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4 Motor Control Methods for Variable Frequency Drives

4 Motor Control Methods for Variable Frequency Drives

The motor control method used for variable frequency drives (VFDs) has a substantial impact on an application’s performance, efficiency and even the cost of operation. Once engineers and operators understand the benefits and limitations of each method, they will be able to choose which is best for their application.

The four major motor control methods are Volts-per-hertz (V/f), V/f with an encoder, open-loop and closed-loop. Find out which method is right for your motor-controlled application.

V/f Control

V/f is a simple, plug-and-play motor control method that is commonly used for multiple reasons:

  • The drive needs very little data from the motor for control
  • An encoder is not required
  • No tuning of the VFD is required (however, it is recommended)
  • Less wiring leads to lower costs

When to Use it

V/f should be used if there is a chance that operations could exceed 1000 Hz. It is often used with machine tool, spindle, fan and pump applications. It is also the only control method that allows multiple motors run from a single VFD. If all motors need to stop and start at the same time and follow the same speed reference, this motor control method should be used.

Limitations:

  • There is no way to ensure that the motor-shaft is rotating
  • Starting torque of motors is limited to 150% of its output at 3 Hz
  • Can only have one V/f pattern programmed at a time

Specifications and Uses

  • Speed regulation is usually between 2-3% of maximum frequency
  • Speed response is rated at 3 Hz
  • Speed control range is 1:40

Users can calculate the minimum running speed VFDs need to achieve to control a motor by multiplying the maximum frequency by the 1:40 ratio:

At 50 Hz maximum frequency
1/40 = .025
50 x .025 = 1.25
A drive using V/f control can control a motor down to 1.25 Hz

Users can use single parameters to select preset V/f patterns in VFD programming that are optimized for specific applications. Users can also program custom patterns for specific applications or motors.

V/f With Encoder

Although uncommon, encoders can be added to V/f control when two conditions are met:

  1. Applications require more precise speed regulation
  2. Applications need the ability to run at a higher reference frequency

Benefits

  • Reduces speed regulation down to 0.03% of maximum frequency
  • Voltage and frequency are preset, allowing for high-speed control without high dynamic responses

Limitations

  • Required encoders and feedback cards increase costs
  • Higher operating frequencies are limited based on the number of pulse-per-revolutions generated by the encoder.

Open-Loop Vector Control

Open-loop vector (OLV) is method of control where motor speed and torque can be controlled independently.

When to Use it

OLV should be used when operators require much more dynamic control over their applications. Because of a higher starting torque at lower speeds, and the availability of four-quadrant torque limits, OLV is very useful for a wide variety of applications.

The four quadrants of OLV are:

  1. Positive torque & Forward motor rotation
  2. Positive torque & Reverse motor rotation
  3. Negative torque and Reverse motor rotation
  4. Negative torque and Forward motor rotation

Torque limits can be set to prevent damage to machinery and can be set independently for each quadrant. Users can also choose to program an overall torque limit into the VFD.

Benefits

  • Produces 200% of its rated torque at 0.3 Hz
  • Speed regulation of ±0.2% of maximum frequency
  • Speed control range of 1:200
  • Dynamic controls that allow users to increase and decrease torque and motor speed and also set limits
  • Quicker speed response (10 Hz) makes it responsive to impact loads
  • Uses a vector algorithm and motor feedback to discover the most optimal output voltage

Limitations

  • Relies heavily on motor data and parameters
  • Requires auto-tuning to ensure it is getting data from the motor
  • Cannot guarantee that the motor shaft is responding because there is no feedback
  • Must be auto-tuned for optimal performance

Closed-Loop Vector Control

Closed-loop vector control is similar to OLV in that it employs an algorithm to determine output voltage, but this method of control requires an encoder to operate. This method of control is best suited for applications that hold loads without moving (cranes, elevators, etc.).

Benefits

  • 200% of rated torque at 0 rpm
  • Speed responses over 50 Hz
  • Speed control range of 1:1,500
  • Zero speed operation

Closed loop control also allows for motors to operate in torque-control mode, which favors torque over speed. This makes this method of control ideal for winders/rewinders, capping and other similar applications.

Limitations

  • Must employ and encoder
  • Must be auto-tuned for optimal performance

 

AMMC can help you find the right Variable frequency drives and control methods for your applications. Contact us today to speak with an automation specialist.