors can be modeled for the power flow solution in several different ways.

  • Constant kVA - This is the most common model. It is conservative, and results in slightly lower voltage values than would be measured on an actual system.
  • Constant Current - This model is generally not used in motor modeling. It more closely matches an induction motor's characteristics in the reactive component than other models, but is technically incorrect because kW is relatively constant throughout the voltage range for an induction motor.
  • Constant Impedance - This model is used for starting induction and synchronous machines, and closely matches motor characteristics during low voltages.
  • kW + j Current - This model is a combination of the above models and more closely matches actual motor characteristics within normal operating voltages.

Scaling Factor

Provides an easy way of adjusting the total motor load used in determining power flows. By changing the scaling factor, the actual Hp (total connected value) entered in the Hp field can remain static. This reduces modeling errors and eliminates multiple databases for different contingencies.

SCADA Model

SCADA data is derived from real time, or metered data, and converted to an ASCII format which can be read into EasyPower. SCADA data is read in as a 100% scaling factor load. The load value is multiplied by the user-defined scaling factor. This provides a way to adjust SCADA loads to form new cases.

kW

The kW value as read in from the SCADA ASCII file.

kVAR

The kVAR value as read in from the SCADA ASCII file.

Load Type

SCADA data can be modeled in the power flow solution in several different ways. SCADA load type is set in the ASCII file, but you can change it.

  • Constant kVA - This is the most common model. It is conservative, and results in slightly lower voltage values than would be measured on an actual system.
  • Constant Current - This model is generally not used in motor modeling. It more closely matches an induction motor's characteristics in the reactive component than other models, but is technically incorrect because kW is relatively constant throughout the voltage range for an induction motor.
  • Constant Impedance - This model is used for starting induction and synchronous machines, and closely matches motor characteristics during low voltages.
  • kW + j Current - This model is a combination of the above models and more closely matches actual motor characteristics within normal operating voltages.

Scaling Factor

Provides an easy way of adjusting the total SCADA load used in determining power flows. By changing the scaling factor, the actual kW +j kVAR read in from the ASCII file remains static; however, the load used in the power flow is adjusted by this factor.

Motor Load

Load Speed Exponent

When you run power flow calculations, the motor load is scaled based on the frequency of the AFD. Speed is proportional to frequency. The simplified load model used is:

Output Power = (Rated Power) * (AFD frequency / System Frequency) ^ x.

Where x is the Load Speed Exponent.

If you have a machine in which the load (power) is independent of the speed, then set the exponent to 0. If machine load (power) is proportional to speed, then set the exponent to 1. Pump loads are closer to speed squared model, so set the exponent to 2. This is the default.

Example:

A 100HP motor rated at 60Hz is fed by AFD at 30Hz. If the Load Speed Exponent is 2, the power output at 30Hz would be:

100 * ( 30 / 60 ) ^ 2 = 25HP.

DC Motor Power Flow Tab

Figure 2: DC Motor Data - Power Flow

Option Description

Scaling Factor

Ratio of actual load to connected (rated) load in percent. This is used in power flow calculations.

Load Type

Choose one of the following models:

  • kW: The kW consumed remains constant even when terminal voltage changes.
  • Current: Constant current is drawn by the motor even when terminal voltage changes.
  • Impedance: The equivalent resistance for steady state is constant, so the kW and current will vary based on the terminal voltage.

More Information