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May 2011 Newsletter
In This Issue
What is Motor Slip?
Invertek Optidrive E2
Control Techniques Commander SK
Control Techniques Unidrive SP
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Issue:13May 2011
Greetings!

Welcome to the latest newsletter from The Motor Control Warehouse (MCW).     

 

This issue of our newsletter focuses on motor slip. Explaining what motor slip is and how our different AC inverters compensate for it.

What is motor slip?

As the load on the motor increases the speed of the rotor decrease, this difference in speed is commonly known as slip.

 

3 phase AC motorWhen an AC motor is running under load the actual rotor speed is less than the synchronous speed of the motor. This difference in speed is called the slip of the motor.

 

In certain applications this speed difference can have a large effect on the performance of the overall system. An AC inverter can help reduce slip in a number of ways.

 

In a closed loop system where the drive has position feedback from the motor, the drive is able to accurately measure the speed of the motor. It will then vary the frequency of the supply to the motor to maintain a constant speed. This allows very accurate speed control, and the speed will not vary when load is applied to the motor.

 

Our open loop drives do not have any feedback from the motor, however they do have algorithms written within the software that will sense when the motor is running at load by measuring the current the motor is drawing and then increasing the frequency to the motor to counteract the slip.

 

Motor slip compensation is essential when the motor needs to maintain a constant speed under different loads. Example of where this is necessary include:

  • Lathes 
  • Saws
  • Conveyors
  • Extruder's
  • Winders

Below are our most popular AC drives; the Commander SK, Unidrive SP, and Optidrive E2 with details of how they work to reduce slip. 

 

Invertek Optidrive E2

A look at how Inverteks Optidrive E2 works to reduce motor slip.

 

The Optidrive E2 is Invertek's very popular, simple to use open loop drive.

 

The E2 has been designed to be as simple for the user as possible. The E2 works on a fixed volts to hertz principle, where the drive generates a linear Volts/Hz output. The output voltage and frequency are held at an exact ratio. 

 

The E2 does have rotor slip compensation in its software.

 

When the motor is under a heavier load, it will draw more current and the drive will increase its output frequency in order to compensate for motor slip. The drive works out how much it needs to increase the frequency to get the required speed using the motor rated speed (parameter 10) and the current that the motor is drawing.

 

Typical speed regulation for open loop systems is in the region of 2 to 3%, and this is fine for the majority of simple applications. 

 

For more information on our Invertek E2 inverters please see the link below:

 

Invertek Optidrive E2 

 

Control Techniques Commander SK

A look at how the Commander SK works to reduce motor slip.

  

The Control Techniques Commander SK is an open loop vector drive. 

 

The Commander SK will use the data required whilst pro-forming an auto tune to control the magnetizing (or field) current in the motor. This allows the drive to produce much better starting torque in the motor than can be achieved in a simple V to F open loop drive.

 

The SK will also use this technology to reduce the motor slip. It achieves this using the motor rated speed (entered into Parameter 07). Achievable speed regulation is in the order of 1%. 

 

For more information on our Commander SK please click on the link below 

 

Control Techniques Commander SK 

 

Control Techniques Unidrive SP

A look at how the Unidrive SP reduces slip in both open and closed loop applications.

 SP family

The Control Techniques Unidrive SP is the most advanced AC drive we sell. The SP can be run in open loop, closed loop, servo, and regenerative mode.

 

In open loop mode the SP has a special feature called Rotor Flux Control (RFC). This is an algorithm built in the drive that gives near closed loop performance in an open loop application.

 

The inverter will use the phase voltages, currents, motor name plate information and parameters generated by the auto tune facilities to simulate the motor and therefore estimate the speed of the rotor. It then uses this data for drive feedback. This will greatly reduce motor slip and achievable speed regulation is in the order of 0.5%.

 

When highly accurate speed, torque, or position control is required then a closed loop system is needed. A closed loop system has feedback from a device fitted to the motor (usually an encoder). The encoder (or other feedback device) will tell the drive exactly how fast the rotor is turning so that the drive can offer speed regulation in the order of 0.01%. The encoder will allow the system to all but cancel out the motor slip as it always knows the exact speed of the rotor.

 

When the drive is in servo mode, the drive will operate in a similar way to a closed loop system.

 

For more information on the Unidrive SP please click the link below:

 

Control Techniques Unidrive SP 

 

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Thank you for taking the time to read our newsletter, we hope that you now know a little bit more about The Motor control Warehouse and the services we can offer. For our contact details please follow the links opposite.
    

Sincerely, Motor Control Warehouse.