"Underspeed a motor" - will it work or fry?

[HomeMadeSin]

OK, I know I can overspeed a motor using a VFD, it just changes the nature of the torque (constant/variable). However, I want to take a 400HZ motor (0.5HP, 22,800 rpm, 5.5A 3Phase) and spin it at 60HZ. No problem as stated, but I want to get the HP out of it at 60HZ ideally. What would happen if I put a 0.5HP load (at 60HZ) on this with full voltage (230V 3 phase) at 60HZ? I have the feeling that it's gonna fry, but I'm hoping for some chance.......

 

[Lewish]

Well, first off, what is the nameplate rating of your 400Hz motor?

Any motor operated below the synchronous speed must have the input voltage reduced. This is the V/F curve. As you go down the curve, your horsepower and torque will decrease. By how much, is a function of the modulation scheme of the VFD. VFDs are designed to accomodate the requirements of the motor at reduced RPMs.

One thing to consider not related to the voltage, is keeping the motor cool at the reduced RPM. The cooling of the motor is designed for the nomial RPM of the motor. In almost all cases, I have had to add external cooling fans when running motors at reduced RPMs.

 

[busbar]

The iron laminations will likely saturate, drawing excessive current with no {or any} load at 60Hz.

 

[mdonner]

In 17 years of military equipment maintenance I have seen several ocations of 208/3/400 motors fed 208/3/50 by mistake (usually 1.5-2HP blower motors), that almost immediately let out smoke.
Maintaining V/F yields 230*(60/400)=34.5V max voltage at 60 Hz. To keep V/F you will need a transformer or an inverter and you will get only a fraction of the shaft power, plus you will need external cooling fan (even for TENV frame).

 

[RadarRay]

The others have stated this but I will state this in a different way.

HP is proportional to Torque and Speed. The Torque obtainable is likely the same across the speed range, up to base speed, depending on the type of VFD and its setting. Therefore the HP is proportional to speed. 60hz gets you 60/400 * 0.5HP or 0.075HP. Make sure you know what you want, torque or hp. After you think you know, work the system out in torque requirements, this is a mistake many seem to make.

 

[electricpete]

Hopefully the conclusion is already clear. At the risk of bearing a dead horse I'll add my two cents.

I believe that unstated in radarray's
calculation is the assumption that votlage still needs to be reduced by 60/400 to avoid overheating the iron. I know radarray is aware of this but the original poster may not be.

Even if you set up this reduced voltage, there will also be significantly reduced cooling from the motor-driven fan, which will create further problems.

So even if you are tempted to try to achieve that tiny horsepower output 0.075hp you still need to reduce voltage and contact motor manufacturer to determine further derating which might be required as a result of reduced internal cooling.

 

[HomeMadeSin]

Thanks for the input. Here's the nameplate info:

0.5HP 3x220V 5.5A 400HZ 22800RPM SF1.6
AMB TEMP 35C CONT.DUTY THERMAL PROTECTED

It is a Grundfos 50mm submersible motor used for sampling. If you are aware of the typical Franklin 4" submersible motors, consider this the ultimate mini-me.

The pump impeller will require a certain hp (or torque) at a given speed. In my case, the impeller requires approx. 0.5 HP at 3450 rpm (or 0.76 lb ft). The motor puts out 0.5 @ 22800 ( or 0.12 lb ft), which is why I think (but wanted to verify) that she will smoke..he he. The impeller power required varies by the cube when realted to speed, so the HP req'd at 22800 would be monstrous.

Perahps the Hp versus Torque mistake is what I'm making per radarray's post. I guess I need to understand the constant torque range versus the constant HP and use torque at a given speed to figure it out. I always assumed that I could overspeed a motor and get the HP I needed (for example a 0.5 HP 4 pole 60 hz nameplated motor could deliver 0.5HP at 120hz which would in effect emulate a 0.5HP 2 pole motor). However, could you get the 0.5HP at a lower than nameplate Hz.

 

[electricpete]

Don't worry about the details of the torque and the power... that obscures a key point:

If you put 230v on this motor it will smoke from overfluxing the iron, regardless of the load.

 

[electricpete]

overfluxing of iron may have been a bad term. The iron will heat from high flux. The copper will heat from high magnetizing current. It will either trip or smoke.

 

[RadarRay]

Electricpete,

You are correct that the voltage has to decrease when the motor speed decreases. I tried to use the catch all "VFD set up" to cover the situation.

I did not say it but perhaps I should have, I was not addressing the motor heating question. Others have had good input, so I did not want to address it.

My only point was,Homemadesin, stated his requirement in HP but then reduced the speed he wanted to run at. If he calculated the HP and then did not gear the system in at base speed, he would not be getting the correct HP output. Not everyone seems to understand that when using HP calculations, it assumes a gear in at base speed ( I am not addressing the issue of direct drive here). It would be a shame for Homemadesin to address his motor cooling problem with an then find out he did not have enough torque to make the application work.

But thanks Pete for bring up this understatement in my post.

 

[HomeMadeSin]

Well gentlemen, nothing proves me wrong than empirical analysis. I hooked up a VFD to the motor, attached the impeller and submerged it. When I fired it up, it......ran great! It has been running for an hour now fine. The VFd is providing the following data: 40V, 4.7 A, 60HZ (3600rpm). The only thing I haven't been able to verify yet is the actual rpm. If it is running at 3450, then the impeller load is 0.5HP (or 0.76 lb ft). Now my only concern is leakage current being sent into the water (this pump will find its way into aquaculture systems).

 

[HomeMadeSin]

I'm still wrestling with how this motor is able to generate this load. If I use the nameplate data, the torque would be 0.115 lb ft (T = HP * 5250 / rpm, where HP = 0.5 and rpm = 22800). If the motor is operating in constant torque at frequencies under base (which is 400Hz), then it shouldn't provide anything over 0.115 lb Ft, right? If so, then the motor wouldn't be spinning at 3450 as the impeller load would exceed the capacity of the motor. And if that is so, I would expect the motor to be running at much higher temps than 54F, which it is doing.......I'm lost.

 

[HomeMadeSin]

Just wanted to add that I was able to measure the speed, which came in around 1588 rpm. After some quick calculations, it seems this makes sense. If the impeller HP required at 3450 is 0.5, it would require 0.05HP at 1588 (or 0.16 lb ft). This torque is amazingly close to the service factor torque of the motor (SF = 1.6). I just can't believe it isn't smoking! It is as cool as a cucumber.

 

[electricpete]

It sounds like your calcs are in agreement with radarray. He predicted 0.075hp at 3450. Reduce that to 1600/3450*0.075 = 0.35. You are loaded to 0.05hp which is a factor of 0.05/0.035 ~ 150% higher. Sounds like you are on top of it.

 

[jbartos]

Suggestion: Check with the VFD manufacturer tech support for the application to your motor. Also, any suitable temperature sensor inside the motor would be helpful to make sure that the motor is not overheated.

 

[GusD]

Hi HOMEMADESIN

I believe you metioned that this motor is thermally protected.If the windings were overheating this feature should protect your windings.Some submersible pumps have great cooling capabilities, so overheating may not be a problem.
The only mistery is the fact that apparently it has not lost any HP or needed TQ.
Interesting.

GusD

 

[HomeMadeSin]

The VFD is a Baldor 10HP unit. I plan to run some tests today and varying the 3pt voltage and frequency. This is how this particular drive allows me to increase the v/hz above linear (0 volts @ 0 hz, full volts @ full hz, and x volts at y hz where I get to choose x and y). Then I'll monitor the speed to see what results I get as well as the skin temp (which has been extremely cool to the touch).

 

[induction]

It will get hot. Cooling unit may work.

 

[electricpete]

I don't understand the mystery based on the analysis that HMS and radarray provided

Trated=0.5*5250/22800 = 0.115 ft-lbf

TserviceFactor=1.6*0.115=0.18 ft-lbf

Load torque at 1600 rpm ~ 0.16 lb ft is slightly lower.

Is the mystery related to whether there should be additional derating (beyond constant torque) due to reduced cooling from motor internal fan? Is it possible that this high-speed low-horsepower motor doesn't even use any fan action for cooling?