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Asked 26th May, 2018

# Is it possible to increase the speed of the motor over its rated speed?

For example motor with 1400 rpm to 2000 rpm by using the inverter (speed changer).

## Most recent answer

I agree with Mohamed EL-Shimy

## Popular Answers (1)

Yes, but with a small amount above the maximum speed under normal rated conditions. Increasing the speed can be achieved by increasing the supply frequency as the speed and frequency are directly proportional; however, there are two constraints. The first is the saturation of the magnetic circuit, but this issue can be significantly avoided by keeping the V/f ratio constant. Consequently, the maximum speed that can be achieved with this ratio kept constant is also constrained by the permissable steady state terminal voltage magnitude of the motor. The second constraint is the mechanical withstand capability in terms of speed and possible vibrations.

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## All Answers (20)

Dear Sir,

If you are using an asynchronous motor, a synchronous motor, or a permanent magnet motor: Yes. You of course need to make sure it is mechanically OK. (Bearing lifetime, centrifugal forces on the rotor.)

Apart from increasing the frequency of the applied voltage, the voltage itself needs to be increased proportional to the frequency.

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Yes it is possible but not necessarily adviceable. You gave a bit limited information, so I am not sure of your application. Assume it was an induction machine, designed for 50 Hz and 1400 rpm. (Often speed of these machines are 1450 rpm for nominal power). By making input of 60 Hz for example by a frequency changer, it would run relative to the new input frequency but you could easily get saturation and heat problems. You have to be sure the motor is designed for it.

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Dear Abbas,

For AC machines, this can be achieved by flux-weakning. This is done both motoring and generation application.

You can have a look on our following paper:

Regards,

Mohamed

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Yes, but with a small amount above the maximum speed under normal rated conditions. Increasing the speed can be achieved by increasing the supply frequency as the speed and frequency are directly proportional; however, there are two constraints. The first is the saturation of the magnetic circuit, but this issue can be significantly avoided by keeping the V/f ratio constant. Consequently, the maximum speed that can be achieved with this ratio kept constant is also constrained by the permissable steady state terminal voltage magnitude of the motor. The second constraint is the mechanical withstand capability in terms of speed and possible vibrations.

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Dear sir

The structure of the motor have to withstand such a high speed. If that is the case, electrically there are two method to obtain speeds more higher than the rated :

1-v/f method (as explained by researchrr above)

2-voltage injection in the rotor circuit this method is applicable for only slip-ring induction motor .

Hope that helpful for you.

Thanks

1 Recommendation

In regard with the rotor injection method of speed control mentioned by Dr. Ezzalden, the induction machine becomes Doubly Fed Induction Machine (DFIM). The method is very effective, but it may be considered a non-traditional method of motor speed control, while it is dominant in wind power generation. You may read,

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28th May, 2018

Université des Sciences et de la Technologie d'Oran Mohamed Boudiaf

Motor over-speed is possible (i.e. over the base motor speed) only in the de-fluxing region that is constant power. Obviously one can increase the speed by increasing the frequency using an inverter but that does not mean that the motor would follow suit that speed with no risks!!! Unfortunately, with the increase of speed at least two conditions should be observed to avoid the failure of the motor:

1-

**Motor mechanical limits**: As a colleague said: is it mechanically OK to go to that speed? I’ll explain more in detail. Obviously that depends on the motor state either from:a) The motor physical mechanical parts aspect such as bearings limits, etc.. (new or old motor may react differently) or

b) The motor special operating aspect resulting from the over-speed which may lead to abnormal behavior and the degradation of the motor performance due to secondary effects such as heating, vibrations and noise ( due to very high centrifugal forces on the rotor resulting in dangerous situations for the operator and the machinery itself).

**2-**

**Driven load:**The increase of speed imposes working in the de-fluxing zone (i.e. flux reduces as an hyperbolic function with increase in speed), obviously this results in a quick decrease of the maximum torque of the motor and therefore the second condition to observe is the operating point or the equilibrium point between the motor torque and the load torque (caused by the resistant torque of load that is to be driven). So increasing the speed beyond the base speed is possible depending on the type of load and its magnitude (that is it depends on the application for which the motor is used). The maximum motor torque in the over-speed region is inversely proportional to the speed. So increasing the speed reduces the torque capability of the motor and therefore may not be able to drive the coupled load.

**NB**: Maybe one has also to take specific actions related to the motor over-speed operation such as forced ventilation, motor derating, etc..

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30th May, 2018

As several people have already stated, it is possible to increase the speed of an ac motor by increasing the input frequency to the motor. This is done in industry by adding a variable frequency drive. Again as several people have mentioned this brings about it's own set of challenges in the form of excess heat generation, reduced torque, and mechanical strain, possibly resulting in premature failure of the motor or driven load. A good rule of thumb used in industry is 25% over frequency is acceptable, but as you approach 50% you need to look for a different RPM motor.

The same is true for a reduction in speed due to the inability to maintain torque and remove heat.

Dear Abbas,

yes, it can be achieved by field weakening or so called (defluxing ) for more clarification you can have a look at the following:

1 Recommendation

Yah, it is possible, For AC machines, this can be achieved by flux-weakning and study the suitable coupling effect its very important

Yes, it is possible; but please be cautious regarding definition of term:"rated speed"; this is nothing but the speed at which the generated back-emf in the stator is equal to rated voltage if the flux density is at its rated value.

As this definition shows, the rated speed is something different from "maximum permissible speed", the latter being confined due to mechanical limitations of the motor and drive system.

I hope this helps.

1 Recommendation

Yes it is possible. Actually the standard is definied by the norm. Formula= Nominal speed * 1.2. (20% higher than rated speed for 2 minutes. To prove mechanical strenght of the rotor. So, acctually you are limited by mechanical strength of designed motor/rotor, plus, you have to keep in mind bearings of the motor. Motor should not work in overspeed conditions.

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Dear Nikola Grgic ,

I think you are referring to maximum permissible speed at steady state, where it can be exceeded for a short duration. But as I mentioned, the maximum allowable (permissible) speed is different from the rated speed: the rated speed is related to the electromagnetic aspects not the mechanical considerations (as I believe).

If we increase the voltage of induction motor ,then it's speed will increase or decrease,

If it increases ,so is it possible it attains synchronous speed

Dear Jitendra Singh Bhadoriya ,

The speed of an induction motor of conventional type (not the doubly fed type) is always below synchronous speed, and the increase of voltage can not change this fact. Also, the voltage can not be increased beyond the rated value, otherwise it will deteriorate the winding insulation.

Kind wishes

Naser

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