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  • October 2013
  • 8 Posts

Can You Run a Servo Motor with a Variable Frequency Drive?

Servo System

Testing Servo Motors and Drives

Can you run a servo motor with a Variable Frequency Drive? First, lets look at how a servo amplifier works with a servo motor.

A servo amplifier actually fires DC voltage into the three phases of a servo motor. Each time a phase is fired, it is completely dependent upon rotor positioning reference to the stator windings to be fired. In other words, you need commutation to run a servo motor. Most servo motors will have an encoder that is coupled to the rotor and aligned to give rotor positioning reference to stator windings.

We have actually ran a Yaskawa servo motor open-loop on a variable frequency drive (VFD) The results were not good. You see, no matter how we tuned the drive, the end result was a very inefficient motor that drew high amps and had no torque.  So the answer is:  yes, but not very well.

Top 5 Strategies to Keep Your Machinery Up and Running

Servo System Equipment

Find out how to keep your servo equipment up and running!

(1) Keep it Clean!!!

The most important strategy to keep your servo motors and industrial electronics running is keeping your servo motors and industrial electronics clean.

The filters, fans and heatsinks are the areas that require the most cleaning, because the filters, fans, and heatsinks are the most common fail points in your automation equipment due to the areas becoming clogged with oil mist and dust.

The heatsink, which its purpose is to draw heat away from the servo automation equipment, can cause overheating if it becomes clogged. When the automation equipment overheats, this can cause stress on the component and machinery, and eventually, the IGBT will blow up.

Spot the Imposter!

Servo Equipment

Find the Servo Equipment Imposter

To celebrate the launch of the Machine Runner Blog, we’re running a contest. Get your detective hat on, and if you’re right, get a Repair Zone “Mini” LED Mag Flashlight.

Below is a list of servo manufacturers paired with servo equipment types. But not all are correct! There’s one imposter.

Allen Bradley 1391 Series – AC Servo Controller – A Legend Lives On!

1391 Drive

Allen Bradley 1391 AC Controller

The Allen Bradley 1391 series was first introduced in 1992 and is still widely used today.  Noted for its stout build and workhorse mentality, the 1391 series has machine owners holding on to them much longer than most legacy servo-automation equipment made by other companies.  And though they are obsolete, parts are still available and companies are supporting them through repair, exchange, and selling reman units.

The 1391 drives come in 2 versions,

in 2 versions, Series A and Series B.  Though they are functionally equivalent, the Series A logic boards cannot be used in Series B Controller.  But, the Series B logic board can be used in a Series A Controller.

The 1391 contains many of the standard features required in a servo system:

  • An easily removable Logic Board for easy inspection, troubleshooting, and diagnostics.
  • Transient voltage protected input.
  • A circuit breaker that opens all 3 AC leads in the event of a short circuit situation.
  • A 300V DC power bus supply with a shunt regulator.
  • A shunt regulator resistor to minimize energy generated by the motor during braking.
  • Velocity loop components that compensate or inertia.
  • A power line/DB contactor.
  • (3) Controller ratings.
  • UL listed.

The Allen Bradley 1391 also contains these user selectable options:

  • Torque or Current Amplifier Operation
  • Contactor Auxiliary Switch
  • External Shunt Regulator Resistor

The 1391 series includes these Catalog numbers (models):

TheThe following is what the nameplate nomenclature stands for on an Allen Bradley 1391:

The 1391 Series drives are used with the 1326ab servo motor series from Allen Bradley.  The power supply is contained inside the unit, so there is no need for a separate one.  These units run for a long time and in harsh conditions, and when they do break down, they can be repaired and put back in a machine and run with further longevity.  It is also rare that one of these 1391 drives is damaged beyond economical repair.  The legend lives on!


Allen Bradley 1336 Drives

1336 Servo Drive

Allen Bradley 1336 Drive

The Allen Bradley 1336 Plus was the jumping off point for the later 1336 Variable Frequency Drives (VFDs); the 1336 Plus drive line was introduced in 1994, and became the most installed VFD base in North America. The entire 1336 Plus line went obsolete on December 31, 2010. The 1336 Plus II was an upgrade from the 1336 Plus, which the 1336 Plus can be converted to the Plus II.

1336 Series Similarities & Differences

Allen Bradley 1336 variable frequency drives (VFD), including the 1336 Classic, 1336 Impact (E), 1336 Force (T), 1336 Plus (S), and 1336 Plus II (F), are built upon a similar base, and each drive has the same basic start/stop control interfaces and communication options.  The similarities of the different Allen Bradley 1336 VFDs are a major advantage when repairing the drives; once the repair technician learns how to repair one model in the 1336 drive line, the repair technicians are able to repair any 1336 VFD.

The main differences between the 1336 VFD models are the drive’s power output and the drive’s dimensions, which depend on the specifications of each model. The different 1336 VFDs’ power output and dimensions vary to fit the specifications of a given machinery application.

What is the 1336 Regen?

One area of confusion when looking into the different 1336 VFDs is the 1336 Regen (R). The 1336 Regen (R) is not a VFD; however, if you are not familiar with the different 1336 series and models, you may not realize what exactly the 1336 Regen (R) is.

The 1336 Regen (R) works with the all of the Allen Bradley 1336 VFDs. The Regen converts the three-phase AC input source to a DC output source, and it acts as a brake output where the line regeneration package limits the amount of inrush current and provides AC voltage and magnitude information to the converter.

1336 Series Today

Today, the Allen Bradley 1336 lines are obsolete, including the 1336 Regen (R), with the last line (1336 Force) becoming obsolete in October 2012; however, most of the 1336 VFD drives can easily and affordably be converted and upgraded to versions of a PowerFlex drive, which PowerFlex drives are the most up-to-date Allen Bradley drives.

To learn more about RepairZone’s Allen Bradley Repair Capabilities, click the button below:

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Allen Bradley 1326AB Brushless Servo Motors: Features & Benefits


Allen Bradley 1326AB Servo Motor

Allen Bradley products are known for being innovative and quality products. One common Allen Bradley product is the 1326AB series brushless servo motor; the servo motor is widely used in industrial automation machinery.

Three different feedback device options exist in regards to the 1326AB brushless servo motor: 1326AB-B____-M2L, 1326AB-B____-S2L and 1326AB-B____-21. The third segments of each different model number (i.e. M2L, S2L, 21) refer to the feedback options. The M2L refers to the absolute multi-turn high resolution, S2L refers to absolute single-turn high resolution, and the 21 refers to the resolver feedback option. The 1326AB brushless servo motor also comes in three different frame sizes: A (108 mm), B (149 mm) and C (194 mm), as well as a torque range of 2.7-50.0 Nm.

Features & Benefits

So, what makes your 1326AB brushless servo motor a great component in your automation machinery?

Features Benefits
Absolute encoder or resolver-based feedback Withstands intense shock, vibration
Withstands high temperatures
IP67 protection option rating Protects from harsh environments
Durable & high voltage windings Reliable operation
Advanced magnet design Has increased flux density
Greater demagnetization protection
Lowers harmonics and losses
Increased slot fill and better frame and magnetic structure Creates higher motor output
Sinusoidal, four-pole design Has predictable linear performance
Square torque-speed characteristic
Offers increased torque at high speed
Smart motor option Simple commissioning
Peak performance


1326AB Applications

The 1326AB brushless servo motor works best in applications that require smooth performance and high continuous output, such as the applications used in the automotive, machine tool and packaging industries.

Overall, the features of the Allen Bradley 1326AB brushless servo motor benefit your machinery by being a durable, reliable, and an efficient servo motor that will keep your machinery running longer with fewer issues along the way.

If your Allen Bradley Servo Motor needs repair or to view all of RepairZone’s Allen Bradley Repair capabilities, click on the appropriate button below:

Allen Bradley Servo Motor Repair Allen Bradley Repair Capabilities


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