VFD's and Motor Starting Currents

[ish1284]

I understand that every VFD probably has a different response but I was looking for some typical information on what kind of current can be seen on the line side of a modern VFD during motor starting.

Are there ever VFD's that allow the full inrush current of a motor to occur or are they almost always a soft start? And for soft starting VFD's, is there a conservative assumption I can use as the (max) inrush current seen on the line side of the VFD when performing protective device coordination with upstream protective devices. This information is difficult to obtain from the vendor.

If you are looking for a specific model I have in question, I am looking at an ABB ACH550 VFD.

 

[drktmplr12]

With respect to inrush current, the VFD will act more like a soft starter-only better. It really depends on how the VFD is configured.

See this post by Jraef

Related article by Peter Novak, May 01, 2009 on EC&M

 

[Besoeker]

I understand that every VFD probably has a different response but I was looking for some typical information on what kind of current can be seen on the line side of a modern VFD during motor starting.

Are there ever VFD's that allow the full inrush current of a motor to occur or are they almost always a soft start? And for soft starting VFD's, is there a conservative assumption I can use as the (max) inrush current seen on the line side of the VFD when performing protective device coordination with upstream protective devices. This information is difficult to obtain from the vendor.

If you are looking for a specific model I have in question, I am looking at an ABB ACH550 VFD.

Pretty much all VFDs have the same power circuit arrangement. An uncontrolled rectifier, smoothing, and an IGBT inverter which outputs the variable frequency (and voltage). There is no inrush as such. The capacitor used for smoothing is pre-charged before a start is permitted.

During run up, the line side current increases with speed.

 

[ptonsparky]

I understand that every VFD probably has a different response but I was looking for some typical information on what kind of current can be seen on the line side of a modern VFD during motor starting.

Are there ever VFD's that allow the full inrush current of a motor to occur or are they almost always a soft start? And for soft starting VFD's, is there a conservative assumption I can use as the (max) inrush current seen on the line side of the VFD when performing protective device coordination with upstream protective devices. This information is difficult to obtain from the vendor.

If you are looking for a specific model I have in question, I am looking at an ABB ACH550 VFD.

I don't think I have ever hooked up a VFD and then had it set to zero seconds ramp. Defeats the purpose of the VFD.
The size of protective devices allowed by VFD mfgs seems to be way over what most of us would even consider. An Altivar 60 hp, allows a 200 amp class j or t fuse. Typically it would be 125 Amp for an across the line start.

Read the manual that comes with them.

 

[ish1284]

I'm more looking for an assumption to make on what an upstream protective device could see, rather than figuring out the size of the protective device.

For example if a motor has a rated inrush current of 600% of it's full load current when not powered through a VFD, the max current that can be assumed during starting through a VFD on the line side of a VFD is XXX% of the full load current of the motor.

This is to set the STPU of an already installed protective device which has an electronic trip unit to ensure it won't trip during starting of multiple downstream loads which are powered by VFD's.

 

[Besoeker]

For example if a motor has a rated inrush current of 600% of it's full load current when not powered through a VFD, the max current that can be assumed during starting through a VFD on the line side of a VFD is XXX% of the full load current of the motor.

Less than 100% as a rule.

 

[GoldDigger]

I'm more looking for an assumption to make on what an upstream protective device could see, rather than figuring out the size of the protective device.

For example if a motor has a rated inrush current of 600% of it's full load current when not powered through a VFD, the max current that can be assumed during starting through a VFD on the line side of a VFD is XXX% of the full load current of the motor.

This is to set the STPU of an already installed protective device which has an electronic trip unit to ensure it won't trip during starting of multiple downstream loads which are powered by VFD's.

Unfortunately, if you are looking for actual numbers and not code requirements, it will depend strongly on the settings (such as ramp time and current limit) of the VFD.
If the VFD is programmed in a less than optimal way the starting current into the VFD could be as much as the across the line starting current of the motor itself.

 

[mayanees]

I'm more looking for an assumption to make on what an upstream protective device could see, rather than figuring out the size of the protective device.

For example if a motor has a rated inrush current of 600% of it's full load current when not powered through a VFD, the max current that can be assumed during starting through a VFD on the line side of a VFD is XXX% of the full load current of the motor.

This is to set the STPU of an already installed protective device which has an electronic trip unit to ensure it won't trip during starting of multiple downstream loads which are powered by VFD's.

This thread caught my attention so I snipped the answer to that question from JRaef's response and I'm copying the folks in my group at work: The line side of the VFD could see 200-250% for the first three seconds and 150% if it doesn't need to start in less than three seconds.

 

[Besoeker]

Unfortunately, if you are looking for actual numbers and not code requirements, it will depend strongly on the settings (such as ramp time and current limit) of the VFD.
If the VFD is programmed in a less than optimal way the starting current into the VFD could be as much as the across the line starting current of the motor itself.

Most VFDs wouldn't survive such currents.

 

[winnie]

The VFD, when configured correctly, will control the frequency and voltage going to the motor so that the current flowing to the motor is much much less than the normal across the line starting current.

Since the VFD itself is capable of trading voltage for current (it acts as a switching converter, so that the input current can be less than the output current), during acceleration the input current to the VFD is reduced even further.

-Jon

 

[Besoeker]

The VFD, when configured correctly, will control the frequency and voltage going to the motor so that the current flowing to the motor is much much less than the normal across the line starting current.

Since the VFD itself is capable of trading voltage for current (it acts as a switching converter, so that the input current can be less than the output current), during acceleration the input current to the VFD is reduced even further.

-Jon

Yes. It somewhat depends on the design of the DC link filter. If that gives you continuous DC current, the input power factor is about 0.95. That is much better than the average induction motor.

 

[GoldDigger]

Most VFDs wouldn't survive such currents.

They could if for some reason the VFD(s) were oversized.
At that point the NEC would have required the OCPD and wire to be sized to the input amp rating of the VFD, but the OP is not concerned with that.

 

[drktmplr12]

is there a conservative assumption I can use as the (max) inrush current seen on the line side of the VFD when performing protective device coordination with upstream protective devices.

OP, the answer to your question is that inrush will, at maximum, be the overload current rating drive can see for the shortest period. As an example, if the drive can withstand 400% current for 1 second, 250% for 3 seconds, and 150% for 60 seconds prior to tripping, the most conservative figure is 400% of the current rating of the drive. The manufacturer should be able to tell you what the drive can withstand and for how long.

Someone please correct me if what I'm saying isn't logical or I'm misunderstanding the problem.

 

[Besoeker]

OP, the answer to your question is that inrush will, at maximum, be the overload current rating drive can see for the shortest period. As an example, if the drive can withstand 400% current for 1 second, 250% for 3 seconds, and 150% for 60 seconds prior to tripping, the most conservative figure is 400% of the current rating of the drive. The manufacturer should be able to tell you what the drive can withstand and for how long.

Someone please correct me if what I'm saying isn't logical or I'm misunderstanding the problem.

Are you taking account of the differences between input and output current?
VSDs are normally rated in terms of output current.

 

[drktmplr12]

Are you taking account of the differences between input and output current?
VSDs are normally rated in terms of output current.

Good point. I can't seem to find what the rated input current is for common drives in our market.

Input current tends to be lower than output current since the power factor on the load side is going to be lower than the line side. Using the output current would be more conservative albeit not entirely accurate. I think not so conservative that you are penalizing yourself. Feels like I am missing something here...

 

[Besoeker]

Good point. I can't seem to find what the rated input current is for common drives in our market.

Input current tends to be lower than output current since the power factor on the load side is going to be lower than the line side. Using the output current would be more conservative albeit not entirely accurate. I think not so conservative that you are penalizing yourself. Feels like I am missing something here...

I think you got it about right with that last comment. For input protection we mostly used fuse-switches rated for about motor current rating. Of course they come in discrete ratings so that limits the choice.

 

[Jraef]

Unfortunately, if you are looking for actual numbers and not code requirements, it will depend strongly on the settings (such as ramp time and current limit) of the VFD.
If the VFD is programmed in a less than optimal way the starting current into the VFD could be as much as the across the line starting current of the motor itself.

Most VFDs wouldn't survive such currents.

I agree with Besoeker here. Most Constant Torque rated VFDs will have a crowbar circuit to shut them down if the motor attempts to pull more that 200-220% of the VFD rating for more than 2-3 seconds (YMMV). So unless you buy a VFD that is 3X the size of the motor, it is never going to deliver locked rotor current.

Back to the OP's issue. The 600% starting current seen when starting Across-The-Line is not directly related to starting TORQUE, in fact the starting torque is only 150% of FLT when the current is at 600% of FLA, and usually torque and current are the same percentage. What's going on when starting A-T-L is that until the motor accelerates to roughly 80-90% speed, most of that current is just reactive, not active, so it's not doing useful work; in other words your power factor at start-up is around .20 until the motor accelerates the load. The VFD will not do that, the displacement power factor as seen from the line side will remain at around .95 and the current going to the motor is going to be mostly active, so the motor torque and current are closer to normal. Hence the VFD can often accelerate a motor while drawing less current from the line, albeit for a little longer.

It should also be noted that if using a "Variable Torque" rated VFD on centrifugal pump or fan, the peak current capability of the VFD is going to be limited to 150% for a few seconds, 110% for 30 seconds.

 

[Meter Junkie]

Here is a good video that shows what the VFD drive pattern looks like. It doesn't talk about inrush current, but most VFD's soft start.

https://youtu.be/5ZHqNW69eCw

 

[kwired]

Here is a good video that shows what the VFD drive pattern looks like. It doesn't talk about inrush current, but most VFD's soft start.

https://youtu.be/5ZHqNW69eCw

What is different from a "soft starter" though is the soft starter only reduces voltage where a VFD reduces both voltage and frequency. But the VFD can be straight V/F ratio - and often is with variable torque loads, but can also have torque - boost features where they tweak that ratio to get some boost in starting/acceleration torque.

 

[Ingenieur]

What is different from a "soft starter" though is the soft starter only reduces voltage where a VFD reduces both voltage and frequency. But the VFD can be straight V/F ratio - and often is with variable torque loads, but can also have torque - boost features where they tweak that ratio to get some boost in starting/acceleration torque.

a solid state SS does not rectify the voltage
it has a set of + and - switching devices which control voltage peak (and therefore current) to the motor