Large Horsepower medium voltage Synchronous motors require special starting techniques and equipment. Synchronous motor starters have proven to be a maintenance headache at times. A medium voltage VFD can sometimes resolve this issue.
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Synchronous motors can be very large. The largest we’ve run across was a 100,000 HP motor at a hydroelectric Dam project.
There are a number of reasons why people use large medium voltage synchronous motors. Synchronous motors run at precise speeds, as opposed to a more standard induction motor which exhibits slip. Synchronous motors have high efficiency, and have a leading rather than lagging power factor. This becomes very important, especially in high horsepower applications, which is where you typically see synchronous motors.
These very large motors are a problem to start. IF these were induction motors, you could just use a reduced voltage starter to limit inrush. These allow the motor to “slip” during starting.
With large Synchronous motors however, you normally can’t do that, so they traditionally use more a more elaborate electro-mechanical starting system. These large horsepower, medium voltage motor starters are complex, expensive, and require maintenance.
In recent years, we’ve seen a trend toward using a medium voltage VFD to get the synchronous motor up to speed, and then use a bump-less line transfer to put the motor across the line once there. In most cases, the motor is started unloaded, so you only have to size the VFD large enough to spin the unloaded motor. Since a VFD never pulls the motor out of synch, it starts smoothly.
We’ve even seen instances where a 13KV motor, was started using a 4KV medium voltage drive via a transformer, and still showed a cost benefit over the old starter system.
EMA sells and services medium voltage drives, and can assist you with upgrading your synchronous motor starter to a much more efficient and maintenance friendly VFD starter. (see our products )
Contact your nearest EMA office by using the contact info on the right, or at the bottom of this page.