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