Protecting AC Drives From Overcurrent or Overload Situations

Another AC Drive gone up in smoke. A common problem we see happen more than it should. We’re going to address one possible cause for why this keeps happening and what you can do to make sure it never happens again.

We’ll be referencing the Allen-Bradley PowerFlex AC Adjustable Frequency Drive throughout, but the reasoning and corrective actions recommended apply to all types of AC Drives.

Common Symptoms

The smoke is likely from the bus capacitors. The surge you experienced likely charged the capacitors beyond their maximum rated voltage and blew their safety seals to shreds. Because of this, it is also likely that the power module failed or is compromised due to the bus capacitors shorting out.

Other possible symptoms include smoking resistors in the regenerative circuit or the power module near the base of the unit had a catastrophic failure.

Standard Protection

Now that we know why this happened, let’s examine the most common ways we can protect the drive from overcurrent or overload situations, so it doesn’t happen again.

1. Instantaneous Overcurrent Trip:
   Most OEMs, especially for newer AC Drives, are kind enough to provide a built-in safety mechanism designed to instantaneously trip or fault the drive if the output current exceeds the allowed value. The value is fixed by hardware and is typically 250% of drive rated amps.
   
   This is not the solution though. Why? Because it’s only letting you know something is wrong, not actually fixing the problem. Yes, the F12 “HW Overcurrent” fault is a good starting point; however, it shouldn’t be the only thing in place to protect your AC drives.
2. Software Instantaneous Trip:
   The Software Instantaneous Trip is for when peak currents do not reach the defined overcurrent value but are sustained long enough and high enough to damage certain drive components. Capacitors and resistors are especially vulnerable here.
   
   If this situation occurs, the drives protection scheme will cause an F36 “SW Overcurrent” fault. The point at which this fault occurs is fixed and stored in drive memory.

Corrective Actions

When it Comes to Voltage

Remember the rated voltage for both the motor and drive should be what’s listed on their respective nameplate. You can find this setting by navigating to…

1. Once at desired group press the enter key.
2. Navigate to the correct parameter using the up or down arrow then press enter.
3. Confirm the voltage listed matches what’s on the motor nameplate.
4. If not, use the up and down arrows to edit this value.
5. Press enter to confirm and press esc key to return to menu.

Check the voltage of the power input leads R => S, R=>T, and S=>T. The measurements should be approximately the rated voltage of the drive and balanced within a few volts.

Don’t forget Ohm’s law (V = I x R) is at work here. One thing off with any one of the variables will have an impact on the others because of the inter-connectivity and inter-dependency of this formula.

When it Comes to Current

An excessive amount of load will trigger “Motor Overload” or “Drive Overload.” You can fix this by continuing to reduce load until the drive output current doesn’t exceed the allowed current listed on the motor nameplate.

Every industrial motor or AC Drive you purchase is designed to operate above normal current levels. This is especially useful during start-up when a large amount of torque must be generated aka break-away torque. However, this is only allowed for a brief moment of time and is never meant to become the operational norm.

But what if you’re absolutely sure the motor and drive selected is capable of handling that amount of load and you still receive this error? You should start by looking for any mechanical issues.

Bonus Information

Overcurrent is far more common but it’s just as easy to not have enough current given the application. Simply put, the variable frequency drive (VFD) must have ample current capability for the motor so that the motor can produce the required torque for the load. Otherwise, expect an “Undercurrent” fault to be headed your way.

Parting Words

Performing these checks monthly and truly taking corrective action before manually clearing the faults goes a long way in building a healthy operation.

Recording drive setup, saving parameters, keeping track of fault/warning queues, and recording the part numbers to any replaced boards during the initial information gathering stage is highly recommended as well. Plus, getting this information upfront makes discussions with other maintenance personal and 3rd party repair or service vendors so much easier.