Two-Speed, One-Direction, 3-Phase Motor Power & Control Diagrams

How to Control a Dual/Tap-Wound 3-Phase Motor for 2-Speed and 1-Direction using Dahlander Connection?

Three-phase motors are widely used in industrial applications due to their efficiency and robustness. Often, there’s a need for controlling the speed of these motors while maintaining a single direction of rotation. This wiring tutorial shows how to control a tap-wound 3-phase motor for two-speed and one-direction using Dahlander connection for contactors with the help of schematic, power, control and wiring diagrams.

Two-Speed Motor Control:

Two-speed control of a three-phase motor involves running the motor at two different speeds, often referred to as high speed (HS) and low speed (LS). This is achieved by connecting the dual or tap wound motor’s winding in a specific configuration (Dahlander connection) to adjust the magnetic field and, consequently, the motor’s speed.

The two-speed control is usually accomplished by changing the arrangement of the motor windings from the main to star (Y) connection and delta (Δ) connection, altering the impedance and hence the speed.

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Components Needed

1. Three-Phase Dual / Tap-Wound Motor
2. 3 Nos. of Contactor
3. Thermal Overload Relay
4. 3-P MCCB
5. 2-P MCB
6. 3 Nos. of pushbuttons
7. Single-Phase & Three Phase Supply
8. Wires & Cables

Wiring & Control of Dual/Tap Wound Motor for 2-Speed Operation

At the initial stage, all contactors are de-energized.

• When K1 is energized, the motor runs at low speed.
• When K2 and K3 are energized, the motor runs at high speed.

Power Diagram

Schematic Wiring Diagram

Control Wiring Diagram

Control Circuit Diagram

Construction of the Power & Control Circuit:

To achieve two-speed, one-direction control of a three-phase motor using contactors, follow these steps:

Power & Control Circuit Diagram

Draw a schematic diagram and wire the power and control circuit as shown in the figures below. This circuit will consist of a power supply, a low speed contactor (K1), high-speed contactors (K2 and K3), a thermal overload relay (OLR), and the three-phase tap wound motor.

Low-Speed Contactor (K1):

The main contactor for low speed (K1) connects the motor to the power supply. When energized, it completes the circuit and allows current to flow through the motor windings. When the K1 is energized, the motor runs at low speed.

The K1 is controlled by a start pushbutton. If you release the pushbutton after pressing it, the contactor K1 is still energized. Thanks to the interlocking via auxiliary contacts. , 

High-Speed Contactor (K2 & K3):

 The high-speed contactor (HSC) and low-speed contactor (LSC) are connected in parallel with each other. These contactors control the motor’s speed by switching between the star (Y) and delta (Δ) configurations of the motor windings. When the K2 and K3 are energized, the motor runs at high speed.

Thermal Overload Relay (OLR) The thermal overload relay protects the motor from overheating. It measures the motor’s current and trips if the current exceeds a predefined value, preventing motor damage. It’s wired in series with the K1 control circuit.

Control Logic:

Design a control logic that ensures the K1 is energized only K2 and K3 are de-energized by interlocking, preventing both contactors from being energized simultaneously. This prevents a short circuit in the motor windings due to incorrect wiring.

Control Inputs Connect start/stop. LOW and HIGH pushbuttons to the control circuit to initiate and halt motor operation. Connect these buttons in series with the K1 and O/L relay coil.

Working & Operation of Two-Speed Motor Control

This control circuit includes three contactors and three push buttons (Stop, S1, and S2). Pushbuttons S1 and S2 each have two contact blocks (one NO and one NC). Let’s analyze the circuit when we press S1 for slow speed.

Low Speed

• The contacts in parallel to K1 change their state from open to closed.
• Simultaneously, the S1 and K1 on the right side change their state from closed to open.
• As a result, only K1 is energized, and the motor runs at low speed.
• If the pushbutton is released by the operator, the K1 contactor will remain energized due to interlocking via auxiliary contacts.

High Speed

• When S2 is pressed, the contacts of K2 close from an open state, energizing the K2 contactor.
• The auxiliary contacts close and energize the K3 contactor.
• This happens because the auxiliaries of K2 and K3 contactors are energized.
• Consequently, the contacts change their state from closed to open on the left side.
• If the operator releases the S2 pushbutton, the motor continues to run at high speed.

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