AC Drive Characteristics
The use of adjustable speed industrial equipment is increasing due to the need for better equipment control and for energy savings where only partial power is required.
The principle of speed control for adjustable frequency drives is based on the fundamental formula for a standard AC motor:
In the formula:
- Ns is the synchronous speed in RPMs
- F is the frequency in hertz
- p is the number of poles in the motor
The number of poles in a motor is determined when it is manufactured and cannot be changed.
An adjustable frequency control regulates the frequency (f) supplied to the motor. The motor speed (Ns) increases and decreases in proportion to the supplied frequency. The output frequency of the control is adjustable by means of a potentiometer or external signal.
The control can automatically maintain the required voltage/frequency ratio for any motor speed. This provides for maximum performance over the motor’s speed range.
Because the frequency output of the control is infinitely adjustable over the speed range, the speed of the motor is also infinitely adjustable.
An AC control is mainly used to operate one standard 3-phase, 60 hertz, squirrel cage induction motor. Groups of motors can be connected to the control if the total full load amps of all the motors does not exceed the control’s amperage rating and if all motors operate at the same speed and are started at the same time. The figure shows how the amperage output of the control is affected by starting motors across-the-line after the inverter output is at full voltage.
A control can operate synchronous induction or permanent magnet AC motors. However because of the lower power factor of these types of motors, the next higher rated control should be used.
When operating a fan-cooled motor at reduced speeds, the ability to dissipate heat is reduced because of the slower speed of the cooling fan. The figure below provides a conservative guide to follow for continuous operation at lower speeds, with a constant torque load.
Typical motor performance is shown in the figure on the previous page. The figure below represents the maximum continuous capability of most AC induction motors when operated with a variable frequency control. Above 90 Hz, horsepower drops off approximately as shown in the figure. If “framing-up” is necessary, use the next size drive. At low speeds, most high efficiency and standard induction motors provide torque approximately as shown in the figure.
A motor operating above its base frequency with the voltage clamped at base voltage is in the constant horsepower range. Most four-pole or slower motors operate satisfactorily in the range of 60 Hz to 90 Hz. Above 90 Hz horsepower decreases. Caution should be exercised on two pole motors driven above 4300 rpm, or any motor driven above 90 Hz. Consult Polyspede or the motor manufacturer if there are any questions.
Motor Torque is defined at four points as shown in the figure above for a typical NEMA design B motor. The numbered items in the figure are:
- Breakaway or starting torque
- Minimum or “pull-up” torque
- Breakdown torque
- Full load torque
The curve represents the motor’s accelerating torque from zero to full speed when rated voltage and frequency are applied. A variable frequency control always operates the motor on the portion of the curve to the right of point 3 or it would trip due to excessive current.
AC Synchronus Induction Motors
Ordering AC Motors using 10 digit part number.
Part Number nomenclature:
* Baldor motors are warranted for 18 months and are made of cast iron frames frames
** Lincoln motors are warranted for 5 years, blower motors are warranted for 2 years.
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