Friday, November 20, 2020

Starting Synchronous Electric Motors and Rotary Converters

This photo is of an old AC generator instead of a synchronous motor but, as discussed further below, they are mechanically equivalent.

Synchronous electric motors were the first common type of motor since their design is equivalent to a generator. (Today, induction motors dominate.) But they are still used today for high horsepower applications or speed critical applications such as steel rolling mill stands.

Andrew Jones posted, cropped
16,000HP roughing mill electric motor at the hot mill of Arcelor Cleveland. Removed during a down turn for cleaning.
Hector Soto: I work in the cold mill at Arcelormittal Indiana Harbor. Our sync motor is 25,000 HP and runs 7 generators.
[Some comments indicate that it is a 450 rpm synchronous motor, probably at 13,200 volts. It helps run a roughing stand in the hot mill.]

When operating, the rotor inside turns at the same speed as the rotating magnetic field generated by three-phase power in the stator field windings. When the rotor and magnetic field are synchronized, these motors can develop a lot of torque. But until the rotor is rotating as fast as the field, they have very little torque. So starting them is a problem.

Some comments about powering an old feed mill that used line shafts offered the following YouTube video. We are used to the frame of a motor being firmly attached to the ground so that the torque of the motor turns the load instead of the motor. In this case, the motor is free to turn. So when it starts, just enough torque is needed to turn the stator instead of the rotor. After the stator starts turning at the synchronous speed, the brake bands around the stator are tightened so that the load starts turning (note the flywheel in the background) and the stator stops turning. Note the buzzing sound when it starts. The electricity is really unhappy until the rotor speed gets closer to the speed of the rotating magnetic field.

As mentioned at the top of these notes, the design for a motor is the same as for a generator. It is a matter of energy flow. If the kilowatts put in the wires is more than the horsepower taken out of the shaft, it is a motor. If the horsepower exceeds the kilowatts, it is a generator. The conversion between horsepower and kilowatts is 1 to 0.7457. The difference between the input and output of a rotating device is the loss due to heat and determines the efficiency of the device. An example of exploiting the dual nature of a synchronous rotating device is a pumped water storage facility. Another example is starting a rotary convertor. The rotary convertor in the video below converts 19kv 25-cycle AC @ from Con-Ed to 650 volt DC for use by the subway system in NYC. Rotary converters were made obsolete by the development of sold state rectifiers. [John L comment on another video]

To start the rotary convertor, the DC side is used as a motor to draw current from the third rail to drive the AC side up to its operating speed. (DC current is being supplied to the third rail by other rotary convertors that are already online.) Once the AC rotor is up to speed and the phase is aligned with the grid, the connection to the grid is closed. Then power through the convertor flows in the other direction and the DC side becomes a generator instead of a motor, and it supplies current to the third rail..

I selected some tidbits from the video's comments. (The indentation has no significance.)

This view shows how the two functions of the big convertors share a common armature. The commuter and brushes of the DC motor/generator are on the left and the three slip rings of the AC idle/motor are on the right. The converter in the lower-left corner is what I'm used to seeing where each device has its own armature and they are connected with a shaft.

A third way to start a synchronous motor is to have an auxiliary motor to start the main motor.

A forth, and I presume current, way of starting a synchronous motor is to add squirrel cage bars to the armature so that the rotor functions as an induction motor during startup. Once the rotor locks into the speed of the rotating magnetic field, the squirrel cage bars are not cutting the magnetic field anymore and they will not have any induced current in them. That is, the motor switches from an inductive motor to a pure synchronous motor.

I'm saving these comments about Steinmetz from a video for future reference. If I am remembering correctly (my undergraduate degree is Electronics Engineering), Steinmetz is the one that figured out how to use complex arithmetic to do AC power calculations. The real number is the resistive power and the imaginary number is the inductive/capacitive power. Thus the terms real and imaginary power. Real power consumed fuel, imaginary power consumed just additional transmission, transformer, power meter, etc. capacity. One of the advantages of synchronous motors is that you could change them from an inductive load to a capacitive load by increasing the current in the rotor. The capacitive load offsets the inductive loads of other motors in the plant and thus reduces the power factor of the plant. And reducing the power factor reduces the electric bill.

This video uses a "motor lab" to illustrate startup using an integral squirrel cage. I didn't bother to understand the wiring diagrams. I'm glad he mentioned that VARS is "the potential energy held in the magnetic field" because I had no clue.

This video is basically a testament to the quality of the bearings and oil. It spends the second half coasting back down to a stop. The 1.5mw rotary convertor in this video was used until 1999. [GeekyGirlEngineer]

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