Speed control of three phase induction motor

Status
Not open for further replies.
M

Muhammad Umer ansar

Member
Location
karachi pakisatan
Occupation
junior electrical engineer
We have squirell cage type three phase induction motors in our plant. I am very interested to know the alternative method of VFD to control the speed of motor. Moreover, can i control the speed of three phase motor using a dimmer?
 
Muhammad Umer ansar said:
Oh Fine. How did people control speed of three phase motor then before the advent of VFD?
Click to expand...
Sub synchronous converter cascade was one approach. Usually fairly large motors at 11kV or 13.8kV although we also made some smaller units at 350kW.
 
Muhammad Umer ansar said:
Oh Fine. How did people control speed of three phase motor then before the advent of VFD?
Click to expand...
For most part you didn't. You either used another motor type that was easier to control speed or you used mechanical speed reduction on the output side of the motor. DC motors or AC motors with wound rotors were probably the most popular speed controlled motor or for lighter duty applications single phase PSC or shaded pole motors.
 
Muhammad Umer ansar said:
Oh Fine. How did people control speed of three phase motor then before the advent of VFD?
Click to expand...
For 3 phase systems, it was (not in this order);
  1. Mechanical Vari-drive (variable pitch pulley) system
  2. Gear boxes
  3. Fluid couplings
  4. Particle clutches
  5. Eddy current clutch drives
  6. Cycloconverters
Versions that require a different TYPE of 3 phase motor:
  1. 2 speed motors (or 3 speed etc.)
  2. Wound Rotor Induction Motors
For the most part, prior to the modern VFD, changing the speed of the LOAD was not done with the motor, the motor was run at full speed and there was something else that changed the speed to the load.

"Dimmers" can only work on a select few types of SINGLE PHASE motors, like Shaded Pole and some PSC motors in specific applications like fans. If you only change the voltage to an induction motor, it just reduces the torque that the motor produces, the speed wants to remain the same because speed is based on frequency. So using a "dimmer" to reduce the voltage will possibly reduce the speed if the load remains the same by decreasing the torque of the motor, but in attempting to get back to speed, the motor may burn itself up.
 
Some confusion here. And don’t forget...what was old is always new again.

First and foremost you can use resistors but efficiency is awful. Variacs are cheap. Since voltage is varying and not frequency it’s variable torque but especially if you have a steady load it’s not a big deal with small motors. There is a way to drive a PSC motor with a VFD but it’s almost not worth it when you can use a voltage doubler VFD and drive a real three phase motor from single phase 120 VAC.

Prior to the 1930s your best bet for variable speed was the wound rotor motor. Still used sometimes today but pretty rare for speed control because it’s really variable torque, not speed. It can be fairly unstable doing closed loop speed control. The best variable resistor is probably the liquid rheostat. This uses electrodes connected to the rotor with some way to raise/lower the level. I’ve seen a couple different versions. One raises and lowers the electrode into a tank. Common in scrap yards and Europe. Another version uses a pump and a valve to control the salt water level. Contactors and resistors are very reliable and popular but keep in mind that method is pretty much limited to starting only. You could use it for speed control somewhat but in all the installations I’ve seen nobody uses it that way. Phosphate mines in Florida used these and varied the contactor settings for pumping but that’s strictly torque control with variable speed/flow as a consequence. It would seem very inefficient regardless but above 1000 HP wound rotor motors are very efficient and since the controls are very simple and cheap they are often competitive with VFDs. Plus they have lifespans far exceeding the 100,000 hour MTBF of VFDs.

Also this is sort of backwards but if you draw power off the rotor on a wound rotor motor through a VFD (slip energy recovery) you can get roughly twice the power from the same piece of equipment. At large sizes this is useful where space is at a premium and the reason modern windmills often use wound rotor generators. Hardly old school. We rebuilt almost a dozen large wound rotor motors and PM double that many every year. Tesla would be proud!

I have seen ONE slip recovery wound rotor VFD in the last 25 years. Generators yes but not motors. It’s very, very rare.

In the 1930s GE introduced the Ward Leonard loop control system. Used on elevators, mining equipment, and some machine tools still even today. This is true variable speed control. Start with a shunt wound DC motor. You can do field weakening and have series fields but these are optional improvements. Now run an AC motor. I’ve seen synchronous and induction, synchronous is preferred. Now mechanically couple a DC generator to the AC motor. Vary the excitation. On small ones you can do this directly with a potentiometer but on larger ones use a second generator. You get roughly 1000:1 gain doing this. The generator created armature current. Armature voltage is pretty close to speed but you can use a tach in modern systems for true closed loop control. No electronics required here. Again very simple and reliable. The downside is you have tons of rotating equipment to maintain so maintenance costs are high. But this was the dominant technology for 25+ years. Even when DC drives came out scaling to large size was uncommon.

The “modern” version substitutes an SCR system for the motor-generator set and drives the DC motor directly. SCRs came out in the 1950s and since all you need is firing angle control which is your “lamp dimmer” you can do it with some very primitive analog controls, still used today. This was the industrial work horse for variable speed from around the 1960s until the 1990s when AC IM displaced it. But conversion costs are almost the cost of one rebuild more or less so it’s hard to get old school plants to convert.

On a dragline excavator for instance the standard motor is say an MD824 with a stall current of 6000 A...almost impossible with electronics but easy with the corresponding generator. And just the MG set produces super clean output. Conversion to AC costs millions where doing controls upgrades is cheap. Only time AC conversion is justified is when a machine is moved or when more power is needed. An interesting variation is UDD but I’m getting way into the weeds here but it brings up an AC/DC issue. In Australia CSIRO paid to upgrade several machines to AC VFD using UDD technology (a special way to rig a dragline where it can dump at any distance). The issue is draglines spend a lot of time at zero speed. IGBTs are rated based on AC RMS ratings so to put out DC the drive needs to be twice as big. Plus the size (roughly 2000 HP motors) requires multiple parallel IGBT modules. The manufacturer thought they were so clever that semiconductor fuses were unnecessary. 18 months later they were proven very wrong when the drives that were under rated by a factor of 1.732 started exploding and launching IGBT shrapnel. Oops! The first SUCCESSFUL retrofit on a mid size Marion machine was my project and we had fuses and drives that were properly sized.

Compared to DC, only in the last decade has AC technology caught up for nearly all applications. The IGBT revolutionized VFDs for 600 VAC and below. High voltage IGBTs have pushed this to around 1500 VAC but thats about it. So medium voltage drives either use tapped transformers to do the conversion in steps (18+ pulse VFDs) or switch to GCTs which can handle higher voltages. Good quality MV VFDs only came out about 10-15 years ago as these technologies did the same thing for MV. Prior to that I’ve seen a kiln that used a 4160:480 transformer to feed a VFD then another to step back up to 4160...DC is attractive doing things like that.

Also if using an AC generator it makes a simple, reliable, clean source of say 120 Hz voltage used in transformer test stands for heat runs, although this is an AC generator, not DC.

Keep in mind, DC drives are VERY reliable and the electronics are very cheap compared to a VFD since fundamentally you have “half” of a VFD. The issue is the motor maintenance costs are horrendous. But at very low speeds (inching) DC drives are still superior to AC. And with brushless PMDC in high torque applications superior to AC. That’s why your expensive battery tools and servos are all PMDC motors.

And...I’m seeing conversion back. With brushless motors DC is making a come back in HVAC, portable tools, servos )(never left), and others. DC is becoming more popular than ever even as AC VFDs have dominated industrial plants in the last 25 years.

Around the same time as DC drives, was the cycloconverter. This is a VFD based on SCRs. Essentially you turn the SCRs on (can’t turn off) as needed to delete pulses of AC so for instance if you only allow every other AC half cycle through you get 30 Hz output. Benshaw has a patent that gives the pulse patterns needed for 0-30 Hz operation but with inductors and some other tricks forced commutation is possible and cycloconverter drives are still used on large 11 kV equipment where VFDs are impractical even today. Benshaw and Phasetronics offer soft starters/cycloconverters as part of their soft starts for inching but our 13.5 kV test stand uses a Motortronics soft start and can go into variable speed mode if needed.

This of course ignores all the mechanical contraptions that produce variable speed.

As an engineer for a large motor shop I get to see most of the very bizarre or unusual systems so I’ve worked on all of these personally. Even VFDs can be very challenging when they do strange things.
 
As a digression, Antrak's North East Corridor (25Hz traction power) has a 20MW(?) cycloconverter at the Jericho Park, MD location to convert from 60Hz. (The NEC power system is fascinating.) I think SEPTA might have one in service, too.
 
We used to install DC drives and motors before the VFD.
I used to service the drives and motors at several plastic extrusion factories in the 70's and 80's.
 
Jraef said:
For 3 phase systems, it was (not in this order);
  1. Mechanical Vari-drive (variable pitch pulley) system
  2. Gear boxes
  3. Fluid couplings
  4. Particle clutches
  5. Eddy current clutch drives
  6. Cycloconverters
Versions that require a different TYPE of 3 phase motor:
  1. 2 speed motors (or 3 speed etc.)
  2. Wound Rotor Induction Motors
For the most part, prior to the modern VFD, changing the speed of the LOAD was not done with the motor, the motor was run at full speed and there was something else that changed the speed to the load.

"Dimmers" can only work on a select few types of SINGLE PHASE motors, like Shaded Pole and some PSC motors in specific applications like fans. If you only change the voltage to an induction motor, it just reduces the torque that the motor produces, the speed wants to remain the same because speed is based on frequency. So using a "dimmer" to reduce the voltage will possibly reduce the speed if the load remains the same by decreasing the torque of the motor, but in attempting to get back to speed, the motor may burn itself up.
Click to expand...
And it works on shaded pole and PSC because they simply can't produce enough torque to reach full speed when the controller basically limits what it can take from the supply. I know you know how they work but for OP these type speed controls don't exactly lessen the voltage so much as they just shave off parts of the AC waveform which lessens effectiveness of what does make it to the motor vs if the full wave made it there.
 
kwired said:
And it works on shaded pole and PSC because they simply can't produce enough torque to reach full speed when the controller basically limits what it can take from the supply. I know you know how they work but for OP these type speed controls don't exactly lessen the voltage so much as they just shave off parts of the AC waveform which lessens effectiveness of what does make it to the motor vs if the full wave made it there.
Click to expand...
It lessens the voltage by reducing the RMS waveform.
 
zbang said:
The best variable resistor is probably the liquid rheostat. This uses electrodes connected to the rotor with some way to raise/lower the level. I’ve seen a couple different versions. One raises and lowers the electrode into a tank. Common in scrap yards and Europe.
....

As a digression, Antrak's North East Corridor (25Hz traction power) has a 20MW(?) cycloconverter at the Jericho Park, MD location to convert from 60Hz. (The NEC power system is fascinating.) I think SEPTA might have one in service, too.
Click to expand...

NASA-LeRC's 10x10 Supersonic Windtunnel uses the moving electrodes liquid resistor scheme, or it did during my tenure. Total was 250,000 HP.

A big issue on the NEC is harmonics. That cycloconverter and the other new toys generate them by the ...trainload..... So a decade ago Amtrak spent $$$ rebuilding the rotary converters in PA. Turns out they eat the harmonics productively, making useful power 25 Hz sine wavesfrom them.

Those of you working on older system would likely flee in terror from the NEC's. Virtually all of it is too old to be in a museum, dating back to PRR's circa 191x electrification.
 
Status
Not open for further replies.
Top