Flux (Φ), magnetizing current (IM), and torque are all dependent on this ratio. Increasing frequency (F) without increasing voltage (E), for example, will cause a corresponding increase in speed. Flux, however, will decrease causing motor torque to decrease. Magnetizing current (IM) will also decrease. A decrease in magnetizing current will cause a corresponding decrease in stator or line (IS) current. These decreases are all related and greatly affect the motor’s ability to handle a given load.
Constant Torque AC motors running on an AC line operate with a constant flux (Φ) because voltage and frequency are constant. Motors operated with constant flux are said to have constant torque.
Actual torque produced, however, is determined by the demand of the load.
T = K Φ Iw
An AC drive is capable of operating a motor with constant flux (Φ) from approximately zero (0) to the motor’s rated nameplate frequency (typically 60 Hz). This is the constant torque range. As long as a constant volts per hertz ratio is maintained the motor will have constant torque characteristics. AC drives change frequency to vary the speed of a motor and voltage proportionately to maintain constant flux. The following graphs illustrate the volts per hertz ratio of a 460 volt, 60 hertz motor and a 230 volt, 60 Hz motor. To operate the 460 volt motor at 50% speed with the correct ratio, the applied voltage and frequency would be 230 volts, 30 Hz. To operate the 230 volt motor at 50% speed with the correct ratio, the applied voltage and frequency would be 115 volts, 30 Hz. The voltage and frequency ratio can be maintained for any speed up to 60 Hz.
This usually defines the upper limits of the constant torque range
Reduced Voltage and Frequency Starting You will recall that a NEMA B motor started by connecting it to the power supply at full voltage and frequency will develop approximately 150% starting torque and 600% starting current. An advantage of using AC drives to start a motor is the ability to develop 150% torque with a starting current of 150% or less.
This is possible because an AC drive is capable of maintaining a constant volts per hertz ratio from approximately zero speed to base speed, thereby keeping flux (Φ) constant. Torque is proportional to the square of flux developed in the motor.
The torque/speed curve shifts to the right as frequency and voltage are increased. The dotted lines on the torque/speed curve illustrated below represent the portion of the curve not used by the drive. The drive starts and accelerates the motor smoothly as frequency and voltage are gradually increased to the desired speed. Slip, in RPM, remains constant throughout the speed range. An AC drive, properly sized to a motor, is capable of delivering 150% torque at any speed up to the speed corresponding to the incoming line voltage. The only limitations on starting torque are peak drive current and peak motor torque, whichever is less.
Some applications require higher than 150% starting torque.
A conveyor, for example, may require 200% starting torque. If a motor is capable of 200% torque at 200% current, and the drive is capable of 200% current, then 200% motor torque is possible. Typically drives are capable of producing 150% of drive nameplate rated current for one (1) minute. A drive with a larger current rating would be required. It is appropriate to supply a drive with a higher continuous horsepower rating than the motor when high peak torque is required
Constant Horsepower Some applications require the motor to be operated above base speed. The nature of these applications requires less torque at higher speeds. Voltage, however, cannot be higher than the available supply voltage. This can be illustrated using a 460 volt, 60 Hz motor. Voltage will remain at 460 volts for any speed above 60 Hz. A motor operated above its rated frequency is operating in a region known as a constant horsepower. Constant volts per hertz and torque is maintained to 60 Hz.
Above 60 Hz the volts per hertz ratio decreases
Frequency V/Hz
30 Hz 7.67
60 Hz 7.67
70 Hz 6.6
90 Hz 5.1
Flux (Φ) and torque (T) decrease:
Horsepower remains constant as speed (N) increases and torque (T) decreases in proportion. The following formula applies to speed in revolutions per minute (RPM).
Field Weakening Motors operated above base frequency can also be said to be in field weakening. Field weakening occurs whenever there is an increase in frequency without a corresponding increase in voltage. Although an AC drive could be setup for field weakening at any speed, it typically only occurs beyond base frequency.
We have seen that below base speed, in the constant torque region, a motor can develop rated torque at any speed. However, above base speed, in the constant horsepower region, the maximum permissible torque is greatly reduced
Field Weakening Factor A field weakening factor (FFW) can be used to calculate the amount of torque reduction necessary for a given extended frequency.
For example, a 60 Hz motor can only develop 44% rated torque at 90 Hz and 25% rated torque at 120 Hz
Selecting a Motor AC drives often have more capability than the motor. Drives can run at higher frequencies than may be suitable for an application. In addition, drives can run at low speeds. Self cooled motors may not develop enough air flow for cooling at reduced speeds and full load. Consideration must be given to the motor.
The following graph indicates the speed and torque range of a sample motor. Each motor must be evaluated according to its own capability. The sample motor can be operated continuously at 100% torque up to 60 Hz. Above 60 Hz the V/Hz ratio decreases and the motor cannot develop 100% torque. This motor can be operated continuously at 25% torque at 120 Hz. The motor is also capable of operating above rated torque intermittently. The motor can develop as much as 150%* torque for starting, accelerating or load transients, if the drive can supply the current. At 120 Hz the motor can develop 37.5% torque intermittently.
The sample motor described above is capable of operating at 100% rated torque continuously at low frequencies. Many motors are not capable of operating continuously at 100% continuous torque at low frequencies. Each motor must be evaluated before selecting it for use on an AC drive
Note) Torque may be higher than 150% if the drive is capable of higher current.
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