AC induction motors can be transformed into a highly controllable machine. With this electronic device, called Variable Frequency Drive (VFD, for short), a motor can have an ideal and directly proportional power-speed ratio while mantaining the torque constant in nominal conditions. In some small models, the input is a monophase connection (two wires) and the output is a three-phase connection (3 wires), which makes them totally convenient for places where three-phase wiring isn't available. Let's see why Variable Frequency Drives or VFDs are highly desirable.
Don't forget to check out what are induction motors and learn about other motors here, in 7 types of Electric Motors that you need to know.
Table of Contents
AC three-phase Induction Motor
This motor owes its popularity to its wide range of advantages over other motors: It has a long service life, low cost, low noise, is efficient and can be operated directly connected to the three-phase power line, without requiring an additional controller.
The operation of the motor is based on alternating current flowing through the stator. To create a variable magnetic field, it induces a current that flows through a non-ferrous rotor. This induced current circulates in these rotor bars to generate a magnetic field. A magnetic field is produced in the air gap, which is rotating at synchronous speed. However, the rotor will rotate at a different frequency and will depend on the number of pole pairs in the stator. A cross section of this motor is shown. There are 2 types of rotors for this motor: wound rotor and squirrel cage rotor. [2] [3]
Torque-Speed Ratio
The typical torque-speed graph from a three-phase induction motor. As it is shown, the torque output depends on the speeds of the rotor. As load increases, speed and and consequently torque decreases, which means that the “force” that the motor can provide is severly affected. The maximum torque occurs at a lower speed than the rated speed of the motor, which is the free running speed without any load. For example, a 1800rpm motor would typically rotate at 1710rpm under no load.
But, with the application of a variable frequency drive (or VFD), this curve changes significally and becomes appealing for industrial machinery.
What is an Variable Frequency Drive?
What makes this device so special? It's an electronic controller that transform the input voltage, which could be either AC or DC, to convert it into a variable voltage and frequency source to connect to the terminals of a AC induction motor.
Besides a power input and output, there is also a controller input and sometimes a feedback output. It serves to an operator an easy-to-reach panel to control mainly the speed of the motor. However, as it is explained here in this post, there are far more benefits than adjusting revolutions per minute in the machine.
When a VFD is attached to an 3-phase induction motor, its advantages and beneficts can be quite similar as an Brushless DC motor or BLDC for short.
Advantages and Savings using Variable Frequency Drives
Control and maneuverability
First of all, it allows an operator to control the speed of the motor. Previously, this wasn't an easy task since the voltage and frequency of the desired speed must be propotional to the nominal voltage and frequency of the motor. Let's take for example an three-phase induction motor with these nominal ratings:
- Voltage: 220V AC.
- Frequency: 60Hz.
- Speed: 1800rpm.
If an operator would like to lower the speed to, let's say 25% of its nominal speed or 450rpm, then a VFD has to adjust voltage and frequency proportionally to match the desired outcome. This table explains how this
| Desired Speed | 25% | 50% | 75% | 100% |
|---|---|---|---|---|
| Voltage (V) | 55 | 110 | 165 | 220 |
| Frequency (Hz) | 15 | 30 | 45 | 60 |
| Speed (rpm) | 450 | 900 | 1350 | 1800 |
The new Torque-Speed Ratio
Using a variable frequency drive, as stated before, respects the ratio voltage-frequency. These two variables affect the magnetic flux of the machine and as long as it remains the same, the torque output of the rotor remains unchanged until the nominal values of the motor. For practical uses, from 0 to 100% of the nominal speed, the motor will always have the same torque. On the other hand, power usage (HP) is proportional to speed.
Slow starts
Typically, an induction motor uses 4 to 7 times more current when the machine startes. This hits the lifespan of any motor in the long run but it affects other circuitry connected to the AC line (like the home appliances) or even the transformer in your home or neighbourhood. Furthermore, if your transformer doesn't have enough capacity or is located far away, then your motor probably won't start.
Remember that there is no rush. The motor could start in 10 seconds from 0% to 100% nominal speed to reduce any peak current consumption.
Regenerative Braking
Some models of VFD incorporate a very interesting feature: regenerative braking, which is a special method of dynamic braking. It consists in returning of the energy back the grid in exchange of the motor slowing down. Its efficiency depends of many factors but it is possible!
Slow and safe rotation switch
Is the motor rotation in the wrong direction? With a VFD, a simple switch could be used to reverse the rotation of the rotor. Typically, once the switch was being activated, the motor will slowly decrease its speed until reaching 0rpm and then it will start in the opposite direction in the same time it took to stop. This time is configurable.
Lower power consumption
Remember the formula for power:
P=t*w
where ‘P‘ is power, ‘t‘ is torque and ‘w‘ is speed. If t is constant, then P decreases as w decreases. This is atractive from the economic standpoint!
Higher Efficiency and more Convenience
Besides the lower power consumption porpotional to the speed, now three-phase have higher energy efficiency which means they require less power to operate with the same torque output. It's notably higher than monophase motors and ridicously higher that fossil fuel motors.
Some models of VFDs have only two input cables and three output cables. It makes a important feature because small industries and condominiums doesn't have a three-phase connection. Therefore, they can leave behind less efficient motors to work with variable frequency drives.
Less maintenance
With less peak current consumption and less heat dissipation if used at lower speeds, VFD can reduce maintenance, downtimes or other complicatios to induction motors.
Variable Frequency Drive with a 3-phase Induction Motor in practice
Let's take a look to a practical benefict that variable frequency drives bring.
Example: Grinder with a motor
The manufacturer of this grinder suggests these two motors be used. Let's add it efficiency next to it.
- Electric monophase motor: 3,7KW (5HP). Typical efficiency: 70%.
- Gasoline motor: 6,7KW (9HP). Typical efficiency: 30%.
- Electric three-phase motor: ??. Typical efficiency: 90%.
Why is this important? Typically, 3 phase connection suitable for 3 phase motors are not available in home and small industries. However, some models of VFD can be powered with only 2 wires from an AC plug and have 3 wires as output for the induction motor. Therefore, variable frequency drives opens a great amount of opportunities. Monophase motors can also be powered with 2 wires from an AC plug but without any of the benefits described here.
How to calculate the power for a three-phase induction motor?
Knowing which motors are recommeded for this machine and knowing it's typical efficiency, a approximately value can be determined for three-phase induction motor with a variable frequency drive applied. For this, we have to apply the inverse porportional rule to find out.
Pi and Pg is power of the induction motor and gasoline motor respectively, ηg and ηi is efficiency of the gasoline and induction motor respectively. So let's calculate it:
| Power P | Efficiency ƞ | |
| Electric Monophase | 3,7 kW | 0,7 |
| Gasoline | 6,7kW | 0,3 |
| VFD + 3-phase | ?? | 0,9 |
Pi=6,7kW*(0,3/0,9)=2,23kW
A 2,23kW (3HP) 3-phase motor could be connected to this grinder and it will be same or better performance than the gasoline and monophase motor. Not only that, but better speed control and torque output.
Conclusions
In this post, the 3 phase motor and its typical torque-speed curve has been explained briefly. Afterwards, the benefits of VFD or variable frequency drives have been shown. Finally, an example of a real life application has been given.
Resources
Disclaimer. Read this first.
Most, if not all, links in this page are affiliate links. It means that if you click a link for a tool and make a purchase, we earn a commission at no extra cost to you. Furthermore, it is a contribution from you to the website and appreciate all the support that you can give us in exchange for helping you succeed. The recommendations given here are based in experience from them. Please don't spend any money on these products unless you believe they will help you achieve your goals or deliver the desired satisfaction.
- Variable Frequency Drive. Wikipedia. Link.
- MPS Monolithic Power, «Appl. Note 047. Brushless DC Motor Fundamentals,» 2014.
- C. Kinsley Jr. S. D. Umans. and A. E. Fitzgerald. , Electric Machinery, 6th Edition, Mc Graw Hill, 2004.
- Induction motor. Wikipedia. Link.
techZorro's Index of Content
Keep Reading!
- 008 – Variable Frequency Drives: how this controller can transform induction motors forever
AC induction motors can be transformed into a highly controllable machine with Variable Frequency Drives or VFD. Click here to listen. - 006 – Regenerative Braking, an awesome Feature found in Electric MotorsThis episode is related to this hidden feature of electric motors called regenerative braking. Click here to listen.
- 005 – 7 types of Electric Motors that you should know aboutThere are several types of electric motors that differs in efficiency, power, cost, torque output, etc. Click here to listen.
- 004 – AC Voltages, Frequencies and Plugs around the WorldLet's talk about electricity! Specifically about how the standards around the world. Click here to listen.
- 002 – RISC vs CISC, how a few Differences are crucial to ComputingToday in the market is found two kinds of processor architectures: RISC and CISC. Both have some advantages. Click here to listen.
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