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Article from TMEIC GE e-news (http://www.imakenews.com/getoshiba_mve-news/e_article000154312.cfm)
May 28, 2003
Drive Time!
by Barry Dick
Introducing "Drive Time", a new column in the TMEIC GE e-news. This column will be
published monthly and will feature topics related to drives, motors, and processes that you,
our readers, introduce. Have a question on your mind? Contribute to this column by asking
questions, sending comments, and participating in the discussion.
To talk to us, click on this article and use the "Letters" link in the left column.
Question:
What is Volts/Hz? How does voltage relate to frequency for a motor operated over a speed range?
Answer:
Understanding the relationship between motor voltage and motor speed begins with the basics. A motor
converts electrical energy into mechanical energy using magnetic fields. These magnetic fields flow in a
magnetic circuit in the stator core and rotor core that has been designed with a nominal value of magnetic
flux. The flux is caused by the flow of current in the stator windings. The winding structure in the ac motor
sets up a four-pole flux pattern that rotates at a speed proportional to the applied frequency.
A simplified magnetic flux diagram is shown here.
The strength of the magnetic fields is a function of the arrangement of motor windings and the current
flowing in the windings. The current, in turn, depends on the applied voltage. The motor must be operated
within the designed limits of magnetic flux in the stator core and rotor core material. If the magnetic circuit
is overloaded, or saturated, applying more voltage to the motor results in no additional output.
Question:
In applying and using a motor with a drive, then, how can one be sure to stay within the magnetic flux
rating? After all, a motor nameplate shows the rated motor current in amps, but no rating for the magnetic
flux.
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Answer:
The motor nameplate does have information on the flux rating, but it is implicit. The rated magnetic flux for
the motor is the nameplate Volts/Hz.
In equation form,
Note that
is the expression for voltage in fundamental meter- kilogram-second units and "webers"
is the unit of measurement for magnetic flux.
Therefore, the motor nameplate does contain information about the magnetic flux rating of the motor, very
simply in the base voltage and base frequency. If the flux in the motor is to be kept constant, the ratio of
voltage to frequency must be kept close to constant. That is, the motor voltage must change in proportion
to the motor frequency as the frequency (and speed) change below the base speed.
Questions:
z
z
z
The voltage and frequency ratios are not always kept constant. What about voltage boost at low
speeds?
What about constant voltage operation above base frequency?
And, isn't a V/Hz drive obsolete now that we have vector drives?
Answer:
These are good questions, but they don't contradict the basic notion of operating the motor at (or near)
constant flux. Voltage boost at starting and very low speeds is needed to push sufficient current into the
motor to meet the torque required to start the motor and load. At very low frequencies, the resistance of
the circuit is higher than the reactance, so the voltage must be boosted relative to constant V/Hz to allow
rated current to flow into the motor. The V/Hz ratio increases quickly at low frequencies, but most of the
voltage drops across the motor and cable resistances so that the internal motor flux does not increase as
quickly.
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Constant voltage operation above base frequency means the V/Hz ratio is less than rated, because the
voltage is staying constant, but the frequency is increasing above base value. Because the motor flux is
lower than at base conditions, this operation is also known as field-weakened or flux-weakened operation.
Field weakening is a term carried over from DC motor days, but it does also have some usefulness with
vector drives. Operating an AC motor in field-weakened condition means the frequency alone controls the
speed, as the voltage and current remain essentially constant in this speed range as long as power output
remains constant.
The flux curve shown below at low frequency is for illustrative purposes only.
Vector drives may not have an explicit setting for V/Hz, but they cannot vary much from the rated flux of
the motor to operate it properly. Older scalar drives simply maintain a fixed ratio of motor voltage to motor
frequency. In general, vector drives can control torque-producing current independently from magnetizing
(or flux-producing) current. However, the vector drive must have some strategy for controlling motor flux,
such as maintaining constant flux below base speed or maintaining constant magnetizing current. In all
cases, the result of the control strategy is a nearly constant ratio of motor voltage to motor frequency,
ensuring the motor magnetic circuit is not saturated.
Of course, the transient response of the vector drive in general, is much better than a scalar drive, and
therein lies the vector drive advantage. The capability of the vector drive to separate the torque current
and flux current means it can change the current in the required axis precisely and not simply change
voltage and frequency in a constant ratio.
In summary, the motor voltage is tied to the applied frequency through the magnetic flux capacity of the
motor. Regardless of whether a scalar or vector drive is applied to an induction motor, it rated flux (V/Hz)
cannot be exceeded.
We hope you have learned something about the relationship of voltage and frequency in an AC motor. And,
if you have feedback, we are easy to reach.
This month's response is provided by Barry Dick. Barry is a senior application engineer with TM GE
Automation Systems LLC.
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The above article is provided free of charge and without obligation to the reader or to TM GE Automation
Systems LLC. By utilizing this article the reader expressly understands that TM GE Automation Systems LLC
does not accept, nor imply, the acceptance of any of liability with regard to the use of the information
provided. TM GE Automation Systems LLC provides the information included herein AS IS AND WITHOUT
WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY IMPLIED
STATUTORY WARRANTY OF MERCHANTABILITY OR FITNESS FOR PARTICULAR PURPOSE. The information is
provided as a general reference to the potential benefits that may be attributable to the technology
discussed. The reader is encouraged to perform independent analysis of the technical and commercial
benefits described here in. If you have any questions regarding your project requirements please contact
the TM GE Application Center at 540-387-8070
Published by TM GE Automation Systems LLC
Copyright © 2005 TM GE Automation Systems LLC. All rights reserved.
TM GE Automation Systems LLC, 1501 Roanoke Blvd, Salem, VA 24153, USA
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