VFD I2t 2

[fab1961]

Dear all,

I would like to know what happens in standard VFDs when motor current drawn exceeds the 160-200%
of the motor plate.
In my application I am changing from a DOL start to A VFD. Motor is a standard 4 pole async 15 kW 400 V.
My concern are the unwanted trips. In fact a standard overload relay chart shows multiples of the current up
to 10 times the nominal one. I' ve been using VFDs quite a lot and I understand that wtih th VFD the starting
current is much less (depending of corse on ramp times, inertias etc) but the fact is that short overcurrents
of say 3-4 times the nominal motor current are "masked" by the overload relay. Currently I am oversizing the
VFD to have more current available (standard products give 200% motor current available roughly speaking)
but to protect the motor I have to guess. I would appreciate a lot some comments.

Thank you

 

[fab1961]

Sorry, the bitmap attached is only an example.
The motor current is 28 A.

 

[itsmoked]

I'm not clear on your question..

Most VFDs now provide all the protect you need. You don't usually add more unless you have multiple motors driven by one VFD.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Subject to correction by jraef;
First you are looking at the wrong curve and I don't mean just the scale.
You should be looking at the motor speed/torque curve..
For a VFD you read the motor chart a little differently.
1 Determine the motor speed. This will be zero at start.
2 determine the actual output speed setting of the VFD.
Subtract motor speed from VFD speed to get the slip speed (often converted to frequency and refered to as the slip frequency).
From the speed/torque curve, measure down from synchronous speed. you will see the expected torque and the expected current.
Example:
Motor rated at 1760 RPM, 1800 synchronous speed.
Motor at standstill. VFD output at 40 RPM (1800RPM-1760RPM). The curve should show 100% accelerating torque and 100% current.
Motor at standstill and VFD output at 60 RPM. The curve is fairly linear in this region and should show about 150% full load current and an accelerating torque of about 150% of full load torque.
To recap, use the actual motor speed and the actual VFD output speed to determine the slip speed and then read down from the synchronous speed of the torque speed curve to determine the expected torque and current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill is right in that you have to "un-think" of your overload protection curve as being relevant to what the VFD does with the motor. The thermal damage curve of the motor remains unchanged of course, but the I²t trip curve of a standard overload relay is intended to match or exceed that thermal damage curve, meaning it will trip befor the curve is exceeded. Because that OL curve must accommodate an Across-The-Line (DOL) start, you NEED the trip curve to allow for those higher currents. When starting ATL however, the high current in the beginning is mostly reactive current because of the high slip, as Bill was describing. The motor still sees it and it still affects the thermal damage curve, but it is not yet doing useful work. The full torque capability of the motor is not reached until 80% speed or higher, but the heating started immediately. Hence the OL must make allowances for it, but still track the overall cumulative effect.

But with a VFD, the drive is controlling that slip and starting off with, in theory, allowing 100% of the available Full Load Torque of the motor from the very outset. That means the motor is always performing useful work (torque), and the motor heating is now only related to overcoming the inertia of the load. VFDs, in days gone by, often did not provide an I²t overload curve, they originally just had a fixed trip percentage, usually at 150% current for 60 seconds, because that was where an I²t curve coincided with the motor thermal damage curve at locked rotor torque (with a safety margin). As the years passed and drive technology progressed, we are now able to make the motor provide full Break Down Torque, which is typically HIGHER that Locked Rotor Torque, at any speed, even down to Zero speed. So if the motor is running at slip speed and you want BDT to reaccelerate it after a step change in load, that 200% BDT will require 200% current which means you are changing the relationship to the motor thermal damage curve. With that also came regulatory changes that resulted in many of the drive mfrs going to the same I²t curve used in OL relays. UL and CSA, here in North America for example, began requiring this in about 2005, but many small inexpensive drive that don't have UL/CSA listing still do not provide an I²t trip curve.

But in reality, you never get to the scenario you described where the motor current may spike to 400% when you run a motor from a VFD. The VFD is never going to allow that to happen because that would damage the VFD transistors (unless maybe the VFD is 2x the motor size). So what the VFD will do is, depending on your programming, either trip itself off with what we call DRIVE Over Current, or go into current/torque limit, in which it overrides whatever speed command is given to it and reduces the speed to reduce the load to protect itself. It will re-accelerate it when it can under the restrictions in place, but it will not spike the current. If, in attempting to re-accelerate, the drive puts out as much current as it safely can for itself, but the motor cannot accelerate the load with that resulting torque, you will run into the I²t curve and it will trip on motor OL.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington