Speed control of three phase induction motor

Question

We have squirrel 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. Can I use a dimmer? Moreover, how did people control speed of three phase motor then before the advent of VFD?

Answer 1

Before electronic VFDs were developed, the speed of wound-rotor 3-phase induction motors was controlled by schemes involving variable resistance inserted in series with the rotor windings or connecting the rotor windings to additional machines (Kramer and Scherbius drives). There were also schemes in which variable frequency was generated by driving an AC generator with some other type of variable speed machine such as a DC motor, eddy-current coupling drive, or a mechanical speed changer (continuous variable transmission).

In many cases, a DC motor, eddy-current coupling drive, or a mechanical speed changer was used to drive machines directly without using AC motors. However wound rotor motors could be built with higher power ratings than other machines, and could be controlled using a lower power rotor circuit control. That was one reason to use those schemes.

Variable-speed AC generator machines were also used to control large quantities of small synchronous motors used for making synthetic fiber. The first solid-state electronic variable speed drives were developed by Borg Warner under contract for DuPont for synthetic fiber spinning applications.

Also, small induction and brushless synchronous motors controlled that way could safely be installed in explosive atmosphere areas.

Before the invention of the transistor, vacuum tubes, mostly thyratrons, were used for variable-speed control of DC motors, eddy-current coupling drives, wound-rotor induction motor drives and perhaps some variable frequency drives.

Answer 2

I can think of several reasons to not use a VFD, and I do own and use VFDs. First, they are expensive. Second, they have to be connected directly to your mains (they must be connected to the live wires coming into your circuit breaker—never plug them into an outlet). Oh, and don’t assume you can connect it. You need filters, and a bunch of other stuff. Unless you are an electrician, we all have zero business trying to connect them. Third, VFDs typically give you way more options than you need, especially if you are just wanting to control motor speed. Fourth, VFDs are the number one electricity polluters (for those who care—I don’t care).
Speak to an electrician. You’d be surprised how frequently a VFD is NOT the industry standard for what you are wanting to accomplish.

Answer 3

VFD meaning: Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.

Can it run on a dimmer? the answer is YES, you can,

But:

• The dimmer only lowering your voltage without any compensation like increasing or lowering frequency.
• This can damage your motor because powering on a lower voltage than intended.
• The motor will be very difficult to control because is very load-depended.
• Can cause overheating the motor because running on lower voltage.
• It's not a recommended way to do it. You will likely break the warranty for your motor.