Running 60-Hz AC motors at 35 Hz

Provided the motor and VSD are correctly set up, there shouldn't be a huge problem running motors at a lower frequency, after all, it is done on pump sets and agitators quite widely. The thing with fan loads is that the power required from the motor varies with something like the cube of the fan's tip speed, therefore reducing the speed of the shaft reduces the power draw from the motor markedly.

Modern VSDs have filtering provisions that limit the harmonics reflected onto the incoming power leads to very low levels, levels that are compatible with radio frequency emission limits. Select VSDs that are CE-marked in particular. There are international standards to be followed that all modern CE-marked VSDs will comply with.

GA PW! VFD's are much better these days than they were 15-20 years ago. The electrical noise would literally show up in the building steel and grounding sensitive equipment was either a nightmare or impossible!

hi

im unsure about air Handling units so not to sure of the method you will be using to control the frequency so i will assume that the AHU contains an inverter (VFD's) then there will be no problem at all. the life expectanncy of a motor can actually be inproved with the use of a VFD as the motor isn't working at full capacity putting less strane on the windings.

bearing in mined that VFD's are designed to keep every thing in balance i.e monitoring for phase in balance and and frequencys so motor will be protected by the inverter

chris

What is mean by VFD?

"Variable frequency drive".

The motors will be able to run at the low frequency since the load on the motor also will be correspondingly reduced. So the motor will not fail that way.

The problem (as an electrical, you may remember) that the Voltage to frequency ratio must be maintained for the ,magnetisation current.

Since you are going at half the frequency, take care of the applied voltage just so that you do not saturate the cores.

Of course the efficiency will be sacrificed, but they are generally with the VFDs.

The motor being primarily inductive, is going to provide sufficient resistance (not correct term electrically I should say impedance ) to the harmonics.

With a bit of filtering you may be able to overcome this issue.

Foot note: as PWS said why not.

Sounds to me like there is an issue with the control system for the AHU that needs to be fixed before you change the speed of any motors. Was the unit not specd properly? Why is it to cold?

Generally speaking, running motors at 35Hz will not damage them if you maintain a sufficient flow of cooling air on the motor. The reduced speed means that the integral cooling fan at the end of the motor might not be providing enough cooling air to keep the motor from over-heating.

If the motor is inside the AHU, the air movement inside may be enough to cool the motor. You'll be in the best position to determine that. Otherwise, you may need to install an auxillary cooling fan for the motor. Since you seem to intend to keep the motor running at 35Hz for extended periods (maybe forever), cooling may be your biggest problem.

As mentioned by others, harmonics is not a problem with the newer generation of drives. You will, however, need to look out for distance between your drive and motor. I can't tell you how to long it can be (it depends on the switching frequency of the drive) but these files may help you (and others as well).

One other thing. It is generally recommended that your motor have a Class F insulation rating if you intend to use a Variable Frequency Drive on it.

Just one question. If you intend to run this motor at 35Hz indefinitely (at least that's what I think you said), why not buy a smaller fan and motor? I prefer to keeps things as simple as possible. Installing a VFD will not simplify things.

regards,

Vulcan

There's a certain area in the mall that is too cold.

This is generally taken care of by adjustment of the dampers in the system for the cold area. Much cheaper than a VFD.

A suitable VFD is unlikely to cause any problems with your motor, but will reduce flow to all areas that fan supplies.

Fan power requirement generally varies with the cube of the speed, flow linearly and head with the square. This is to adjust to the same point on your fan curve.

The actual operating point will vary according to the pressure/flow characteristics of your system (ducting, dampers distribution vents, etc)

Adjustment by damper is cheaper and usually much less prone to trouble.

hello,

frequency control method has some limits.firstly simply it is not easy as to change ac frequency .we have to convert ac to dc then using an inverter convert back to ac with required frequency.if u can afford this better go for v/f control.this method is good to keep the flex density maximum inturn torque and efficency.

consider the case , while motor works at 35 hz and supply voltage reduces ,then the motor wont be working fine.forecast that cases too.

Hello Dear,

What i understood with your question that you want to run the motor at 35 Hz instead of 60 Hz for ruducing the spped of motor. This will not harm the motror winding untiland unless the temeprature rise of the motor is much more tahn the limit.

May be you are using the VFD so this method is also correct.

Thanks WR

Kamlesh