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I am trying to understand the switching pattern of Inverter and the Brake chopper while a 3-phase AC motor is in regenerative mode for example in Elevator case:

  1. When its Cabin is full and it is moving downward OR
  2. When its Cabin is empty and it is moving upwards.

In case (1) the Cabin is heavier than the counter-weight so it will accelerate downwards on its own. In Case (2) the counter-weight is heavier than the Cabin so it will accelerate downwards on its own and Cabin will move upwards.

There are 2 things that I cannot understand so far:

  1. How does the Elevator Controller differentiate that the motor is in Braking mode or Driving mode? That is how does the above two cases are detected? Is it that the Controller always assume that it has to drive the motor and it starts switching the Inverter in drive mode but if the motor currents are not as expected then it reverses its mode and goto the braking mode?

  2. Once the Controller has detected (or decided) that the motor is in Braking mode then what is its switching pattern for the Inverter and the Brake-chopper? One possibility that comes to my mind is that all 3 HIGH side switches will be turned ON and all 3 LOW side switches will be turned OFF and the Brake-chopper will be turned ON until the ride is complete.

My understanding is very basic so far. Looking for its approval or corrections.

I am going through following sources:

  1. Regenerative braking
  2. 4-Quadrant operation

Elevator 4-Quadrant operation

ajeebx
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2 Answers2

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How does the Elevator Controller differentiate that the motor is in Braking mode or Driving mode?

In a 4-quadrant drive, motoring vs. braking would be distinguished by determine whether positive or negative torque is required to maintain the required speed and direction.

Once the Controller has detected (or decided) that the motor is in Braking mode then what is its switching pattern for the Inverter and the Brake-chopper?

There is no chopper in a 4-quadrant drive. Choppers are only used when braking energy is dissipated in resistors. The inverter switching pattern does not change, between motoring and braking. For motoring, the fundamental waveform frequency is above the mechanical frequency (speed) and the motor slip is positive. For braking, the fundamental waveform frequency is above the mechanical frequency and the slip is negative. The control strategy used to accomplish this varies somewhat from one manufacturer to another.

Added Information re Comments:

Motor current feedback alone is not sufficient to determine the torque direction. Ultimately, the controller needs to "know," at every instant in time, where the operating point is on the motor's torque vs. slip curve. That requires a motor shaft encoder and a means for determining the motor current and voltage phasor values.

It is possible to make a 4-quadrant drive that uses a braking resistor rather than returning braking power to the utility. That is inefficient, but it might be done for an elevator control to allow braking during a power outage. However, that would not eliminate the need of mechanical emergency braking. Elevator control is a very specialized area that uses specialized equipment, not standard industrial motor control equipment.

Braking may not be needed in situations such as counterweighting at maximum load and high ratio worm gear drive. There may be some need for braking if the dc bus is overcharged due to high energy power line transients such as ringing transients caused by power factor correction capacitor switching.

  • "..Whether positive or negative torque is required.." Electrically this will be determined by sensing the motor current feedbacks? – ajeebx Apr 30 '19 at 21:50
  • So if there is a Brake-resistor in an (Elevator Motor) Inverter would it be safe to say that its not a 4-quadrant drive? – ajeebx Apr 30 '19 at 21:52
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    How is positive or negative torque predetermined if number of passengers is unknown? More likely a type of VCF motor control is used with incremental and absolute position and motor current feedback for servo control. Although they may also have strain gages for safety. – Tony Stewart EE75 Apr 30 '19 at 22:02
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    It can only "know" by position error feedback and thus from command direction, velocity, acceleration , the expected DC current can be either polarity and must stay within expected control limits with many fault detection, fault correction, & safety circuits. (Quad redundancy). – Tony Stewart EE75 May 01 '19 at 00:01
  • I have seen some elevators which are using Geared ACIM motors (Reduction Worm gear ratio 40:1). It means the Cabin or Counter-weight can never free-fall. Weather the Cabin is full or empty, or it is moving up or down in all cases the motor has to drive the traction system. So does it mean that the Brake resistor will almost never be used because motor will never go into regeneration mode? – ajeebx May 01 '19 at 14:16
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    @Sunnyskyguy EE75: I am a bit out of my depth on the determination of positive/negative torque. Is the position feedback in this case ultimately a determination of torque angle? –  May 01 '19 at 15:29
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    For accel. vel. a,v=0 for any position, the servo uses the initial condition of current and applies an incremental change to accelerate in either direction regardless if present weight balance requires + or - current torque for static balance ( with safety brake). Depending on how sophisticated the elevator servo is, it may regulate a fixed acceleration , fixed max speed, fixed brake deacceleration, (using relays) or may control Jerk (da/dt) as well and have a higher velocity. – Tony Stewart EE75 May 01 '19 at 15:43
  • Deleted my comments covered in answer revision. –  May 02 '19 at 02:03
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Is not the lift controller that determines if motor is motoring or regenerating. This task is accomplished by the VVVF (variable frequency variable speed).

For two quadrant operation (this is the most common elevator lift, but there is also four quadrant inverter) the inverter senses the DC BUS voltage. If it increases above a preset level (perhaps 800 VDC on a 400 V supplied system) the internal BRAKE CHOPPER (an IGBT connected on one side to the negative of the DC BUS link and the other side is connected to brake resistance) begins working until the regenerative energy is dissipated safely into heat inside the brake resistance and the DC bus link voltage returns to the normal value. Otherwise the inverter cuts the six motor IGBTs and the motor runs in freewheeling mode. In this case only one the braking resistance ohmic value is not sufficient or the deceleration time is too fast.

Transistor
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toufik
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  • Welcome to EE.SE. It looks like English is not your first language. You should add your native language into your user profile as it may help us fix some of your translations. I tidied up your post. I'm not sure what you are trying to say in the last sentence. You can review the changes by clicking on the "edited" link. – Transistor Jul 09 '19 at 19:13