How to Bypass a VFD and Run a Three-Phase Motor Using DOL Starter?
Variable Frequency Drives (VFDs) have become a cornerstone technology in modern industrial applications, providing precise control over motor speeds and energy efficiency. However, there are situations where a motor might need to operate without the VFD, necessitating the use of a Direct-On-Line (DOL) starter for bypassing the VFD. In this article, we will show how to bypass a VFD and run the Motor through DOL starter using switches and 8-PIN DPDT power relay with the help of power and control wiring diagrams.
VFDs (Variable Frequency Drives) are electronic devices that control the speed and torque of an AC induction motor by adjusting the frequency and voltage of the power supplied to the motor. This precise control helps in reducing energy consumption, improving process control, and extending motor lifespan.
Direct-On-Line (DOL) Starter
DOL (Direct On-Line Starters) are widely used for direct starting of motors. They consist of a contactor and overload relay that provide a simple and cost-effective way to start a motor at its full load current. However, starting motors directly can lead to higher inrush currents and mechanical stresses.
The Need for VFD Bypass with DOL Starter
There are instances when VFDs need to be bypassed, such as during VFD maintenance or in emergencies. During these times, a motor can be connected directly to the power supply using a DOL starter with a VFD bypass feature, allowing it to operate at full speed temporarily.
- Three-Phase Motor
- 3-P MCCB
- 2-P MCB
- Variable Frequency Drive (VFD)
- DOL Starter (based on Contactor & O/L Relay)
- 8-PIN DPDT Power Relay
- Single-Phase & Three Phase Supply
- Selector Switch for manual ON/OFF and DOL/VFD Operation
- Start and Stop pushbuttons
- Wires & Cables
Wiring of VFD Bypass DOL Starter
- Connect the three phase motor’s terminals (U1,V1,W1) via DOL starter (contactor and thermal overload relay) from 3-P MCCB to the 3-Phase 415V AC.
- Connect the VFD Input Contactor and Output Contactor to the VFD and output phases of O/L relay respectively as shown in power diagram.
Wire the control circuit as shown in below figures to bypass a VFD and run and control the motor via DOL starter.
VFD Bypass Dol Starter using Switches
VFD Bypass DOL Starter using 8-PIN Relay
Operating Sequence of bypassing VFD for DOL Starter
As the motor runs and controlled automatically by VFD. It can be manually run and control via DOL starter bypassing the VFD manually as follow.
- Operator initiates the start pushbutton (After selecting the DOL mode in selector switch).
- The control circuit activates the contactor and relay, connecting the motor directly to the power supply via DOL starter.
- The motor starts at full speed and runs without VFD control.
- When the stop push button is pressed (or moving the selector switch to the OFF position), the control circuit deactivates the contactor, disconnecting the motor from the power supply.
- Again, the motor can be run and operate by VFD if the operator moved the the selector switch to the VFD position.
Benefits and Limitations:
- Benefits: VFD bypass DOL starters offer a seamless transition between VFD-controlled and direct motor operation. They allow maintenance or emergency operation without requiring a separate bypass circuit.
- Limitations: Direct starting can result in higher inrush currents and mechanical stresses on the motor and driven load. Additionally, energy efficiency is compromised during direct operation.
VFD bypass DOL starters find applications in industries like manufacturing, pumping, HVAC, and conveyor systems. They are particularly useful in scenarios where continuous operation is critical.
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