Electric Motors in low current Circuits
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Electric Motors, Trends over the last 50 Years
For those born after 2000 and electric motor is seen as just another product of technology, just like the cell phone. Electric motors have been with us for the last 200 years, personal handheld phone (Motorola and Martin Cooper) about 50. There is a link of course, that being switching devices.
Although it is an accepted fact that inverters and variable frequency drives are with us to stay, older technology relied on some pretty innovative ideas to increase and/or lock an electric motor’s speed.
Commutator and Inverters for Electric Motors
Going back over 100 years the trend was really all around the commutation circuitry and DC control of electric motors and even today the techniques used were robust and ultra reliable. Although it is out of the scope of this article one need only to understand the elements used in electric locomotion for our train and tram transport systems.
When our last generation electronic engineers were first bitten by the switching power supply bug and here I would be looking at “static inverters” and my own experiences in the 1970s – they were just not that reliable. Using snubbing circuits to protect the SCRs it was not all that uncommon to have catastrophic failure requiring a total rebuild. Now of course we have MOSFETS and IGBTs, ultra reliable and being able to pass hundreds of Amperes.
The common link between most technologies used in electronic circuits today is going to be the H-Bridge driver and Variable Frequency Drive. There has been a substantial effort to focus on brushless motors, powering a motor through multiple phases and having a variable frequency output to adjust speed. This is not only seen in automobiles but in consumer electronics, air conditioners, vacuum cleaners, washing machines etc. Drones and electric powered R/C vehicles are all following this trend.
Not that many years back all speed control was DC related and yes, power tools have proven reliability and are cheap to repair. Using universal motors which are series wound they can run on both AC and DC. Unlike an induction squirrel cage motor they are not designed for prolonged use.
Toys thirty years back were invariably powered through those little 3V to 12V electric motors with commutators and brushes. Brushed motors can easily be controlled through variable pulse length circuits as used in drilling machines, keeping high torque at low RPM. (or in simpler circuits, by varying supply voltage).
Squirrel Cage Motor
Audio equipment, tape deck transport systems and turntables were either DC motor powered or AC, the better equipment would all have squirrel cage motors moving over to variable frequency poly-phase motors for higher end equipment. Of course this is one of the reasons making the Technics 1200 famous, powerful and very, very reliable. Modern audio equipment is mainly digital and does not require motors except possibly in remote control e.g. mixers and for level adjustments. And no, electric motors are more popular than ever before…
Servos, Steppers and Converters
Thirty years ago a servo motor for a marine radar (PPI display driver) would cost in the vicinity of R2000.00. Just for the motor. Nowadays an entire servo motor with loop circuitry can be had for the same price. Stepper motors are in abundance and is a big export from China. Your entire DIY CNC market relies on servo and stepper motors, almost bomb proof and wallet friendly. And all electronically controlled.
In my chosen field, marine electronics, motor alternators were very common and were more aptly known as frequency converters. The gyro and radar sets were powered from these devices. Primitive for the 70s and 80s.
An interesting topic often arising on the forums is whether VFD in consumer electronics is all that cost effective. The general tone would be that for any country or city with power outages, brownouts and or spikes and surges, definitely not. VFDs are with us to stay, they will just get more reliable.
And of course we get to grid power versus consumer electronics. From the early 1900s we had DC power then AC which was more efficient. Electric motors were mainly DC (commutator). Now the argument is that we need HVDC and VFD. We were always taught that it is cheaper and more efficient to convert AC (transform) to AC and then to DC. Seems like someone is wrong.
Although electric motors have been with us for going on two centuries the theory remains the same, it’s just the means to power them which has changed.
Induction Motor – By S.J. de Waard – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17313755
Further Reading:
- Popular Mechanics on brushless motors used in power tools
- 400Hz motors – what’s the deal? – All about Circuits
- Mating existing motors to VFD Drives – EC&M Web
- Traction Control in Diesel Locomotives – ee.co.za
- Motor & Drives – 30 Years of Innovation – Processing Magazine
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