Inverter duty

[Mwater]

motor losses are fixed losses + variable losses (I2R). I have around 100 amps output current from the VFD at different motor frequency. the current output from the VFD is equal to the current drawn by the motor? thus the heat generated at the motor is almost the same at different motor speed and fan speed. so if we are running the motor at 20% of it's normal speed the fan could not cool the motor.

I contacted some motor dealer and 50% percent say that with inverter Duty motor or any type of motors is not recommended to go below 50% of the nominal speed, but the other 50% they say you can go up to 20:1 it depend on the motor frame.

what you recommendation for inverter duty WPI motor used for water pumping? known that we can go up to 20:1 because of pump requirement.

there is other type of motor that you suggest? bands?

 

[jraef]

The real crux of the problem is, the term "inverter duty" is unfortunately not a recognized and enforceable standard by which something is tested, it is a marketing term. It has different meanings with every manufacturer. In this case, I am surprise that you only got 2 answers (50/50), I would have expected more like 6 answers, none the same.

A better way is to look at specific definable industry standards which address as many of your concerns as possible and reflect the criticality (word?) of the application. One that I often recommend is IEE 841 Severe Duty, but that does not address the issue of inverter use. So I also recommend backing it up with a clearly stated caveat to suppliers that you will be using it with an inverter, what type, how far away from the drive, etc. etc. In addition if you need to run your motor at 5% speed, define that as a stated requirement to your suppliers and have them certify, in writing, that their offering will conform.

In other words, don't engineer something out of a brochure. Use that Engineering degree!

 

[Mwater]

Thanks for the advice

 

[jraef]

By the way, I didn't mean to sound condescending in that last statement, it's just a trap we all fall into once in a while, especially if we are working outside of our field (I just noticed that you are a Civil).

By the way, being a Civil you may not know this but it would be important to consider. If you are running a pump at 5% speed, it must not be a centrifugal, most likely a PD? In that case, make sure that when selecting a VFD that you specify it is a "Constant Torque" application, rather than just say it is for a "pump". Sloppy suppliers will see the word "pump" and assume it is centrifugal, then supply you with a Variable Torque rated VFD, which will not survive the rigors of running a PD pump.

 

[DickDV]

As for motor inverter ratings that have real meaning, I trust an MG1 Part 31 endorsement. As jraef points out, terms like "inverter duty", "inverter ready", and "inverter rated" are marketing terms and are useless without any backup data.

Expanding a bit further on jraef's discussion on motor cooling, it all starts with the load, in this case, some kind of pump. You must identify the load torque characteristic over the speed range that you intend to operate. That information will then give you the motor load torque requirement which you can translate into motor input current. The input current determines the motor heating over the speed range and the motor manufacturer can identify how far down in speed you can go. This point is highly variable and is a function of cooling method (TEFC, ODP, or aux. blower), horsepower (typically, a 250hp TEFC motor is only good for 2/1 where a 5hp TEFC motor is good for 5/1), and general design (extruded aluminum or cast iron, high or premium efficient).

Once you have identified the continuous current and peak current required for the motor, select a drive with output current ratings that meet or exceed that level. A variable torque drive has only 10% peak overcurrent capacity and a constant torque drive has 50% peak overcurrent capacity. One is not necessarily more reliable than the other as long as you have sized it to cover the motor currents as described above. Actually, drive terms like Variable Torque and Constant Torque are somewhat misleading. Both kinds of drives get used on both kinds of loads. The terms simply refer to the amount of short-term overcurrent capacity they can produce.

 

[Barry1961]

If this is a positive displacement pump with a 20:1 speed range you will probably use a drive with vector mode torque control. One thing to be aware of with some “inverter duty” motors is that the electrical and mechanical characteristics may not be standard. The motor may be larger than a standard motor to dissipate more heat. For example an inverter duty 10 hp may be a de-rated 15 hp motor.

This can cause some drives to perform poorly. Changing the motor size parameter to the next larger hp in the drive can solve this.

Barry1961