Motor modification for VFD

[TobyParticipant @toby]

fwiw, the motor on my Meddings drill is 0.75HP and is rated at 1.7A FLA for 440V star and 3A FLA for 240V delta. Running it as delta from an inverter (with about 240V out) and the inverter (a yaskawa) reads 1.7A no load and around 1.8A when drilling a 1" hole through steel. So it never gets anywhere near its 3A FLA.

So yes, 5A means there is something wrong…….

(btw the inverter I use is a Yaskawa J1000 (0.75kw/0.4kW)

Edited By Toby on 21/03/2017 13:30:34

[Neil WyattModerator @neilwyatt]

Posted by Toby on 21/03/2017 13:29:24:

fwiw, the motor on my Meddings drill is 0.75HP and is rated at 1.7A FLA for 440V star and 3A FLA for 240V delta. Running it as delta from an inverter (with about 240V out) and the inverter (a yaskawa) reads 1.7A no load and around 1.8A when drilling a 1" hole through steel. So it never gets anywhere near its 3A FLA.

So yes, 5A means there is something wrong…….

Running the motor on a low voltage may be part of the problem, if the motor isn't getting up to speed it will draw extra amps despite the lower voltage.

Neil

[MuzzerParticipant @muzzer]

Current is basically proportional to torque – with a small additional magnetisation current – and the output voltage is roughly proportional to speed. Assuming you have set up the VFD with the correct nameplate info (particularly the number of poles or base speed and the phase-phase voltage), there isn't a whole lot to get wrong.

You can develop a significant motor current (torque) at low speed but the corresponding motor voltage will be low, so the net corresponding (real) power at either the input or the output will be low. The % efficiency of a modern VFD will be in the high 90s at rated output and obviously will be zero at stall.

The reason for specifying VA is because motors are fairly inductive (poor power factor) and the phase current isn't in phase with the drive voltage. The shaft power is developed by the real power (W), while the reactive power is basically recirculated. The power factor at the VFD input is fairly close to unity (not least due to legislation), so the big discrepancy between VA and W only really arises on the output.

If the VFD load were purely resistive, the VA and W would measure the same. Conversely, if it were purely inductive, the VA would be high and the W would be zero – and the VFD input power would only have to cover resistive losses.

When you are designing a VFD, the DC bus voltage is generally well defined and the main critical parameter is the output phase current. With IGBTs, it is the average AC phase current that matters. With a FET output stage it would be RMS current – but 200V and 400V class VFDs use IGBTs.

Murray

[Brian OldfordParticipant @brianoldford70365]

Posted by Neil Wyatt on 21/03/2017 13:42:40:

Running the motor on a low voltage may be part of the problem, if the motor isn't getting up to speed it will draw extra amps despite the lower voltage.

Neil

I'm gradually getting there. Quite right Neil, the extra current is caused due to the lack of back emf because of the low revs.

[Brian OldfordParticipant @brianoldford70365]

Posted by Muzzer on 21/03/2017 14:30:41:. . . . .

The reason for specifying VA is because motors are fairly inductive (poor power factor) and the phase current isn't in phase with the drive voltage. The shaft power is developed by the real power (W), while the reactive power is basically recirculated. The power factor at the VFD input is fairly close to unity (not least due to legislation), so the big discrepancy between VA and W only really arises on the output.

. . . . .

Murray

For those with an electronics background CIVIL.

[Brian OldfordParticipant @brianoldford70365]

I'm in the process of borrowing another (higher rated) inverter and (in my bits box) have found a spare 1/3HP 3 phase motor which will easily re-wire to delta. I'll report back when I have some more results. Thanks again for all the valuable input.

[Anonymous]

Posted by Brian Oldford on 21/03/2017 14:56:47:

I'm gradually getting there. Quite right Neil, the extra current is caused due to the lack of back emf because of the low revs.

For an induction motor surely the speed is primarily determined by the supply frequency (and number of poles), not by applied voltage. So if the motor is not getting up to the speed expected for the supply frequency then there is something wrong, probably with the motor. Does the motor get hot?

Andrew

[Neil WyattModerator @neilwyatt]

Posted by Andrew Johnston on 21/03/2017 15:05:43:

Posted by Brian Oldford on 21/03/2017 14:56:47:

I'm gradually getting there. Quite right Neil, the extra current is caused due to the lack of back emf because of the low revs.

For an induction motor surely the speed is primarily determined by the supply frequency (and number of poles), not by applied voltage. So if the motor is not getting up to the speed expected for the supply frequency then there is something wrong, probably with the motor. Does the motor get hot?

Andrew

My guess is that a motor run at less than half voltage (110 instead of 240) will slip a lot and therefore be a lot less efficient.

N.

[john fletcher 1Participant @johnfletcher1]

Are you absolutely sure you have the motor configured in DELTA. I know you said in para 3 that you had been careful , a mate of mine said the same when he altered a motor, then we found he was wrong, we all make mistakes. I have the same inverter (pre owned as the car salesman would say) running an old 746 watt motor, no problem. Have you got all the setting correct, especially the voltage. As some member here may recall, I had the inverter for 4/5 years but couldn't get motor to run below 500rpm, try as a may. Later, I requested assistance here and a member told me I had a particular item set at 50 instead of zero, job done, I was very grateful, I had overlooked the same problem many times.John

[Joseph Noci 1Participant @josephnoci1]

Hi again Brian…Joe again…

You have not answered any of the questions I put to you…I think you need to at least verify that the inverter is set up OK, esp the Voltage versus Frequency curve. That should at least be set to the 50Hz general application factory settings to start with. Then a good test is to set the inverter output to 50Hz and try to measure shaft RPM. If it is what you expect, then all the issues mentioned related to motor not achieving speed, back EMF, etc are moot.. In which case there is either a mis-wiring issue, which I would strongly suspect.

It would help the diagnosis though if you could answer some of my questions related to the fault displayed, and the mode of the fault…

And of course a connection to utility mains supply would help eliminate the inverter as a culprit..

Generally ( within limits) applied voltage does not affect speed, but reducing the applied frequency ( hence rotational speed) does require that the applied voltage be reduced as well ( that's why there is a V/f curve that needs tailoring for motor types..) in order to match the back EMF, and that is why the torque falls off so badly at low RPM.

Regards

Joe

[Brian OldfordParticipant @brianoldford70365]

Right gents. I'm 99% sure I've cracked the problem. Once again thanks for all the ideas and input.

I rewired the 1/3HP motor to delta, connected it to the inverter, set the relevant parameters so they passed a sanity check, powered it up and it spun up perfectly and I was able to vary the speed both below and above 1425rpm by varying the output frequency.

I then put the inverter back onto the 3/4HP motor and did a bit of voltage monitoring and also discovered the motor is failing to spin up properly. This phenomena is something I missed the first time round. What I'm getting is massive voltage droop (because of the excessive load) preventing the motor spinning up to produce a decent back emf to limit the current. i.e. the inverter was trying its darndest to deliver the goods but the motor was just that little bit too much and was eventually having to self protect its transistors.

Not having experience of the soft start properties of these inverters produce added to the confusion in my mind.

I'll report back again, hopefully with a happy ending to conclude this tale, when the bigger inverter arrives.

[Brian OldfordParticipant @brianoldford70365]

Just a quick note as an epilogue to this tale/learning curve. The larger VFD is connected to the 3/4HP motor and, once the VFD was configured correctly the motor spun up perfectly and was controllable from less than a few hundred rpm to 20% over-speed (60Hz).

The smaller inverter has been allocated to the Dore Westbury mill.and will be brought into use once the existing single phase 1/4HP motor has been replaced with the 1/3HP three phase one. Fortunately all motors were four pole.

Once again thanks for all the useful hints and tips from the many posters on this thread.

[SillyOldDufferModerator @sillyoldduffer]

Thanks for the feedback Brian – well done you!

As a matter of interest how much current does the new vfd say the motor is pulling?

Ta,

Dave

[Brian OldfordParticipant @brianoldford70365]

Posted by SillyOldDuffer on 26/03/2017 15:31:01:

Thanks for the feedback Brian – well done you!

As a matter of interest how much current does the new vfd say the motor is pulling?

Ta,

Dave

A quick check "off-load" says it's less than 1A/phase when driven at 50 Hz.

I'll live with than

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