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.

Motor Considerations

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.


 Motor Performance

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

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:

  1. Breakaway or starting torque
  2. Minimum or “pull-up” torque
  3. Breakdown torque
  4. 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.

HP RPM ENC’L FRAME Part Number ENC’L FRAME Part Number
0.3 1800 TEFC 56C F00030CNAC TENV 56C N00030CNAB
0.5 1800 TEFC 56C F00050CNAC TENV 56C N00050CNAB
0.75 1800 TEFC 56HC F00075CNAC TENV 56C N00075CNAB
1 1800 TEFC 56HC F00100CNAC TENV 143TC N00100CNAB
1 1800 TENV 145TC N00100CNAC TEBC 143TC B00100CNAB
1.5 TENV 143TC N00150CNAB
1.5 1800 TENV 145TC N00150CNAC TEBC 145TC B00150CNAB
2 1800 TENV 182TC N00200CNAC TENV 145TC N00200CNAB
2 TEBC 143TC B00200CNAB
3 1800 TENV 184TC N00300CNAC TENV 184TC N00300CNAB
3 TEBC 184TC B00300CNAB
5 1800 TENV 213TC N00500CNAC TENV 184TC N00500CNAB
5 1800 TEBC 184TC B00500CNAC TEBC 184TC B00500CNAB
7.5 1800 TENV 215TC N00750CNAC TENV 254TC N00750CNAB
7.5 1800 TEBC 213TC B00750CNAC TEBC 213TC B00750CNAB
10 1800 TENV 256TC N01000CNAC TENV 256TC N01000CNAB
10 1800 TEBC 215TC B01000CNAC TEBC 215TC B01000CNAB
15 TENV 254TC N01500CNAB
15 1800 TEBC 254TC B01500CNAC TEBC 256TC B01500CNAB
20 TENV 284TC N02000CNAB
20 1800 TEBC 256TC B02000CNAC TEBC 256TC B02000CNAB
25 1800 TEBC 284T B02500CNFC TEBC 284TC B02500CNAB
30 1800 TEBC 286T B03000CNFC TEBC 286TC B03000CNAB
40 1800 TEBC 324T B04000CNFC TEBC 324TC B04000CNAB
50 1800 TEBC 326T B05000CNFC TEBC 326TC B05000CNAB
60 1800 TEBC 364T B06000CNFC TEBC 364TC B06000CNAB
75 1800 TEBC 365T B07500CNFC TEBC 365TC B07500CNAB
100 1800 TEBC 405T B10000CNFC TEBC 404TC B10000CNAB
125 1800 TEBC 444T B12500CNFC TEBC 444TC B12500CNAB
150 1800 TEBC 445T B15000CNFC TEBC 445TC B15000CNAB
200 1800 TEBC 445T B20000CNFC TEBC 445TC B20000CNAB
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