frequency settings


Thread Starter

ravi chandran

Dear sirs,

I am wondering that most of the inverter manufacturer have given that motor speeds can be increased by increasing the frequency. For example if the motor is rated at 50 Hz , 3 phase 415 VAC, 1440 rpm. If we increased the freq. at the inverter ( ex. 100 Hz ) what are the possible damages will happen to the motor.

Thanks in advance,
ravi chandran

Adolfo Jimmy Saldivias Valarezo

Ravi Chandran:

Most of the damages that can occur by operating the motor at a speed much higher than nominal will come from the mechanical part attached to the
motor. You can create a torque for which the machine was not designed.

More worrysome than high speeds are low speeds. Motors are designed to be auto ventilated.

That means that they will get rid of the heat generated by their own blower. If you decrease the rotating speed of the motor, this blowing will not be enough to get the heat out of the motor.

Heat inside means that the isolation of the motor will suffer.

Suffering means that it can fail sooner and short circuit the motor (burn it).

You can avoid this if you use forced ventilation to maintain the motor cool though.

There is rule of thumb used by several manufacturers: you can increase or decrease speed within 25% of the nominal speed without many concerns.

As all rules of thumb you may want to be careful with this one.

Normally the customer/user is the one who knows best his/her process and is the one who will define if he wants to ac drive to increase the speed to a certain limit.

Beware of customers/users who do not know the mechanical limitations of their own systems though.

I think you can benefit from a series of articles written about ac drives technologies.

One set of those very interesting articles is at: and there is another set of good information on:

Hope it helps

Jimmy Saldivias
[email protected]
If the bearings can take it and the fan . . . no damage will occur to the motor, but perhaps damage to the connected load?
You should check with the motor manufacturer about max. allowable speed e.g. bearing limitations, balance limitations and windage losses. Also, note that above base speed the V/Hz ratio is decreasing which results in the field
weakening effect like dc motors. The torque approximately drops in proportion to the speed ratio above base speed. Another way to say the same thing is that this is the constant power region so 2x speed = 1/2 torque. One trick around this limitation is the motor in low voltage connection and modified V/Hz knee point. However, the drive must now be oversized accordingly.

Tom Gianni

Vitor Finkel

Out of the top of my head:

Possible motor damages:

Voltage insulation would need to work under aprox 830 Volts ( twice frequency means twice speed and twice the voltage, if you need to keep torque, current and power ), else sparks and motor winding short-circuit, or short to ground.

Centrifugal forces might implode the rotor or "release" parts of the rotor with heavy motor damage/danger to bystanders, if (when) it reaches approx. 2880 RPM

Vitor Finkel [email protected]
P.O. Box 16061 tel (+55) 21 285-5641
22222.970 Rio de Janeiro Brazil fax (+55) 21 205-3339

Adolfo Jimmy Saldivias Valarezo

Ravi Chandran:
I quote from: "Selecting Motors for Adjustable Frequency Drives"

"Although standard three-phase motors are used in most adjustable frequency drive applications, motors should not generally be selected, sized and
applied in AF applications as if they were to be operated from 60 Hz sine wave power. The nature of the adjustable frequency power source and the
complete range of operating conditions must be carefully considered to ensure that the application will be successful. This article attempts to provide an understanding of the principles of selecting and applying AC motors in adjustable frequency drive systems. "

You can check the whole article at:

Jimmy Saldivias
[email protected]
Responding to Ravi's query related to doubling frequency.

From a pratical standpoint, the motor's impedance will double. Hence, without increasing voltage too, it is very unlikely that the motor will
develop any useful torque.

Phil Corso, PE
Trip-A-larm Corp

Trevor Ousey

A sawline I worked on had VF drives, and the chipping heads would run up to 100hz plus a bit. Sounds great with the logs going into it. Two problems we looked at was the bearings used and the rotor construction. The motor manufacturer should be helpful here. We used ABB motors and ABB Sami Drives, they were 75, 90 and 132 kW units.
Trevor Ousey.
[email protected]

Kevin Corliss

At one of my customers we had a problem on two 1/2 HP drives. The motor leads were in excess of 100' and we were experiencing what is called "reflected wave phenomenon". Upon start-up we were spiking the motor with excessive voltage and this was breaking down the insulation on the motor windings. In the installation
directions for new drives it will give you a recommended motor lead length. Also there are products available to address this problem. Such as line reactors, and using special cabling for your motor leads. This problem has come into the spotlight with the evolution of drives using IGBT technology.

I would recommend talking with your distributor and follow their specifications closely to eliminate any possible start-up issues.

Kevin Corliss
Corliss Automation and Design
[email protected]
I'm pretty sure you can work out the max frequency you motor can handle by using the following formula. (This is going back to my college days)

Max Frequency = (RPM*No of Poles) / 60

You should find the RPM and No of Poles on the motor.
If I'm wrong please let me know.

Richard Bartrug

One word.......Don't

Running a motor beyond it's rated frequency not only creates a risk of damaging the motor, but also the inverter. As a repair tech, I have seen the results of experiments like this on my bench more times than I care to remember.