Complications with Multiple Motors on One VFD

| Scott Savage

There are so many instances where original equipment manufacturers incorporate multiple motors on one AC drive as their design. Kilns are a typical example of many drives on one VFD.  Another example is cooling towers that need to run at the same speed, leaving the design simple and the motors operating at the same hertz value from the drive (same speed).

The design is simple and cost-effective. However, as the system ages, it can be difficult to troubleshoot. There are things we will discuss in this blog that owners should be aware of during and after the installation.
This blog will review facts about multiple motors wired to one VFD (Variable Frequency Drives) and discuss solutions to these reoccurring problems.

The first topic of discussion is the lengths between each motor and the drive. Operations are dealing with most systems after the fact and have reoccurring issues with motor failure. It may take months or even longer periods of time for this to happen.

Why is Lead Length Important with Multiple Motors on One VFD?

Many blogs discuss the simplicity and cost savings of one drive to multiple motors and do not discuss some of the other issues that may occur. Let’s take a deeper dive into some of the requirements involved with these systems.

  • Measure each motor lead from drive
  • Total the various motor lead lengths and sum the total
  • Observe the type of cable used at installation
  • Inspect the grounds in the conduit box of the motor should be clear of paint and corrosion 
  • Review your ground paths from conduit box to each motor ground should be a direct homerun path back to drive if needed - add a ground bus isolated from cabinet panel to avoid noise to the drive panel. 
  • Use a PVC coated cable or armored with PVC coat to avoid contact with frames in building 
  • Land the ground directly to the drive ground screw on drive from motor grounding bus (if added or present) with all motor grounds
  • The second ground screw located on drive should go to a solidly grounded system on the supply transformer (solidly grounded Wye secondary)
  • Grounding path is extremely important in these applications if done at installation it avoids rerouting the paths
  • Avoid grounds from motors clustered out in the field away from drive cabinet these are more points of failure with corrosion and contaminates
  • Avoid THHN or THWN wire, the concentricity of the conductor inside the insultation is not consistent and thermoplastic insultation absorbs moisture PVC type insulation is toxic in case of fire
  • Use a VFD cable with cross linked polyethene and shielding NFPA79 requires this – as alternative to PVC insulation, XHHN or XHHW at minimum thermoset insulation (i.e. cross linked Polyethene)

How to Know the Difference

THHN/THWN-2 is a thermoplastic product, while XHHW-2 features thermoset insulation.
Thermoplastic:

  • Cheaper than Thermoset
  • Nylon Coating – Required for the Insulation System’s Rating
  • Uses Thinner PVC Insulation
  • Lower Overall Electrical Insulating Properties

Thermoset:

  • Hospital Grade (low leakage insulation)
  • Dual Rated 600V/1kV
  • Uses XLPE (cross-linked) Insulation Instead of PVC
  • Substantially Higher (better) Insulation Resistance

Now you have inspected the cable or hopefully followed the ground rules before installation. Let’s understand the concept of motor lead lengths and how they affect drive/motor challenges.
Now you know the total distance of all the motor leads here are basic rules you need to follow (ballpark figures) when it comes to multiple motors on one VFD drive:

  • MG1 MOTORS with 1600VAC rating should be a standard good for approximately 200ft distance without output protection (this is total distance of the system)
  • dv/dt filters should be placed on output of drive if between 300ft and 900ft please gather information available discussed in the sine wave bullet below
  • Sine wave filters should be added above 900ft - please have your engineering staff check with a Rockwell specialist or TCI (Transcoil) to verify all the physical components such as wire type, drive type, ambient temperature, PWM frequency (carrier frequency of drive 2Khz-8Khz) run the PWM carrier frequency to manufacturer’s suggestion
  • Beware of capacitive current consumption to allow overhead capacity on the drive:  1mA per foot unshielded and 2mA per foot for shielded cable

Why is the Total Length a Concern?

Reflected wave is a phenomenon with long lead lengths to the motor. Voltage doubles and sometimes triples at the first turn of the motor. This will cause corona (bubbling of the dielectric in the windings) ground faults will occur and fault the drive. Non-shielded cable that is not twisted has great capacitance – then the waveform strikes the motor which is high in inductance and reflects back on the output wires.  Even wires near the motor may exhibit heat issues and even corona as well as the motor windings.

The second phenomenon that happens is common-mode noise, the longer the distance the greater the magnitude of noise. It is important to practice good grounding practice. Noise shortens the life of the drive and disrupts instrumentation and controllers (PLCs or BMS systems). Poor shielding and grounding allow high frequency circulating currents through surfaces of wires, frames of equipment, and yes - without shielding broadcast airborne noise. These currents need to go back to the point of origin (the drive) to make all currents cancel to a value of “0”. Good grounding directs the noise back to the drive and not throughout the system.
The system shows sign of degradation over time and burdens the maintenance staff. They are frustrated and repeatedly placing quick bandages on a hemorrhage.  

Many articles promote this system design; there are consequences in taking this approach. The fact of the matter is with individual drives (which now costs the same or less) the one drive fits all approach may be something to consider on an upgrade. This is one solution versus adding dv/dt filters or sine wave filters.

Ground Fault Scenario

  • Technician is called on a ground fault on one drive without many motors say 10 motors.
  • Where does he start?  Conditions are a hot summer day - it is 95F degrees.  It could be in a kiln with excessive heat where motors reside.  It could be on a roof for a cooling tower in extreme heat from sunshine.  They have a lot of work ahead of them.
  • Is it the drive (noise degradation)?  Is it the wire leads in conduit boxes on motors (x30)? Is it the wiring itself or splices in the system (may not know they have splices)?  Most start with a mega meter and start with one motor (guessing game).  It is not that motor so let’s try another.  You can quickly see this could be a while…wire nuts in conduit box are taped and damage may not be obvious, they could be burned due to reflected wave.
  • They find the problem after hours of searching – things are operating but more problems will happen.  Why?  The reasons listed above.  The technician decides to leave the company due to harsh conditions. (true story)
  • Rough waveforms (from VFD) and long distances promote these failures
  • “It’s going to be different this time” no it will not till you fix the problem
  • Months go by and the issue happens again – Humans are notorious to forget about things, especially negative things

One Drive Configuration

One drive and multiple motors must run in V/hz mode, Sensorless Vector and Flux Vector are one motor-only models. Shown below is the example normally seen. The motor overload relays must be added to each motor lead to protect the motor from excessive amps. This adds to the cost of the installation. A drive on each motor drive protects the motor and overload is not required. If the system is retrofitted with individual drives the branch protection (circuit breaker) does not change and the overload relays would be removed. Now you know which motor has the ground fault. Downtime is severely reduced, and the entire system does not go down and can be run at reduced capacity till problem is solved. (some have done exactly this with no problems afterward). Motors do not have to be the same HP.

Sine Wave Filter Retrofit

Your facility decides not to go with individual drives.  “We have never had problems in the past”. Motors fail and is routine maintenance they say.  Please have the motor examined by an EASA motor repair shop and determine the cause – are the first turns in the motor burned up?  If they are it is reflected wave.  All the motors are decaying due to long lead lengths and rough waveforms.

A sinewave filter will reformulate the rough waveform back into a sinusoidal waveform. Companies like TCI (Transcoil) build a sine wave filter called MotorShield that can be wired close to the drive output and then go to the motors. Reflected wave does not exist after the retrofit. Horizon Solutions has helped customers install these units and they have run for many years without a problem.

  • Sinewave filters generate lots of heat, you may want to get the filter in a NEMA3R enclosure and mount outside the building if it is a large unit
  • Sinewave filters do not eliminate Common Mode Noise – shielded cable and especially good grounding practices eliminate this challenge
  • Sinewave filters are amp rated devices so match up with the amps on your drive
  • Check with your manufacturer of the filter and provide all information cabling, distances, size of drive etc. before you purchase a unit 

Grounding

Grounds to Multiple Motors on One VFDs Need to Have a Direct Path to The Drive

There is just not one proven solution if you have an existing system. Please consider all the topics discussed in this blog and decide what works best for you. If you are designing a new system, or upgrading an existing system, these ground rules (excuse the pun) could save hours of aggravation and downtime.  Horizon Solutions encourages you to practice good grounding not just for electrical code but both code and noise mitigation (the hidden problem). Reflective wave is destructive and not so hidden. If motors are failing dig deeper and hopefully you find this article on the web.
Enjoy this discovered article and contact us for more information.
Special thanks to Rockwell Automation®, automation.com, Istock, and OmniCable for images.