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The motor control approach you choose for your variable frequency drive (VFD) can considerably impact costs, efficiency, and the application’s overall performance. Understanding which method works best for your situation is integral to a successful application.
There are multiple methods for motor control, the most common being volts per hertz (V/f), V/f with an encoder, open loop, and closed loop. Keep reading to learn more about these methods and pick which is best for your motor-controlled application.
V/f Control
Volts per hertz is the simplest form of motor control of the four. People commonly refer to it as a “plug-and-play” method because of how simple and ready to use it is. For example:
- The drive needs little to no data from the motor for control.
- An encoder is not required.
- The VFD does not require tuning (although it is recommended).
- Less wiring leads to fewer and lower costs.
Best Practices
V/f is most often used with machine tool, spindle, fan, and pump applications if there is a chance that operations could cap 1,000 Hz. It is also the only approach that lets multiple motors run from one VFD. If all your motors need to start and stop at the same time and follow the same speed, this method is the best for your situation.
Disadvantages
- The motor shaft is never guaranteed to be rotating.
- Torque motor startup is limited to 150% of its output at 3 Hz.
- Only one V/f pattern is allowed to be programmed at a time.
Applications
- Speed regulation between 2–3% maximum frequency
- 3 Hz speed response rating
- Speed control range 1:40
The minimum running speed that VFDs must achieve to control a motor can be calculated 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
Single parameters to select preset V/f patterns can be used in the program that are enhanced for certain applications. Custom patterns for distinct operations can also be programmed by users.
V/f with Encoder
V/f with an encoder is more obscure than V/f Hz but can be added to V/f motor control under two circumstances:
- Precise speed regulation is needed.
- Applications require the capability to run at higher reference frequency.
Advantages
- Speed regulation reduces to 0.03% of maximum frequency.
- The voltage and frequency are present. This allows high-speed control without high dynamic responses.
Qualifications
- There are increased costs from required encoders and feedback cards.
- The number of pulses per revolution generated by the encoder can limit the higher operating frequencies.
Open-Loop Vector Control
Using open-loop vector (OLV) motor control allows speed and torque to be controlled separately.
Applications
OLV is the preferred method for users needing more custom motor control over their applications. This method suits a wider variety of applications because of a higher starting torque at much lower speeds and the availability of four-quadrant torque limits.
Four OLV quadrants:
- Forward motor rotation and negative torque
- Forward motor rotation and positive torque
- Reverse motor rotation and negative torque
- Reverse motor rotation and positive torque
Using torque limits can prevent damage to your machines and can be set separately for each quadrant. Operators can pick a program with a torque limit into the VFD.
Advantages
- 200% rated torque produced at 0.3 Hz
- ±0.2% speed regulation of maximum frequency
- Speed control range of 1:200
- Allowance to increase or decrease motor speed and torque with dynamic controls and capability to set limits
- Responsive to impact loads with a quicker speed response (10 Hz)
- Motor feedback and vector algorithm to discover the most optimal output voltage
Disadvantages
- Motor data and parameters heavily required
- Must be autotuned for best performance and to guarantee the motor is translating data
No feedback, never ensuring that the motor shaft is responding
Closed-Loop Vector Control
Like OLV, closed-loop vector (CLV) motor control utilizes an algorithm to establish output voltage. However, a downside to this method is the requirement of an encoder. CLV motor control is best for applications that hold loads without moving (elevators, cranes, etc.).
Advantages
- Zero speed operation
- Speed control range of 1:1,500
- 200% of rated torque at 0 rpm
- Speed responses over 50 Hz
Torque-control mode is available in CLV motor control, which favors torque over speed. This application is best for winders/rewinders, capping, and other similar uses.
Disadvantages
- Requires an encoder
- Must be autotuned for best performance
Motor control methods for VFDs are imperative for everyday operations in facilities everywhere. It is important to know which is best for your application to achieve better performance overall.
No matter your specific application, Motion Automation Intelligence is a perfect starting point for finding your perfect motor control method. We have experts on standby to help whenever you need for whatever you need. Contact Motion Automation Intelligence today to speak to a specialist.
