# variable speed drive

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### #1 Guest__*

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Posted 12 January 2006 - 05:17 PM

sir,
how will be the current when measured at the input and output of the inverter?will it be low at the input compared to the output.
why ordinary meters are not able measure correct output voltage and current .
kindly any one reply on the above issue
thanks
ram

### #2 marke

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Posted 12 January 2006 - 06:08 PM

The inverter uses a PWM output voltage waveform. Most meters will measure the peak voltage and calculate the average from that. In order to measure the voltage, you need to use a low pass filter to remove the high frequency component.

At frequencies below 50Hz, the output voltage from the VSD is below the line voltage. As VAin = VAout, the output current must be higher than the input current. i.e. at half speed, the output voltage is half and so the output current is double.

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### #3 jraef

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Posted 12 January 2006 - 08:59 PM

Except for the losses in the VFD. At 1/2 speed the output current is typically slightly less than twice the input current because the losses are greater in the circuit at that speed (maybe 5% or even 10%?), but the input current will include those losses.
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### #4 ram14375

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Posted 15 January 2006 - 05:40 PM

sir
in the v/f drives if we limit the current limit to 125% of drive rating,how will drive behave in case of short circuit in outgoing cables or ground in motors. whether the inverter will limit the current only up to 125%
thanks
ram

### #5 ram14375

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Posted 15 January 2006 - 05:45 PM

sir,
if the motor kw is 75 what is the maximum limit to which the variable frequency drive can be selected ?
what will happen if we select a drive in excess of motor rating. will it have impact on motor windings and mechanical equipments?
thanks
ram

### #6 marke

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Posted 15 January 2006 - 05:55 PM

Hello ram14375
QUOTE

in the v/f drives if we limit the current limit to 125% of drive rating,how will drive behave in case of short circuit in outgoing cables or ground in motors. whether the inverter will limit the current only up to 125%
The current limit feature will limit the average current but not the peak current. Most modern drives will shut down if there is an output short circuit. Some of the older ones will let the smoke out.

QUOTE

if the motor kw is 75 what is the maximum limit to which the variable frequency drive can be selected ?
In most cases, there is no problem using an oversized drive on a motor. If the drive is too large, the drive may not be able to measure the motor performance current etc accurately. In some drives that are closed loop, sensorless vector and Direct Torque Control, there may be an issue (due to measurements) if the drive is much to large. The manufacturer will advise you of this. I would not expect any problems caused to the motor or the driven load.

Best regards,

### #7 ram14375

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Posted 15 January 2006 - 06:06 PM

Thanks sir,
i have got one application where i use a variable freq drive to drive one 415v,3 phase gearised motor (Full load amps of motor 3.5 amps).apart from the overload in the drive i have used one overload relay in the output on the inverter.
The problem is the motor trips frequently due to the overload relay(output). where as the drive current shows very less. i used a clamp on meter (local meter)it showed only 0.2 amps and around 800volt. what could be the problem ? how much i have to set the overload relay.
ram

### #8 marke

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Posted 15 January 2006 - 06:18 PM

Hello ram14375

Measuring voltage and current on the ouptu of a drive is always a problem. Most meters read wrong due to the output being a PWM output rather than a nice clean sinewave.
Some overloads also have problems with the output of a drive. Make sure that the overload that you use is not an electronic type, bimetal only, and set the trip current to the motor rating.

Best regards,

### #9 ram14375

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Posted 15 January 2006 - 06:25 PM

sir,
The overload in the above particular application i am using is a bimetallic type and i have set the current to motor FLC.The drive runs at very low rpm(10hz) rated 50hz /1000 rpm. Is it due to cooling which causes this tripping. the motor is a gearised motor used for rotary airlock application.The motor trips only due to thermal overload relay and not the drive relay setting.
thanks
ram

### #10 marke

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Posted 22 January 2006 - 08:59 PM

Hello ram14375

I have heard of similar situations where there is no apparent reason for the thermal overload to trip, but I would comment that you do need to be very careful operating a motor continuously at around 10Hz because the cooling on the motor is very low and you can get overheating of the motor just due to the iron losses alone.

Best regards,

### #11 jraef

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Posted 23 January 2006 - 05:43 PM

Marke's advice is right on regarding running a motor that slow.

If you have already dealt with that issue however, output OL relay tripping is sometimes caused by the harmonics in the VFD output causing a disproportionate heating of the bi-metal OL relays. I have seen this quite a bit, and it has always (so far) been solved by simply adding a load reactor in between the VFD and the OL relay.

I'm curious as to why you have a bi-metal OL relay on the output anyway. Is this an older VFD that does not have a built-in OL protection circuit? Most newer drives do, and it is far more accurate than any bi-metal OL relay anyway.
"He's not dead, he's just pinin' for the fjords!"

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