Three Phase Inverter Motors

Ben Kemink

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At the end of this training session you will have learned;

1. There are different three-phase measurements.

2. Pulse Width Modulation.3. Inverter Driven Applications mess-up with

Period Time T.

Three Phase Inverter Motor

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Three Phase Systems

A One-Phase System, also called a Single-Phase (SP) System works very much similar to a DC network and can be considered so far at any particular moment in time.

Electrical Systems comprising two or more phases are also called Multi-Phase or Poly-Phase (PP) systems.

The voltage amplitudes and frequencies in the individual system circuits are the same, but they have definite phase differences.

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Why Poly-Phase?

The technical complications and economical investments are small compared to the enormous advantages of a Poly-Phase system:

1. Same size generator or motor, higher output.

2. Same energy transmission less copper required.

3. PP motors have uniform torque, SP have pulsating torque *).

4. PP motors are self-starting, SP are not *).

5. Polyphase generators work fine parallel, SP not.

*) Single Phase commutator motors are exception.

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Why 3 Phase and not 7?

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Three Phase is ideal

Source: Advanced Electrical Technology by H.Cotton

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Three-Phase Three-Wire

Total Power calculation based on Two-Watt-meter method

3 wires – 1 = 2 power meters required

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Three-Phase Four-Wire

4 wires - 1 = 3 Watt meters are required

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Three-Voltage Three-Current

Calculation based on Two-Watt meter method.

Third Phase measurements are added.

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Three-Phase Three-Wire with PT & CT

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Introduction

What is Three phase?

Three phase = three signals Phase shift: 1200

Figure 1: Phase shift 1200

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Pulse Width Modulation of a Sine Wave

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Demonstration circuit

Demonstration circuit consists of:

Frequency inverter; 3-Phase Electromotor; Power meter;

Figure 2: Demonstration circuit

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Demonstration circuit

Freq. inverter

Step 1: AC → DC Step 2: DC → 3-Phase

Control unit

T

L1(mains)

50 Hz

T

0 Hz →

Figure 3: Block diagram 1phase inverter

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Demonstration circuit

AC → DC Performed by rectifier; DC-signal is

smoothened by capacitor;

In 3phase mains the output of the rectifiers is connected to the same capacitor;

t →

Example: Single phase rectifying

Freq. inverter Step 1: rectifying

Figure 4: rectifier

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Demonstration circuit

Freq. inverter Step 2: regeneration of 3-Phase-signals

Six switches Generate periodic pulse-trains; Insulated Gate Bipolair Transistor (IGBT); High frequency switches;

U

tt

Figure 5: Inverter step 2: regeneration of AC-signals

New Period T

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Demonstration circuit

Freq. inverter

YEF-HQ Test Motor

Figure 6: Output of Freq. Inverter: 1-phase, current (green) / voltage (yellow)

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Running the motor

Direction Direction (CCW or CW)

Can be changed by manipulating the pulses

CW

CCW

U

V

W

U

V

W

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Running the motor

Speed

Depends on the switching frequency

Slow

Two times faster

U

V

W

U

V

W

T

T

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Running the motor

Faster 1s time window

Speed:voltage (yellow) and current (green)

Slow 1s time window

Figure 7: Output of Freq. Inverter at different speeds

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Now you have learned;

1. There are different three-phase measurements.

2. What Pulse Width Modulation is.3. Why Inverters mess-up with Period Time T.

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Thank you for your attention