Ou3 Over Voltage in Nidec & Control Techniques NE200 Drive

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Getting the Ou3 fault on your Nidec/Control Techniques NE200 VFD? Learn why overvoltage happens at constant speed and how to fix it with power checks and braking units.

Ou3 Over Voltage in Nidec & Control Techniques NE200 Drive

 

If you are using a Nidec or Control Techniques NE200 series drive, you may encounter the Ou3 fault code. While most overvoltage trips happen when a motor is slowing down, the Ou3 code is unique because it indicates an Over Voltage condition while running at a constant speed.

This can be particularly frustrating because it interrupts your process during normal operation. This fault means the internal DC bus voltage has exceeded the drive's safety threshold. Here is a guide to understanding the causes and how to get your machine back in production.

What Does Ou3 Mean?

In the NE200 drive, "Ou" stands for Over Voltage. The "3" specifies the timing of the fault:

  • Ou1: Overvoltage during acceleration.
  • Ou2: Overvoltage during deceleration.
  • Ou3: Overvoltage during constant speed.

Primary Causes of Ou3

When the motor is not changing speed, there are two main reasons why the voltage inside the drive would suddenly spike:

1. Abnormal Power Supply

Since the motor isn't slowing down, the excess energy is likely coming from the "front door"—your utility power. If your facility experiences a voltage surge, or if you are located near large machinery that switches on and off, the incoming AC voltage can rise. The drive's rectifier converts this high AC into an even higher DC voltage, triggering the Ou3 trip.

2. Large Load Inertia (Overhauling Loads)

This is a common mechanical cause. Even at a constant speed, a load with high inertia can "overhaul" or "push" the motor. For example:

  • A downhill conveyor where gravity is pulling the belt faster than the motor wants to go.
  • A large fan being moved by a strong external draft or wind.
  • An unbalanced centrifuge or reciprocating machine.

In these cases, the motor acts as a generator, sending electricity back into the drive (regeneration), causing the DC bus voltage to climb until it hits the Ou3 limit.

Step-by-Step Troubleshooting & Remedies

1. Inspect the Input Power Source

Before assuming the drive is at fault, verify the quality of your power supply:

  • Use a multimeter to measure the AC input voltage (terminals R, S, T). Ensure it stays within the drive's rated tolerance (typically ±10% or 15%).
  • Check for transient spikes. If your power is "dirty" or unstable, you may need to install an AC Line Reactor to smooth out the incoming voltage.
  • Verify if any large capacitor banks in your factory are switching on/off, as this is a frequent cause of constant-speed overvoltage.

2. Add Suitable Braking Devices

If the fault is caused by mechanical inertia (regeneration), the energy needs a place to go. Since the DC bus capacitors can only hold so much, you must "burn off" the excess energy.

  • Braking Resistor: Install a braking resistor on the (+) and (PB) terminals (check your specific NE200 frame size for the correct terminal labels). This allows the drive to dump excess DC voltage into the resistor as heat.
  • Verify Braking Parameters: Once a resistor is installed, ensure that the braking functions are enabled in the drive settings so it knows how to use the hardware.

3. Adjust the Overvoltage Suppression Settings

The NE200 has internal logic to handle voltage rises. Check your manual for parameters related to Overvoltage Stall Prevention. Enabling this allows the drive to automatically adjust its output frequency slightly to "ride through" the voltage surge without tripping, provided the surge isn't too severe.

Summary Checklist

Checkpoint Required Action
Input Terminals Measure R-S-T voltage for spikes.
Mechanical Load Check if the load is "pushing" the motor (gravity/wind).
Hardware Addition Install a braking resistor and/or braking unit.
Electrical Shielding Ensure the drive is properly grounded to prevent noise interference.

 

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