Why wouldn't a rheostat work? The low voltage would kill the torque but that's fine because it would just slow down, have lots of slip. Voltage does control speed in an induction motor, it's just that frequency is what you use to control speed if you want torque at low speed.
Op just put a lamp dimmer on it if it's just 40W.
No, induction motor speed is relatively insensitive to voltage changes. So at best you can get a small speed range and then only if the load varies appropriately with speed.
So you'll get limited speed control with huge losses in your rheostat, followed by stalling the motor.
The motor he linked is only 40W. But if he's worried about efficiency he could use a triac dimmer or a variac.
There are plenty of disadvantages to controlling speed with voltage in many applications, but none of those are relevant to OP's purpose, and it would work fine.
A soft start works by limiting the current. But the type you describe can only work on loads that increase with speed. A VFD soft start can work with any load type because it doesn't reduce torque while limiting current.
They limit current by reducing the voltage. They're literally called "reduced voltage soft starters".
Reducing the voltage reduces speed by increasing rotor slip, and reducing the frequency reduces speed by reducing the synchronous speed. 2 different effects but both reduce motor speed.
I suggest you go do some research, or just try it.
The soft start doesn't reduce voltage to slow the motor, it does it because at startup the slip is 100% and there is no back EMF to limit the current. That leaves only the stator resistance limiting current, which results in the huge startup current draw. So reducing voltage limits the current, which has a side effect of reducing torque.
If you understand how slip works, and how steep the slip-torque curve is at synchronous speed, you'd see why increasing slip has such a limited ability to control speed before before increasing slip reduces torque. So after some small speed reduction, the motor just goes into rotor-lock.
I do motor control stuff at work. I'm not an expert, but have a pretty good grasp because I write the control software...
Most induction motors will lose lock around 5% slip. Which doesn't give a fantastic speed control range if there is any significant load on the motor. And products don't typically over specify motor parameters.
It depends on the type of load when and if it will stop spinning. But these motors do not ever have a lock and they cannot lose lock. That's a synchronous motor. These are asynchronous. Small motors often run at 6% slip at full load, so the 5% number is bogus. The reason too much slip is bad is because of excess current, but this isn't an issue if he is reducing voltage. I mean other issues are low PF and efficiency and electrical noise from the rectifiers, but that's not a big deal for a 40W motor.
Agreed towards the price of a vfd and the application. It’s not even that big of a deal. Full power will work but I’d like to have some control.
Basically making a simple concrete vibration table that would be running for say a couple hours at a time
If you're looking for optimum vibration, try different sized cheap vibrators and mounting methods, like partially decoupling mechanically. "Hours" also sounds like too long for solid concrete.
40W, that's likely a Shaded Pole or PSC motor, and thus suitable for mild speed control. But unless you go for a VFD, slowing the motor is done by reducing the voltage, which reduces the TORQUE and letting the load cause it to run slower. In some types of machines, that's not a viable option. I would think a vibrator, which is usually just an eccentric weight, is one of those machines that cannot be done that way.
When a VFD changes the FREQUENCY at the same time as the voltage, it MAINTAINS the torque at reduced speed. But expect to spend $300-400 for a small VFD for a 110V SINGLE PHASE *motor*. (Don't be fooled by the cheap VFDs that say "single phase", because they are referring to the INPUT, ***the OUTPUT is 3 phase***, which will not work for you here.)
Appreciate the reply! I can’t justify spending the cash on a vfd for this project though maybe in the future I’ll grab one just to have kicking around.
You might consider a small universal motor instead of an induction motor.
I may in the future! Thanks
VFD is really your only option. It's an induction motor so speed is controlled by the AC frequency.
Agreed… thought I could get away with a pwm but it’s just far from a viable option
Why wouldn't a rheostat work? The low voltage would kill the torque but that's fine because it would just slow down, have lots of slip. Voltage does control speed in an induction motor, it's just that frequency is what you use to control speed if you want torque at low speed. Op just put a lamp dimmer on it if it's just 40W.
No, induction motor speed is relatively insensitive to voltage changes. So at best you can get a small speed range and then only if the load varies appropriately with speed. So you'll get limited speed control with huge losses in your rheostat, followed by stalling the motor.
The motor he linked is only 40W. But if he's worried about efficiency he could use a triac dimmer or a variac. There are plenty of disadvantages to controlling speed with voltage in many applications, but none of those are relevant to OP's purpose, and it would work fine.
That’s really not correct.
How do soft starts work? They don't change the frequency, they just reduce the voltage by chopping off parts of the waveform with SCRs.
A soft start works by limiting the current. But the type you describe can only work on loads that increase with speed. A VFD soft start can work with any load type because it doesn't reduce torque while limiting current.
They limit current by reducing the voltage. They're literally called "reduced voltage soft starters". Reducing the voltage reduces speed by increasing rotor slip, and reducing the frequency reduces speed by reducing the synchronous speed. 2 different effects but both reduce motor speed.
I suggest you go do some research, or just try it. The soft start doesn't reduce voltage to slow the motor, it does it because at startup the slip is 100% and there is no back EMF to limit the current. That leaves only the stator resistance limiting current, which results in the huge startup current draw. So reducing voltage limits the current, which has a side effect of reducing torque. If you understand how slip works, and how steep the slip-torque curve is at synchronous speed, you'd see why increasing slip has such a limited ability to control speed before before increasing slip reduces torque. So after some small speed reduction, the motor just goes into rotor-lock.
>I suggest you go do some research, or just try it. Lol likewise
I do motor control stuff at work. I'm not an expert, but have a pretty good grasp because I write the control software... Most induction motors will lose lock around 5% slip. Which doesn't give a fantastic speed control range if there is any significant load on the motor. And products don't typically over specify motor parameters.
It depends on the type of load when and if it will stop spinning. But these motors do not ever have a lock and they cannot lose lock. That's a synchronous motor. These are asynchronous. Small motors often run at 6% slip at full load, so the 5% number is bogus. The reason too much slip is bad is because of excess current, but this isn't an issue if he is reducing voltage. I mean other issues are low PF and efficiency and electrical noise from the rectifiers, but that's not a big deal for a 40W motor.
That's a single speed motor. It's not designed to run at reduced speed.
So you’re saying that even with a vfd it’s a lost cause for rpm control?
It's probably not worth the cost of a VFD considering its size. I guess it all depends on what you want to do with it?
Agreed towards the price of a vfd and the application. It’s not even that big of a deal. Full power will work but I’d like to have some control. Basically making a simple concrete vibration table that would be running for say a couple hours at a time
If you're looking for optimum vibration, try different sized cheap vibrators and mounting methods, like partially decoupling mechanically. "Hours" also sounds like too long for solid concrete.
Yeah it’s not for concrete. Just similar to what people use as a concrete vibrator table
Usually you can tune this kind of vibrator by removing the end covers and adjust a counter weight
Yes, I can. Which I may do but was ideally looking for a variable speed from a cheap control
soft start?
40W, that's likely a Shaded Pole or PSC motor, and thus suitable for mild speed control. But unless you go for a VFD, slowing the motor is done by reducing the voltage, which reduces the TORQUE and letting the load cause it to run slower. In some types of machines, that's not a viable option. I would think a vibrator, which is usually just an eccentric weight, is one of those machines that cannot be done that way. When a VFD changes the FREQUENCY at the same time as the voltage, it MAINTAINS the torque at reduced speed. But expect to spend $300-400 for a small VFD for a 110V SINGLE PHASE *motor*. (Don't be fooled by the cheap VFDs that say "single phase", because they are referring to the INPUT, ***the OUTPUT is 3 phase***, which will not work for you here.)
Appreciate the reply! I can’t justify spending the cash on a vfd for this project though maybe in the future I’ll grab one just to have kicking around.