Apart from the engineering units, electrical engineers also use per-unit to measure quantities in physics. A per-unit system expresses system quantities as fractions of a defined base unit quantity. The per-unit system scales the actual value of a physical quantity, such as current, voltage, and speed, with a selected constant of the same measuring unit. The ratio of the actual value to the constant is the per-unit value of the physical quantity. The selected constant, also known as base value, has the same measuring unit as the actual value.

The following equation explains the relationship between a per-unit value and a base value.

Per-unit Value = Real Value (any unit) Base Value (same unit as the Real Value)

For example, suppose you have two voltages, U1=99 KV and U2=110 KV. When selecting 110 KV as the base value of voltages, you can express the per-unit value of U1 and U2 as follows.

U 1 = U 1 U 2 = 99 110 = 0 . 9

U 2 = U 2 U 2 = 110 110 = 1 . 0

The results show that the real value of U1 is 0.9 times of the base value, while the real value of U2 is the same as the base value. In other words, the per-unit value of U1 is 0.9 and the per-unit value of U2 is 1.

The per-unit system is ideal for fixed-point implementation. Per-unit helps to make the fixed-point model usable for various motors. All quantities are multiples of the base value that you select.

Troubleshooting Improper Base Value Error

When you use the Electric Motor Simulation VIs to generate FPGA data for electric motor simulation, an error occurs if improper base values lead to overflow. Complete the following steps to resolve the error.

  1. Keep the value of speed base as default.
  2. Gradually increase the value of current base. Keep the per-unit value of the maximum current within the range from 0.1 to 10. For example, suppose the maximum current of an electric motor is 300 A, you can select a value in the range from 30 A to 3000 A as the current base.
  3. Gradually increase the value of voltage base. Keep the per-unit value of the maximum voltage within the range from 0.1 to 10.
  4. Repeat steps 2 and 3 until the error stops occurring.