# Why are induction motors not run at high frequency ?

#### firebreather

Joined May 27, 2013
19
As frequency increases, induced emf and hence current and torque in the rotor coil increase, for a given input current to the stator coil. The point is that we can operate such motors at very high frequency like 10 kHz, at low input currents like 100 mA, thereby reducing the power consumption of the motor, but at the same output torque.
Then why is it not practically implemented, instead of typical low frequencies like 50,60 or 400 Hz ?

(If the problem is that at high frequencies, reactance increases, thereby producing low current to the stator,we can easily eliminate this by inserting a capacitor in series ,to resonate the circuit and eliminating reactive part totally. As frequency increases,capacitor size also reduces.)

#### shortbus

Joined Sep 30, 2009
8,809
Induction motors are run at a higher frequency in some areas of need. But no where near your 10kHz. Aircraft and some places like oil field work and mining use 400Hz. This is for lighter weight, since the higher frequency cuts down on the amount of iron and copper.

10kHz would be very hard to use in anything other than sub fractional HP induction motors. And there isn't much call for them, in the real world. Because a DC motor fills that area very well. Then you would have the expense of a Dc to AC inverter in the mix.

#### crutschow

Joined Mar 14, 2008
27,734
Motors run at the main's frequency so the question becomes why are the main's frequency not higher? That's primarily due to the increase in various loses, including long transmission losses, at higher frequencies. Thus the mains are mostly run at 50Hz or 60Hz. In the early days some mains ran at 25Hz.

#12

Joined Jul 18, 2013
23,630
It is not only the frequency that determines rpm but pole count.
You can obtain higher rpm by using a P.M. rotor motor, such as AC synchronous and BLDC.
You will find the 400Hz 2 pole 24,000rpm induction motor popular now in the DIY CNC router users.
But due to the small dia rotor to obtain this rpm safely, the low inductance restricts them to around 100hz min.
Max.

#### firebreather

Joined May 27, 2013
19
But i didn't mean that the motor should rotate at 10 kHz !
Instead, what i meant was that, keeping the stator current constant, say at 100 mA, since induced current in rotor is proportional to rate of change of magnetic field from the stator, but independent of stator current, so we have a higher starting torque and the angular velocity increases as high as friction and external load permit.
Here we have power consumption of the device = stator current X Mains voltage
= 100 mA X 220 V rms = mere 22 W !
And morever, producing this high frequency can be done by external electronic tank circuits.

#### Ramussons

Joined May 3, 2013
1,060
As frequency increases, induced emf and hence current and torque in the rotor coil increase, for a given input current to the stator coil. The point is that we can operate such motors at very high frequency like 10 kHz, at low input currents like 100 mA, thereby reducing the power consumption of the motor, but at the same output torque.
Then why is it not practically implemented, instead of typical low frequencies like 50,60 or 400 Hz ?

(If the problem is that at high frequencies, reactance increases, thereby producing low current to the stator,we can easily eliminate this by inserting a capacitor in series ,to resonate the circuit and eliminating reactive part totally. As frequency increases,capacitor size also reduces.)
As frequency increases, so does the Impedence of the Rotor, current will actually reduce.
Higher frequencies will lead to higher eddy losses.
And, irrespective of Frequency, the Motor Output can Never be Greater than the input.

Ramesh

#### firebreather

Joined May 27, 2013
19
Of course impedance will increase at higher frequencies, but we can balance it by inserting a series capacitor so that the net impedance is zero/very low, leading to the same input current as that with low frequency.
O.K that o/p can never exceed i/p, but where is the flaw in this design ??

#### Ramussons

Joined May 3, 2013
1,060
As frequency increases, induced emf and hence current and torque in the rotor coil increase, for a given input current to the stator coil.
Corrected:

As frequency increases, induced emf increases
and
(hence)
current reduces
and
therefore torque in the rotor coil (increase) remains same, for a given input current to the stator coil.

As shortbus said, 400Hz is a standard in aircraft for reasons mentioned, but at higher frequencies, feeder losses and Eddy losses becomes an overriding factor - at least for commercial applications.

As far as higher frequencies are concerned, I remember reading some time back about motors, the size of a pin head operating at 40 KHz - they were stepper motors for some very specific application.

Ramesh

#### DMahalko

Joined Oct 5, 2008
182
Many modern 3-phase induction motors ARE run at high frequencies, using a variable frequency drive.

It is possible to save up to 90% power using a VFD with a 3-phase induction motor, to only run it as fast as needed for an application, rather than all-out all the time and for example restricting pumped flow with a gate valve.

The drive usually lets the end-user set the maximum operating frequency of the drive, which is preferably higher than 1-8 kHz because it is audible to humans as a constant droning high pitch whine coming from the pole faces of the motor.

As mentioned, there is waste energy at higher drive frequencies, so it is a tradeoff. With a higher drive frequency a 3-phase motor will run a little warmer from eddy current losses, though with less annoying shrill audible noises for people that have to work around it.

I don't know if single-phase VFD's exist. It is possible, but probably not usually done because normal single-phase motors must include out-of-phase starting windings or capacitors, which would interfere with the actively driven VFD input.

(Yah, I've worked on improving the Wikipedia Variable Frequency Drive article....)

(Oh, and you can play music on industrial 3-phase motors using a specially-designed VFD and an audio source...)