How VFD limits the starting current of the motor?

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karachi pakisatan
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I have a 5 HP three phase motor, i am going to install a vfd for speed control, i know that VFD is used for soft starting of motor. My question is how VFD limits the starting current of the motor? Plus is the input current to the motor same as the motor current?
 

Besoeker3

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UK
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Retired Electrical Engineer
I have a 5 HP three phase motor, i am going to install a vfd for speed control, i know that VFD is used for soft starting of motor. My question is how VFD limits the starting current of the motor? Plus is the input current to the motor same as the motor current?
You don't need it for soft starting the motor. Normally, the input current is significantly lower than the motor current.
 

winnie

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Springfield, MA, USA
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Electric motor research
The VFD does not directly limit the starting current of the motor. What the VFD does is select operating frequencies and voltages where the motor current is naturally low.

For a given supply frequency and voltage each motor has a torque/speed curve, and also a current/speed curve. With an across the line start motor the starting current is very high because of the huge starting slip.

A properly programmed VFD will select a low starting frequency and voltage, so that even at standstill the motor is on a low slip portion of the current/speed curve. (Note: I am using slip in the frequency sense and not the fractional sense, a separate discussion. ) it is because of this drive frequency selection that current is controlled.

If the VFD were to apply the wrong frequency or voltage, then the motor would try to draw very high current. Generally this will cause the VFD to 'trip' and shut down to prevent damage.

Finally because the VFD is in some sense acting like a 'buck' switching power supply with the motor the line current supplying the VFD can be lower than the current going to the motor. This is especially relevant when starting a motor; the motor gets low voltage and normal running current at low frequency, the VFD input is normal voltage at low current.

Jon
 

paulengr

Senior Member
It is not soft starting.

Think of the current and torque at normal operating frequency. It is almost a straight line from no torque at synchronous speed to full torque at breakdown, about 2 to 3 times name plate. In that range the flux current (feeds the magnetic field of the stator) is nearly constant. The rest of the current is converted into torque and a tiny amount of losses.

The operating point depends on the load torque. Up to breakdown the current is determined by the load torque.

Now what I did not mention is the frequency and voltage. If we reduce the voltage the breakdown torque is reduced. The same relationships still exist but torque and thus current is limited.

At this point we can see that adjusting the voltage controls torque and current although they are not independent. This is what a soft start does.

Up until this point I haven’t mentioned frequency and speed yet. Synchronous speed is the frequency seen by the motor. With a soft starter ignoring cycloconverter mode we can’t change it, except that the percent of synchronous where breakdown occurs does go down a little wire voltage.

But in a VFD we can simply change the synchronous frequency. Now our whole torque/current line shifts. By changing the frequency we can adjust the torque/current and speed almost independently.

Normally we have multiple conflicting goals with motor control. We want to maintain a certain speed (or torque), accelerate (or decelerate) as quickly as possible, and limit torque or current, or indirectly thermal load. Often these goals conflict. For instance with a large fan we want to get it up to speed without burning up the motor. The motor can’t survive the heat load of starting for say 2 minutes. A soft start seems like a solution but all it does is make a bad problem worse because it also decreases torque and adds more starting time, exacerbating the problem. A drive can be set to limit current to say name plate. It can just let the motor run on the edge of stall for as long as it takes to get to speed while manipulating the frequency so that we get name plate torque at any speed.

That doesn’t mean soft starts are obsolete. VFDs do bad things to your costs in many cases, and have other negative issues such as reflected waves, shaft currents, shorter life, and greater cabinet heating that soft starters don’t have. So depending on the application soft starters are preferable to VFDs when they can do the job, just as basic starters are preferable to both.
 

kwired

Electron manager
Location
NE Nebraska
It isn't soft starting yet it effectively is.

They can tweak volts/frequency some to get some desired results but generally if you halve the frequency you have half the voltage, and generally keep that same V/F ratio regardless what speed you are operating. This means motor running at half speed only has about half output rating it has at 100% speed as well.

VFD's generally can't handle the current you would see @ full voltage starting, so they typically will ramp up voltage while starting the motor instead of just hitting the motor with whatever the desired output is. If motor is at tenth of rated speed it will typically see a tenth of rated frequency and volts, but in true constant torque situation would still be drawing same amps as it does at full speed.

Where this becomes effective soft starting is the fact that the input to the drive is not seeing a huge surge of current like across the line starting normally does, and the driven load doesn't see a big shock load during acceleration like across the line starting does either.
 
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