Motor Speed

As u increase the frequency the skin effect resiatnce of widing conductors increases resulting in increased copper losses and heating.the bearings can take some amount of over speed .You need to be more specific as to what is the speed rated and increased speed, etc to give a better answer

If you consult the motor manufacturer, they will tell you what is permissible & what is not, regarding motor overspeed.

I would be hesitant about doing this with an old motor or a motor that is of "open" construction i.e. has holes in the frame to facilitate ventilation. It is well known that "open" frame motors absorb lots of moisture when they are shut down for any length of time. They also tend to be quite dirty as a result of their "open" construction. Under situations of extra stress (e.g. overspeed), they may fail quickly.

Any increase in supply frequency above that of normal (as specified on the motor rating plate) will place extra stress upon the windings & therefore the insulation due to the increase in heat (hysteresis losses).

What you ask about can be done but I would certainly be contacting the manufacturer of the motor before you do anything.

At a sawmill where I used to work , we used VFD's to speed up motors over design frequency. I was told by a VFD tech most common rotors should be able to turn the speed of an equivalent two pole motor. ie 3600 rpm in most cases. We speed up our planer motors to unreal speeds using stators wound 440 V, connected to 600V VFD's. It was very common practice for us to run 7 1/2 Hp Baldor TEFC 1200 RPM motors at 90-110 hertz. We used the 1200 RPM motors as they braked better than an 1800 RPM motor in our uses. They did have to be wound VFD spike rated though. At over design frequency the torque does drop off quite quickly.

Vic

It is generally true that motor manufacturers typically only use one set of mechanical components for all of their motors, i.e. the bearings, races etc., so that rule of "2 pole design" is a popular idea. But it is dangerous to ASSume it to be true 100% of the time. Some manufacturers who make low cost components will in fact use cheaper components on 4 and 6 pole machines knowing that they should not be run faster than the synchronous speed. If you over speed them, they could come to pieces! It is always best to simply ask the motor manufacturer, they will tell you, and if not, don't use their motors!

Torque drops off because as you increase speed over the design, you cannot at some point keep increasing voltage, because you get to max. voltage at max. speed. That is why, in the above example, they are using 4380V motors and 600V drives (assuming a 600V supply as well). Motors need a constant V/Hz ratio to provide rated torque. a 460V 60Hz motor is designed around a V/Hz ratio of 460:60, or 7.67:1. If you are able to increase the voltage to 600V, you can increase the speed to 600/7.67 or 78Hz before torque would begin to drop off.

Using the same principal then, one trick often employed by people who need high speeds is to use a 240V motor and supply it with 480V from a VFD. Then set up the VFD to provide full voltage (480V) at 120Hz. That way you will get full torque at double the speed, making it double the HP. Again, you must ensure that the motor can mechanically take that speed, and of course the VFD must be sized for the maximum 240V current.

I think it will be better to tell the size and class of the motor and its application, and type of bearing.

The life of ball bearing is related to speed of shaft, axial, radial load and lubrication condition on the bearing, as long as the three quantities comply with the manufacturer's recommendation and lubricants are applied properly, there shouldn't be any problem with the bearing. However, if the motor is hotter in the new operating condition, and the heat is transfered to the bearing, the grease in the bearing may be affected and cause bearing to suffer from a shorter life(defects on inner and outer raceways and rolling elements).

Dear all,

The motor with unbalanced counter weight is used for creating vibration in hopper.The following are the motor name plate details.

Make: FRIEDRICH, 1.7KW, 240/415V, 5.38/3.11,50Hz, 960 rpm,Delta/Star, pf 0.77, IP 65, F class insulation,Bearing-NJ 310 EC4

Having idea to increase the frequency upto 60Hz.

Forget about the warranty from the manufacture, if you have a proper thermal protection for the winding, and you think it is required by your application, if I were you I will just try it out, it shouldn't damage the winding.

However The motor will be operating at higher slip value, hence a higher rotor current frequency, which will tend to overheat the rotor, broken rotor bars or deformed rotor may be encountered if you are not lucky, especially when you are driving an unbalanced load.

It might not help you by increasing frequency without increase voltage, and it will depend on your load characteristics and the new motor torque and speed curve. Stalling or crawling of motor is possible due to reduced torque.

Dear friends,

Thank you for all of you.Just now I have visited the manufacturer (FRIEDRICH)of vibrator motors website and downloaded operating manuals.

It is not advised to run the motor more than 50Hz. Because at 60Hz the vibration increases by 44%, which would heavily affect the lifetime of bearing.

But there is no problem to winding, because it doesn't need higher starting torque, and it consumes 60% of rated power only. So reserve capacity is there.

But still planning is there to increase the frequency a bit (max-53 Hz) , since we need higher TPH.

Hi,

60 Hz will not at all be a problem.

We often drive motors rated to 50 Hz at 100 Hz.

Be cautious to have an adequate filter (inductance) to remove most of the voltage-transients as these will damage the insulation. And be sure that cooling is adequate.

If the motor is failing in the windings after some time then it is highly recommendable to get a motor with insulation class H or C (cost some % more but will endure much longer) at worse conditions.

May be the bearings fail as load from unbalance rises with the square of frequency! If so then look at the bearing-manufacturer's information, it is very likely that you can replace the ball-bearings with roller-bearings that have higher load-capacity.

RHABE

Both