How can a joystick control the speed of an AC motor

Thanks!

For an induction motor, any minimum frequency limit to make it start?

Is a soft starter same as to a Star-Delta motor starter?

Did you mean "an asynchronous induction motor"? I know there are synchronous motors, but asynchronous are much more common.

I disagree an induction motor and a synchronous motor are two distinct devices.

An induction motor has a wound stator connected to the supply and a rotor in which the windings are no more than a shorted series of bars in the iron rotor. They will not achieve synchronous speed and always have some slip so a 2 pole motor will run at 2850RPM or less when run on 50Hz. The reduction being the % slip. Some motors employ wound rotors from which the slip frequency can be gathered, then rectified and finally chopped back to the supply frequency to improve the efficiency.

A synchronous motor will have a stator containing the windings but the rotor will have a magnetic field from either a fixed magnet or a variable voltage DC coil such that the field locks onto the rotating field in the stator and runs at synchronous speed. Small size think clock motor, large size think dragline generator prime mover.

The only way to change the speed is by changing the frequency supplied to the stator windings

The joystick can be used to control a VFD to change the frequency of the stator excitation voltage and both types of motors will be able to run at a varying speed as a result of the change in frequency.

FYI. Large synchronous motors on mining machines usually start as induction motors and accelerate to near sync speed and then the field is applied which locks the rotor to the sync speed of 3000RPM for a 2 pole sync motor. Over or under excitation can be used on the rotor field to change the power factor of the machine.

Regarding changing the power factor by changing the excitation of the rotor on a synchronous motor--

Many older factory designs designated a large motor as a synchronous motor with a controller that automatically or manually changed the excitation to maintain the factory's net power factor close to unity. In this way they were referred to as synchronous capacitors, because there was no need for a separate capacitor bank and all the contactors and controls needed to apply the correct amount of capacitance for power factor control.

--JMM

Also known as Synchronous Condensers this side of the pond. Often used on long power lines feeding many mines to correct the power factor. There is a very large one outside Moranbah, Qld feeding the "nasty, dirty, coal mines."

Using an induction motor, you need to control the speed with power frequency (VFD). Alternatively, you can replace the induction motor with a DC or universal motor to use voltage control.

Youngken,

There is an "old school" way to indirectly do this without a VFD. Using a wound-rotor induction motor, the joy stick can provide an input to control the value of the secondary resistor wired to the rotor via its slip rings. This can control it from about 1/6 speed to full speed (with no change in the input frequency). However, this is an impractical approach because it has so many other parts and would be pretty-much a "one-of-a kind" application no one else would understand how to operate or repair. Wound rotor motors are kind of scarce. Asking to find all the other control components will be met with many puzzled looks or "not here" statements.

So, I agree with the idea of using a VFD (variable frequency drive). This, however, has problems if trying to drive to a speed slower than about 1/3 of line frequency unless there is a separate blower for cooling the motor. Then with a vector-control type of VFD you can get nearly down to zero speed with proper torque (as well as to speeds above nameplate). You would also probably need an encoder to provide a speed feedback to the drive. Your joystick would provide an analog input to the drive's frequency control. Of course, you would need to restrict the rate of speed change to what the motor and VFD can handle. DC bus voltage controls may be an issue (or using a drive that can regenerate back into the incoming line). Lots of interesting technology gets involved.

Besides using an inducation motor, you can change to a stepper-type motor, or a DC motor, or some other possibilities I have forgotten. There has even been a motor type with slip rings at one end and two sets of brushes on the commutator at the other end where the relative positions of these sets of brushes can change the motor's speed at full torque and full efficiency. (I worked on one on a printing press--it had 3 auxiliary motors, one to change the brush positions, one as a cooling fan, and one on the same output shaft working as a pony motor to run the press at a constant very slow speed for setup, etc.)

--JMM

I have seen a webpage with variable speed 3 phase motors. Is that reliable?

https://www.ato.com/variable-speed-motor

Depends on what you mean by reliable. The information posted on that webpage is reliable. The motors identified on that webpage can be misused in a number of ways to make them unreliable.

Can we get 2 speeds without using a VFD as the video discribed?

https://www.youtube.com/watch?v=AhT_3CuGAgs

The video you posted shows the typical wiring for a two-speed 3-phase induction motor. The video appears to have a minor error because it does not show the winding shorting contactor closed when it is energized, but that does not detract from the reality of what is being done.

You have no choices but to operate at one speed or the other, with no in-between and no ramping from one to the other. This imposes on the driven load the mechanical stresses of a sudden speed change. This also limits you when your application would be better matched by an intermediate speed.

Energy efficiency is best when one avoids indirect ways to control pressure, flow, or movement with things such as dampers (in air handling systems) or pressure reducing valves (in hydraulic systems). These ways have a long-history of use, but at a cost.

Unless you only need either of two speeds, it would be better to use a motor with an external cooling fan and a VFD with proper components to mitigate the possible problems of noise, bearing degradation, and distance.

Not mentioned before are mechanical ways to vary the speed, such as with a drive belt and variable sheave or eddy current drives. These have their own problems (wear, heat, efficiency, not being automatic).

--JMM

Thanks JMM.

Can this wiring provide the same torque in both speeds?

Same Torque? No. Look down a ways at the site that you referenced (#12). It shows the torque, current draw, and shaft speed for the different connections of the motor.

Some loads are constant torque (such as a conveyor or mixer), but others are variable torque (such as a fan or pump). These are things characteristic of different load types. Motors and drives are selected with these in mind: Torque, HP, Speed. You can find many good references to help with this, such as from NEMA (National Electrical Manufacturers Association) in the USA, or good engineering texts, or even Wikipedia.

--JMM

Most loads present more torque to the motor as the shaft speed increases. For a fan, for example, the torque demand rises with something like the cube of the tip speed.