External Operation [EXT] of a Mitsubishi D700 VFD to Control a 3phase Motor

Realise 3 speed operation of a motor using a VFD


External Operation of a 3phase Motor Using Mitsubishi D700 Inverter The Mitsubishi D700 Inverter is a very handy variable frequency drive for the electrical engineer. This model, though discontinued has been used in a variety of connections for different types of loads. In today’s experiment, I want to show you how to control a small 3phase motor using the D700 to make the motor run in 3 different speeds, and change direction. All this control shall be done in EXT mode on the variable frequency drive. For those new to the variable frequency drive, you can have a look at this introductory piece on variable frequency drives them come back here. In EXT mode, it simply means that the variable frequency drive is externally controlled via some switches.

Hardware and their States:

  • 3phase 415V asynchronous motor, star connected
  • Mitsubishi D700 VFD
  • Voltmeter and Ammeter(Digital/Analogue)
  • AC 415V power source


  • Connect your motor to power source and add the ammeter and voltmeter as per the diagram below. Take care when working with 3phase voltage
  • Connect your variable frequency drive as per the diagram below. If you are using another type apart from the D700, please check its manual on how to make the connections.
  • Set Pr. 79 =“3” (EXT/PU combined operation model)
  • Use the D700’s operation panel to set the correct frequency.
  • Switch terminal STF(STR)-SD on to give a start command.
  • Now that the EXT PU LEDs are on set the following parameters: Pr.79=3; Pr.7=3.0s; Pr.8=3.0s.
  • Turn the dial to set the frequency to 30Mhz
  • Connect the 0V terminal on SD terminal. Operation
  • The STF, STR are Start Forward and Start Reverse switches
  • The RH,RM and RL are Run High, Run Middle and Run Low are speed selection switches
  • Since the Inverter has the EXT/PU combined start the motor using the ON/Start button on the Inverter panel.
  • The motor will not run at first, but when you toggle RL the motor will start in slow frequency/speed then when you toggle the RM switch it will pick up speed.Finally toggle RH and the motor will run at the maximum speed/frequency setting available. The values of the different speeds depend on what you set in the beginning for the parameter.
  • The motor can also change direction by toggling the STF and STR switches respectively. Note that if both switches are ON the motor will not run, instead come to a halt.



PU/EXT mode comes in handy especially when you want to retain control of the variable frequency drive from the front panel and also from other switches which may be in a different location.

That’s all for now, on the next post I will describe how to completely control the variable frequency drive externally without operating the variable frequency drive from the front panel. Later on I will cover about how to control the variable frequency drive from the PLC.