Vfd's ??

[Mike01]

Mechanical equipment calls for a vfd to control multiple fans, (one large vfd / multiple fans) the fans will be running at 96hz however what happens when the vfd fails and the bypass kicks in at 60hz the output produced by the fan will be reduced but will this damage the motor at all??

 

[a.bisnath]

I don not think so

I don not think so

once the motor windings are inverter duty it will just run slower should look at motor nameplate or motor spec sheet to see that frequency(60HZ) is within operating range of motor,seen this in furnace fans running at 95hz not very common though

 

[ron]

An inverter rated motor should be able to still cool itself at 2/3 speed.

 

[Jraef]

I'd be more worried about the motors running at 96Hz. They are designed to run at 60Hz. Running at 96Hz does NOT increase the HP unless they have increased the VFD output voltage (and by default the line) to roughly 770V. Otherwise you have a constant HP from the motor but a HP demand from the load (fan) that is 60% higher than it was at 60Hz. So unless those fan motors were seriously over sized, they are going to overload at 96Hz.

If they considered all this in advance, then absolutely no problem running the motors at 60Hz in bypass as far as the motors go. But you will have less fan flow.

Also just FYI, hopefully you have separate OL relays and short circuit protection for each individual motor, regardless of how they are powered.

 

[Besoeker]

I'd be more worried about the motors running at 96Hz. They are designed to run at 60Hz. Running at 96Hz does NOT increase the HP unless they have increased the VFD output voltage (and by default the line) to roughly 770V. Otherwise you have a constant HP from the motor but a HP demand from the load (fan) that is 60% higher than it was at 60Hz. So unless those fan motors were seriously over sized, they are going to overload at 96Hz.

If they are centrifugal fans, and most are, the torque required will go up with the square of the speed and power as its cube so the power at 96Hz would be around four times that at 60Hz speed. The other side of the coin is that, if the fan is designed for 96Hz operation, then at 60Hz it will do very little. Bypass operation just might not be worth the hassle.

Also just FYI, hopefully you have separate OL relays and short circuit protection for each individual motor, regardless of how they are powered.

Totally agree.

 

[Strahan]

Mechanical equipment calls for a vfd to control multiple fans, (one large vfd / multiple fans) the fans will be running at 96hz however what happens when the vfd fails and the bypass kicks in at 60hz the output produced by the fan will be reduced but will this damage the motor at all??

I'm not certain of what your asking; so I'll go in a different direction than the rest. If at the time the bypass kicks in you are running at 96HZ the motor will experience a "speed difference jerk" it will instantly slow to 60hz. Will this create mechanical problems for the motor or the load on the motor I'm not sure but it would be something I would want to investigate and watch closely.
Some VFD's actually have a parameter built into them to prevent this from happening (of course when your running off of the vfd) called Flying Start. This parameter when enabled will adjust the frequency of the output pulses to match the motor speed in order to prevent speed difference jerk. Example VFD gets a stop command and motor is still coasting at 30hz, vfd then receives a start command under usual circumstances the vfd will ramp from 0hz which would mean for a time the motor would jerk to a much slower speed. The flying start would see the motor was coasting at 30hz and would accel from this point. Is this application that critical that you need a bypass? Why not come up with some way of alerting people of VFD failure so it can be addressed a.s.a.p

 

[Mike01]

thanks

thanks

thanks for all the great responses I am not a motor guy however am trying to understand the basics of VFD's and how they affect motor operation by adjusting the frequency, from what I have found that motors need to be NEMA MG1 inveter rated that way when operated at the high or low end the insulation is rated for the higher voltages and spikes where in a standard motor you would have insulation breakdown and failure of the motor, and to get additional CFM by changing the frequency the fan will rotate faster?? thus producing a higher CFM, motors have indivigual overlaods and short circuit protection, is it common to power multiple motors from a single VFD??

 

[Mike01]

faster rotation?

faster rotation?

another question if the fan is rated at 60hz and 1800rpms and you adjust the frequency to 90 hertz the new speed would be apprix. 5400rpms what is the max speed of the motor rpms before adverse damage would occur? also the volts per herzt ratio would drop from the 7.67 @ 460 to about 5.1 so how would that affect the motor / drive??thanks again

 

[ptonsparky]

Check with your motor mfg but I asked the posed the same question for a 30hp 1750 rpm motor a year ago to my local motor shop. He & I were both suprised with the answer of 180hz for the motor. Torque is baaad at this point but the motor will stand it. Your driven equipment is another problem.

 

[Besoeker]

another question if the fan is rated at 60hz and 1800rpms and you adjust the frequency to 90 hertz the new speed would be apprix. 5400rpms

At 90Hz the motor speed would be about 2700 rpm, not 5400.

what is the max speed of the motor rpms before adverse damage would occur?

Cage motors are fairly robust. I have routinely run 50Hz machines at 80Hz with no adverse effects. Ptonsparky has had a figure of three times rated frequency and that doesn't altogether surprise me but that's information you'd need to get from the motor manufacturer.

also the volts per herzt ratio would drop from the 7.67 @ 460 to about 5.1 so how would that affect the motor / drive??thanks again

First, a couple of points.
Power is speed times torque. This is a simple but quite fundamental and important relationship.
For normal operation of an induction motor, speed is proportional to frequency to a close approximation.

Assume that the motor is rated for 460V at 60Hz. Base speed.

At up to 60Hz with constant v/f ratio, (0-460V, 0-60Hz) the motor can produce rated torque at any speed. This is known as the constant torque region of operation. Because power is speed times torque, the available power varies with speed. Half speed is half power.

Above 60Hz, the voltage stays the same and the v/f ratio reduces as you already pointed out. The motor available torque reduces in proportion as the frequency (and speed) increases. In this mode, you get less torque and more speed. Twice the speed gives you half the torque. Again note that power is speed times torque. So, twice the speed and half the torque gives you the same power. This is known as the constant power region.

 

[Mike01]

why??

why??

so why would you want to operate at a higher frequency? higer fans speed (hvac application) so more air flow?? but by operating at a higher frequency the voltage remains the same correct? however the tork will drop, by increasing the frequency would this not also create additional heat required to be disipated?

 

[Besoeker]

so why would you want to operate at a higher frequency? higer fans speed (hvac application) so more air flow?? but by operating at a higher frequency the voltage remains the same correct? however the tork will drop, by increasing the frequency would this not also create additional heat required to be disipated?

Taking your points in order:

The need or preference for running a motor at higher frequency (above base speed) is to suit the application. We have done this for multi-motor applications on textile lines and on steel mill roller tables where higher speeds are required without the need for higher power. Typically these have been 150% to 200% of base speed on fairly small motors (<5kW). We have also done this for some pumps where the required operating speed was above base speed or it fell between 4-pole and 6-pole motor speeds.
Fans would give more air flow at higher speeds but I can't recall any application where I seen that done. Maybe for the situation you mentioned it was done to allow a physically smaller fan to be used?

Yes, operation at above base speed would be at constant voltage. There are, as always exceptions but these would be generally considered as special cases.

Yes, above base speed the available torque drops.

Would operation at higher frequencies increase the losses? TBH, I hadn't given much thought to that before. If it was higher frequency and full flux, there is little doubt that it would increase iron losses. But, above base speed, the V/f ratio reduces so the flux reduces and I think that would prevent any great increase in losses. Also, if the motor has a shaft mounted cooling fan, cooling will be greater the faster the motor turns.

 

[Jraef]

This is why I questioned this at the outset. It appears you're not the one who designed this but at least you are smart enough to ask questions. It's good that you are, somebody needs to. Lots of people have misconceptions about what you can and cannot do with VFDs and someone may have misapplied them here. It happens a lot unfortunately.

When you run a motor above base speed, you are running in Constant HP mode, which means, as noted, your motor shaft torque is dropping proportionally to the speed increase.

In a centrifugal fan, the load torque requirement is INCREASING with speed. If you want to move more air, you need MORE power to run the fans.

So in this application, these two issues are at crossed purposes. At 90Hz you have cut the motor torque in half while the HP at the shaft remains the same, and at the same time you have created the situation with the fan that it is now demanding 337.5% of the HP it would at 60Hz (power increases at the cube of the speed change in a centrifugal load, so at 1.5X speed, the fan will required 1.5 x 1.5 x 1.5 horsepower). The only way this will work is if the fans, and the VFDs running them, are over 3X the size you would use at 60Hz. So in other words if you needed 3/4HP at 60Hz, you have better be using a 3HP motor to run it at 96Hz.

If your motor is not over sized, running it with lower output torque by running over speed puts it at risk for running at higher than rated slip, which indeed increases current draw and therefore heat in the motor. While it's true that some types of motors with fan cooling will get more cooling as well, there is always a chance it cannot keep up. If you have a TEFC / TENV motor it may not be able to take it at all.

 

[weressl]

I'd be more worried about the motors running at 96Hz. They are designed to run at 60Hz. Running at 96Hz does NOT increase the HP unless they have increased the VFD output voltage (and by default the line) to roughly 770V. Otherwise you have a constant HP from the motor but a HP demand from the load (fan) that is 60% higher than it was at 60Hz. So unless those fan motors were seriously over sized, they are going to overload at 96Hz.

If they considered all this in advance, then absolutely no problem running the motors at 60Hz in bypass as far as the motors go. But you will have less fan flow.

Also just FYI, hopefully you have separate OL relays and short circuit protection for each individual motor, regardless of how they are powered.

Due to the limited short circuit delivery capability of drives, SC protection is a useless excercise on that side. The bypasses should be individual combination starters for each motor.

 

[weressl]

At 90Hz the motor speed would be about 2700 rpm, not 5400.

Cage motors are fairly robust. I have routinely run 50Hz machines at 80Hz with no adverse effects. Ptonsparky has had a figure of three times rated frequency and that doesn't altogether surprise me but that's information you'd need to get from the motor manufacturer.

First, a couple of points.
Power is speed times torque. This is a simple but quite fundamental and important relationship.
For normal operation of an induction motor, speed is proportional to frequency to a close approximation.

Assume that the motor is rated for 460V at 60Hz. Base speed.

At up to 60Hz with constant v/f ratio, (0-460V, 0-60Hz) the motor can produce rated torque at any speed. This is known as the constant torque region of operation. Because power is speed times torque, the available power varies with speed. Half speed is half power.

Above 60Hz, the voltage stays the same and the v/f ratio reduces as you already pointed out. The motor available torque reduces in proportion as the frequency (and speed) increases. In this mode, you get less torque and more speed. Twice the speed gives you half the torque. Again note that power is speed times torque. So, twice the speed and half the torque gives you the same power. This is known as the constant power region.

As a general rule, the larger the motor is the less it will be cappable of operating above its nameplate rating. I'm thinking in 100's and 1000's of HP.
Lubrication can be a problem both on the high side and low side of speeds.