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VFD Frequent Questions

Modified on 2008/04/06 22:06 by Administrator Categorized as Electrical, FAQ

Gee, this seems too good to be true. There has to be something wrong.

This is one of those rare cases where it IS good and it IS true.

How many more motors fail per year when powered from a 230 volt VFD?

None. The actual number of motor failures that occur have no relationship to whether or not the motor was powered from the power grid or a VFD. Put another way, if you have 3000 motors and experience a 1.3% failure rate per year when powered from the power grid, in general the failure rate will remain unchanged after the change to an AC Drive aka VFD. In a few studies, the failure rate actually decreased slightly. The causes for this increase in lifespan are generally attributed to proper overload protection and soft starting of the motors.

How difficult is it to use my existing controls to control the VFD?

Not difficult at all. Wiring diagrams are available for every type of common motor control in use. Just remember that the VFD control lines must be used to stop and start the VFD. Never put anything between the VFD and the motor other than a piece of wire.

What is dynamic braking and what will it do for me

Dynamic braking basically uses the variable speed feature to slow down the motor at a user selectable rate. The rate is usually adjustable from 0.1 seconds to 99 seconds. What it will do is to stop that spinning blade or tool much faster thus reducing the risk of accidental physical contact. My bandsaw has a significant amount of moving mass due to cast iron wheels and pulleys. Dynamic Braking has lowered the blade stop time from over 1 minute to less than 7 seconds. Motor braking is nothing new. In the 1980s, Delta Machinery supplied a 3 hp single phase motor for their Unisaw which has a mechanical brake built into it. Some saw motors will brake electrically.

How long can I expect a VFD to last

The minimum MTBF I have seen to date is 100,000 hours. I have seen some manufacturers rate their drives at 500,000 hours. A VFDs are designed for the industrial user, not the consumer and as such they last a long time. 100,000 hours MTBF breaks down like this.

8 hrs a day times 5 days per week is 40 hrs. 40 hrs times 52 weeks

2080 100,000/2080 hr per year 48 years

If the motor and VFD are run 24x7x365 then the drive is expected to last about 11 years.

Can I use a VFD to power multiple machines?

If you mean turning on and off two motors at the same time, the answer is yes. Just hook them up in parallel.

If you want to have two or more devices turn on and off at will then the answer is maybe.

I wrote a PLR (Programmable Logic Relay) program earlier this year so that one could safely share a VFD between machines. I was done in about 4 hours.

The constraints to switching a VFD's output are formidable when one tries to use just contactors and switches to handle the switching. Here are the constraints as I see them.

  1. The output of the VFD must be connected to the motor before the VFD is told to turn on the motor.
  2. The VFD must be told to turn off and braking completed before disconnecting the motor from the VFD.
  3. Once a motor or motors are selected and the VFD turned on, no other motor may be connected to the VFD's output. In other words a lockout must occur.
  4. The fwd/rev, start/stop, and variable speed controls must be switched from station to station.

Is a VFD better than a RPC (Rotary Phase Converter)?

That depends on what you want. I made a little scorecard that will help sort it out.

Start with 10 points for the VFD and the RPC.

Is motor 2hp or less -3RPC Is the motor greater than 2 hp? -1 for VFD Motor greater than 5hp -5 VFD Is the machine a lathe? -5 for RPC Drill press? -3 for RPC Bandsaw -2 for RPC Shaper -2 RPC Can build RPC? -1 VFD Have a surplus 5-10 hp 3 phase motor lying around? -2 VFD Do you work with metal and wood? -2 RPC Several motor starts and stops? -1 RPC

If you have a 2hp 3ph motor on your Unisaw and have no other 3 phase tools then the obvious choice is a VFD.

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