Overload Protection by VFD

I do not know what "bimetal relays" are; are you referring to a "contactor" or "motor starter"?

Part of your question is answered by the particular electrical code that is enforced by your local authority having jurisdiction:

For installation of a motor, you will typically have either a motor starter or a vfd. A motor starter can be an electrically controlled, motor rated contactor, or a manual motor rated switch. You can also select a vfd for motor control. Whether you select a vfd or motor starter, the circuit to the motor and protection is mandated by the particular code that is enforced in your area. If you are in the U.S., then National Electrical Code likely applies and proper motor protection is prescribed in article 430.

In accordance with article 430, the circuit conductor size is generally rated 125% of full load motor current and the circuit breaker that protects this circuit may be a maximum of 250% of full load current. Additionally, the motor requires thermal protection and a local disconnect switch.

If you purchase a vfd as a packaged unit, sized for the particular rated motor load, then it should come with built in thermal protection, and will likely also have a built in disconnect switch. If you have only the vfd unit as a separate device that you are installing in an enclosure, without thermal overload and disconnect switch, then I would advise purchasing a motor rated switch with thermal overload. The thermal overload, or "heater" as they are sometimes called, is selected from the manufacturer's chart for the particular motor rating. The switch with thermal overload is connected in series with the vfd and motor.

Thank you very much to all for replying. I put my quention by giving an example. We are having 110kw conveyor drive driven by recent 132kw abb vfd. Conveyor maximum running full load current run around 140amps against motor rated current of 180amps. If i want to protect my equipment by keeping overload around 155 amps, providing conveyor should start on loaded condition. I have got similar applications with siemens vfds too. I have tried a lot but did not get satisfy. Bimetal in this case works absolutely fine and gives good equipment protection. Please share your views.

Jraef gave you a superb answer and you must read carefully.

The VFD is programmed against the motor data nameplate. If you programmed this data in, then the vfd has built in overload-overvoltage-torque-Lock rotor....etc. monitoring and tripping points built in.

What make and model of VFD do you have? With most VFD's if not all, electronic (solid state) overload will not work and will cause false tripping issues. If you insist on a thermal overload, you can but need to size properly.

The point of rules and regulations toward these types of installation is to protect conductors, devices and equipment and personnel. If you are trying to perform this through programming of the VFD, you would have to lower the programming data points for FLA and this I highly suggest you do not do!!!!!!! unless you truly understand all factors associated with these changes.

Go with the bimetallic if you need that additional comfort, but, otherwise, if programmed properly, your VFD should do the work of protection. The first tell tell of being "alright" begins with the running amps. If the amps is lower than Nameplate data, then you motor-conductors-...etc. if sized properly are not under duress.

If the running amps is higher than nameplate, then begin investigation of probable cause. If suspect is VFD, repair and or replace..................Good luck

Download one of the manuals from the Misubishi range of VFDs, some 200 pages, but a good read, lots & lots of information on overloads as mentioned above. I use a motor overload circuit breaker to feed them just in case something goes wrong, but they are so well made and sophisticated now, they are never needed.

Both ABB and Siemens make excellent VFDs and also offer cheap simplistic VFDs. If yours does not have the features you need to accomplish this simple task, you probably opted for the cheap stripped down models. You get what you pay for. Another possibility is that you are not reading the entire manual, or someone has not provided you with access the the "advanced programming" parameters in the VFDs that would allow you to do this. That would be something to take up with your suppliers. If you bought cheaply over the internet and have no support available to you, again, you get what you pay for. This is not a difficult task for most good VFDs.

Thank you very much to all for your positive inputs.

Talking about vfd models i am using, they are reasonably advanced supplied by Siemens and ABB. I have gone through manuals too. I have consulted drive manufactures too. But i did not get proper solution.

Function of VFD is to limit the current if it programmed for overload or overcurrent purpose. Yes, vfd does well through thermistor if used in motor. Without temperature detector, vfd's motor overload function is not accurate. Generally vfd's does not distinguise overload and overcurrent, which is quite misguiding. Even some times we do not find any overload alarm or/and fault in advanced vfd's too.They give only overcurrent alarm and fault which is the part of overload also.

Overload relays( IDMT based) in motor starter takes care of equipment nicely than VFD's in case of overload. Vfd is excellent at speed and torque control as per application. I am putting my concern to save equipment through current based overload protection.

In conveyors starting torque demand and running torque demand is far different. In such scenario, vfd's overload function is not meeting the task what IDMT relays of motor starter is providing.

I hope you all got my point. Please correct me if anything concluded wrong.

"Function of VFD is to limit the current if it programmed for overload or overcurrent purpose. Yes, vfd does well through thermistor if used in motor. Without temperature detector, vfd's motor overload function is not accurate."

This is just plain inaccurate. The motor overload protection algorithm in every single MODERN VFD I am aware of is Inverse Time (IDMT) based. You are either misinterpreting the manuals of your VFDs, or you have some other agenda in continuing to denegrate the capabilities of the VFD protection schemes.

Thank you very much JRaef.

you have some other agenda in continuing to denegrate the capabilities of the VFD protection schemes.

No quention of carrying any agenda of putting down vfd's protection capabilities. In fact, i am a big fan of vfd's. My intention is to get the information which i did not recieve from drive manfacturer's.

The motor overload protection algorithm in every single MODERN VFD I am aware of is Inverse Time (IDMT) based

Please share exact algorithm or parameters of MM440 drive of Siemens family related to IDMT based overload. I will be grateful to you if you provide me the information.

Please reply.

Don't be ridiculous, I'm not privy to Siemens' internal motor protection algorithms. I wasn't even when I worked there and if I had been, I could be sued for divulging them. But I assure you, in all of the Micromaster drives they are I²t based. All of their literature says so by the way.

The programming parameters are not directly specific to the protection algorithm, that is internal. But when you program ALL of your motor data into the proper parameters, starting at P0300, the protection will be as advanced as you need it to be based on how many of the 30+ motor data parameters you want to enter. If you ignore some, the mP will sometimes fill in estimates or standard motor data based on Siemens motors, which may or may not match your motor, but that's the way they are...

They also have INVERTER current protection features, in and around P0290, and the MM440 can be programmed to trip on a specific amount of current or torque as a way to protect mechanical components at or around P2170. Were you perhaps confusing those features with being the motor overload protection scheme? They are separate ADDED features.

From a Designer's view the over load protection is so designed that it closely matches the heating curve of the motor. The thermal time constant of a motor can be of the order of 15 - 20 minutes hence for protection from overloads the relay should have a time constant slightly lower than this.

A thermistor is directly in the vicinity of the winding, provides an accurate account and does not necessarily be associated with a VFD as is being projected.

What the thermistors are capable of are exactly matched by a Thermal Image Relay which does not requires a probe inside the motor frame. In this case both the +ve and _ve sequence currents are monitored by a relay to accurately asses the temperature of the motor windings.

IDMT based over current relays, VFD inbuilt mechanism as well as bimetallic overload relays are all approximate methods, none of these are superior to each other, only accurate adjustments which is a matter of hit and trial or everyday experience provides the protection which is normally considered adequate but there is no scientific criteria since the thermal time constant varies from motor to motor.