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Ron Nombri
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Maximum Demand Calculation: VSD + Motor

06/23/2014 5:51 PM

Should VSD rating be accounted for in maximum demand calculations?

If so, in practice, by what percentage of load factor on a VSD rating should a VSD be accounted for in maximum demand calculation, apart from the motor rating?

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Ron

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lyn
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#1

Re: Maximum Demand Calculation: VSD + Motor

06/23/2014 10:13 PM

Some study is in order. Your question is unclear.
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Ron Nombri
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#2
In reply to #1

Re: Maximum Demand Calculation: VSD + Motor

06/23/2014 10:55 PM

Hi,

Say a VSD of 18kW is connected to a 15kW motor. I'd use the VSD FLA rating in the maximum demand calculation and size the power cable to the VSD FLA rating, and size the motor power cable to the motor FLA rating.

A couple of my colleges hold the opinions that;The maximum demand shall be calculated based on motor FLA, and 45% of VSD FLA. The power cable to the VSD shall be sized based on the motor FLA, and 45% of VSD FLA as well.

Q: What is your opinion?

Haven't come across any standard that talks about maximum demand calculation on VSD connected motor-drive systems: If you know of any like standard, please advise.

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PWSlack
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#3
In reply to #2

Re: Maximum Demand Calculation: VSD + Motor

06/24/2014 2:50 AM

British Standard 7671.

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TonyS
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#7
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Re: Maximum Demand Calculation: VSD + Motor

06/25/2014 4:12 AM

What about it?

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Tornado
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#8
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Re: Maximum Demand Calculation: VSD + Motor

06/26/2014 10:27 PM

There are so many kinds of BS that they are now assigned numbers for convenience.

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ticker
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#4
In reply to #2

Re: Maximum Demand Calculation: VSD + Motor

06/24/2014 7:25 AM

The load should be the motor full load plus the losses in the VSD, you need to contact the VSD manufacturer for this specific information (about 6%).
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Phys
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#6
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Re: Maximum Demand Calculation: VSD + Motor

06/24/2014 2:54 PM

NEC ARTICLE 430 X. Adjustable-Speed Drive Systems 430.122 - requires one to size the feeder to a VSD, or as they call it "power conversion equipment included as part of an adjustable speed drive system," based on 125% times the FLA potential of the VSD.

Don't know where that extra 45% came from. Glad that doesn't apply here as I once had to provide a 150HP capable VFD for a 50HP load. (customer had expansion plans in mind) The feeder to the VFD was noticeably larger then the motor feeder, and looked quite strange indeed.

That covers the feeder calculations. This section also covers motor overload protection, but skips any details of the VSD protection, referring you back to the basic sections on motor circuit protection.

Also don't know if the NEC means a thing in your location, but it is a starting point.

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JRaef
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#5

Re: Maximum Demand Calculation: VSD + Motor

06/24/2014 10:17 AM

Maximum Demand calculations are specific to each and every utility, so the only relevant answer will have to come from them. That said, motor loads are typically a category in and of themselves, with few exceptions (i.e. intermittant loads such as lifts, hoists, motorized valves etc.) but there is no allowance or adjustment that I have ever seen with regard to a motor being powered from a VFD or anything else. Max Demand calcs typically require simply using the motor full load power ratings, not the actual load on the motors, which then provides for starting power requirements. On a VFD, it is theoretically possible to start with no added demand beyond full power, so if you have multiple motors, use the largest Across the Line (DOL) motor as your first motor in the calc, then include the VFD supplied motor in the typical "50% of full load power of all remaining motors" portion. If you have just the one motor, using the standard values will ensure more than enough starting power.

You may be able to get by with less if you are generating your own power, in which case all the talk of Maximum Demand calcs was pointless.

By the way, using "VSD" is actually too vague, because a Variable "SPEED" Drive could be AC, DC or even a mechanical drive system. VFD would make it specific to Variable Frequency Drive for an AC motor.

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JRaef (1); lyn (1); Phys (1); PWSlack (1); Ron Nombri (1); ticker (1); TonyS (1); Tornado (1)

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