Inverter motor question 1

[place1234]

I'm testing no load with VFD + Reliance inverter motor). Reliance motor information is below:
50hz/5HP/2915RPM/380V/FLA7.63A
HZ RANGE: 5-50HZ. INVERTER TORQUE: CONSTANT
INVERT HP VALUE: 3 INVERT NP AMPS: 5.3
CODE:J

But when I started the VFD at 5 HZ, motor current was 8.5A. While my VFD set the limit 6A, so VFD alarmed and stopped. The current is supposed to be 5.3A at 5HZ on the nameplate.

How can this happen, anyone could advise. thx alot !

 

[ozmosis]

assuming the motor is free running, I would first check the wiring to the motor to make sure there is no mistake in the connections. It doesn't sound like a short-circuit as the drive would trip immeadiatly on over-current, but check all the same.
Then look at the settings within the VFD. If your acceleration time is very fast, even on an unloaded motor, it could produce a high current on start-up. If this is ok (>15 secs should be ok on an unloaded 5hp motor) then check the settings in the VFD. Most makes of drive allow you to input the motor parameters and if these have been input incorrectly, then the V/F(volts/frequency) could be wrong for the motor you are connecting to and cause over-fluxing in the motor and therefore overcurrent seen at the VFD.
These are first steps and there are a number more after this, but take it step by step.
Refrain from running the VFD until you have checked the basics as there is obviously something not right and you don't want to keep stressing the problem.

 

[CJCPE]

If the VFD is set up properly for the motor, the voltage boost may still be set too high. Voltage boost is an increase above constant V/Hz (380V/50Hz) that is applied at the starting frequency to compensate for IR drop in the motor. With an oversize motor on a small drive, that setting can easily cause enough overcurrent to trip the drive.

With a VFD, the normal idle current at low frequency can be nearly as much as full load current. As load is applied, the power factor increases, but the current may not increase very much. Reducing the voltage boost will likely help. The ideal setting is a compromise between getting enough torque and getting too much current.

With a vector drive, the VFD should tune itself to the motor and dynamically control the boost voltage automatically.

 

[jraef]

The current is supposed to be 5.3A at 5HZ on the nameplate. [/quote

Where do you get that from? The motor nameplate says that the motor FLA is 7.63A, on a 5HP motor, which is correct. You have also apparently listed the VFD nameplate information and it is a 3HP VFD with a maximu output of 5.3A. Your VFD is too small for that motor.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[buzzp]

Jraef is very correct unless wrong information was posted about the drive. The 3HP drive will never work unless the 5HP motor is very lightly loaded. Even then, it may not provide adequate current.

 

[ozmosis]

jraef, as buzzp says, your right. Must be my age, but I missed the bit at the end showing the VFD details.
place1234: yep, here's your answer for the VFD showing overcurrent.

 

[DickDV]

I would think a 6amp VFD is big enough to test a 7.6amp motor running unloaded.

I suspect that the poster is using a clamp-on ammeter to read motor lead current and is seeing 8amps on his meter from a drive with a 6amp current limit! I would suggest using the VFD display to indicate motor lead current. There will be far less error.

Also, there is no mention of the motor shaft turning. At high current and 5Hz, I would expect the shaft to turn, at least try to turn. If it doesn't, the motor is either wired wrong or has defective windings.

Maybe the original poster can fill us in on this point.

 

[aolalde]

To me as Jraef stated the driver is too small. The motor accelerating current for the v/hz ratio and probable boost compensation can not be handled by that 3 HP driver.

Why is people always attempting to abuse the equipment capacity and then blaming it for the problem created?

 

[aolalde]

A motor is a black box with internal impedances based on the motor parameters. If a 1000 HP motor is started to run 1 HP load it will start like 1000 HP motor not as 1 HP one!

 

[jraef]

DickDV,
You make a good point, I have in fact tested VFDs with larger unloaded motors successfully. But as aolalde says, if the default settings for the VFD included torque boost at 5Hz and under, as many do, that could explain his problem. Starting an oversized motor with a VFD and running one at 5Hz are two different things as far as the drive goes.

The clamp-on ammeter issue is a good point as well, that came up in another recent thread too. thread237-143590

place1234,
We need some clarifications fo you postted data to help you any further. Is that the VFD nameplate info? If not, what is that? And how were you reading the current?



Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Marke]

aolalde

A motor is a black box with internal impedances based on the motor parameters. If a 1000 HP motor is started to run 1 HP load it will start like 1000 HP motor not as 1 HP one!

This is very true, but with a VSD, the motor never runs under high slip conditions, so it never really starts in the classic sense, it runs from zero Hz.

The limit to the use of a smaller drive is often related to the magnetising current. On small motors, the magnetising current is high relative to the rated current of the motor when compared to large motors. Any small change in the effective v/Hz ratio on small motors will have a major effect on the current due to the typical operation closer to saturation flux.
On medium to large machines, the magnetising current is as low as 20% of the rated current and a VSD can be well undersized relative to the motor and still work well up to the inverter rating. On small machines, the design flux is much higher and the magnetising current is commonly in the range of 40 - 60%. This reduces the potential to undersize the drive.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

Hi Marke.

Given a voltage and frequency is not the “magnetizing current” finally sized by the motor impedance?
It does not mater if the start is with high slip or very low slip, the motor reactance is inversely proportional to the motor size, even starting with the no load current, the larger the motor the larger the current demand.
I agree that when the slip is 1, the current is 6 or more times the full load current and when the operation is close to zero slip, ideally the motor will draw the no load current. Still torque is required to accelerate the rotating parts and provide the load torque demand, normally full load torque which drives around full load current.
Are you able to start and operate successfully a larger motor-load with a smaller size rated drive?