Electric motor power and startup

[PaulJohnson]

Hello
I'm building a device for testing speed sensors and have until now been using a small bench grinder as the speed source which spins happily at a very stable 2950 rpm and is 150watt 240v.
I then required higher speeds up to 5000rpm so made a pulley system with a separate shaft running in a couple of housed bearings.The motor didn't have enough power even to overcome the friction of the drive belt and bearings!
I then purchased a 450w bench grinder which still struggles to spin the shaft and only gets up to full speed after a few minutes and won't easily accelerate when cold and I had to fit a slightly larger start capacitor.
I'm surprised that a 3/4 hp motor struggles to drive the shaft with no load that can easily be spun by hand.
I also built in a simple speed controller which sort of works but reduces the power so not very accurate
Is there a better way to do this ?
I even though about using a 12volt engine starter motor

 

[waross]

Way too much belt friction.
Try running the belt very loose.
You can't get 3/4 HP out of 540 Watts.
Many capacitor start motors of that size don't respond well to speed control.
For a dependable controllable system use a small three phase motor driven by a single phase VFD. (Variable Frequency Drive, single phase in, three phase out.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

+1 on the suggestion of using a small VFD and a 3 phase motor. Most small (3HP and under) VFDs will accept single phase input and provide a 3 phase output to a motor, there are even some that will take 120V single phase input, i.e. your wall socket*, and provide a 230V 3 phase output. If you get a VFD with what is called "Sensorless Vector Control" you can maintain very accurate speed control, i.e. .01% or better, which I would imagine is important if you are testing sensors. You can likely put together the entire thing, motor + VFD, for under $250.00.

*Your use of the term "HP" instead of kW leads me to believe you are in North America somewhere, hence the reference to a 120V wall outlet.

"Will work for (the memory of) salami"

 

[electricpete]

I'm surprised that a 3/4 hp motor struggles to drive the shaft with no load that can easily be spun by hand.

I'd be surprised to. A few things to consider:
* the torque is higher on the on the low speed side than on the high speed side by the speed ratio, but that's a relatively small number
* is the bench grinder driving anything else
* how much inertia is being accelerated
* is the voltage drooping at the motor terminals during start.. that will cause reduction in torque
* if the starting cap drops out of the circuit before motor is above let's say 50% speed, torque will be reduced at that point (the torque speed curve of single phase motor goes through zero at zero speed and is also low at low speed...moreso than 3-phase motor)

There may be a number of ways to address starting:
- look at terminal voltage
- look at cap size and timing of dropout
- A little bit off the wall solution to the starting problem .... if there is provision to SAFELY move one of the machines while running then start with belts completely loose to get motor up to speed, then move to tighten while at speed. But it is a safety nightmare on a number of counts.... machine comes loose while adjusting position...belt breaks while you're standing nearby etc.... not good.

But if you did figure a reasonable way to get the machine up to speed and loaded, an induction motor without vfd doesn't really have any way to control the speed. As the wise men before mentioned: vfd does, and handles the start as well.



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(2B)+(2B)' ?

 

[PaulJohnson]

Thanks for the replys
I played around a little more with it and I think the motor just doesn't have enough power and the capacitor originally supplied was a 5 microfarad and I'm not even sure whether it's a start cap or a run cap.Anyway I attached a regular 240v hand drill to the shaft on the mechanism and it spun the whole lot including grinder motor to full speed effortlessly.
If I could get a motor the same size and power as the drill which is 2000w ish with a 12mm shaft and decent mounting frame then that would be good as there is no load to drive except belt friction and it's not on constantly just a few minutes at a time.
Ideally I would like a motor with speed control that would reach 5000rpm so I wouldn't need pulleys but I haven't seen anything that goes over 3000rpm.
I shall research the VFD
I just can't believe that the bench grinder motor is so weak but hey it was only £60 new and I will put the grind stones back on and use it for it's original purpose

 

[squeeky]

Dear Paul,

The VFD or VSD, sounds like your best option. If you buy a single phase to three phase unit, you will then be able to drive a three phase, fractional HP, motor from a wall plug. These are relatively inexpensive. You will need speed so look for a two pole motor. This starts you at a nominal speed of 3000rpm, or about 2900rpm. The VSD should be able to go to 400 Hz or about 8 times nominal speed. Running over 6000rpm will need you to look at special bearings and the fan won't really be working so watch out for over heating. Look at your designed point of failure. Set the drive overload very close to the running current on a trial run. Guarding needs to be substantial. Most drives will allow you to wire in an external stop button. If you wire this into a deadmans switch, you can just let it go and it will guide to a stop. Getting fancy, you can add breaking functions and even an external breaking resistor to stop it really fast. I find that the WEG drive easy to use and understand. If you still need help let me know. Good luck.

 

[PaulJohnson]

Thanks for the reply Squeaky.
Interesting about how the frequency dictates the motor speed and now I know why all the motors i try spin at 2900rpm as up till now I have tested the sensors at this fixed speed.
My latest plan is to construct a two shaft frame with pulleys and drive the larger pulley with a Black and Decker drill which has variable speed trigger switch and a button lock.The bench grinder states 585 watt motor but seems weak yet the drill rated at 550watt has a lot more power which I can't understand as it spins at 3000 rpm so not geared down. I think I can get a safe speed on the driven shaft of 4500rpm which will probably do for now

 

[squeeky]

The speed control on a drilling machine works on voltage. The slower you go the less voltage. The torque is proportional to the square of the voltage (simple terms for technical people who will jump down my throat). This means the slower you go the less torque you will have. May not turn and give you the desired result.

Speed in rpm = frequency x 60 / number of pole pairs.

In the real world (not USA ) the frequency is 50 Hertz or cycles per second.
The frequency x 60 converts revelations per second to minutes.
Pole pairs a little tricky for a lay person.

2 pole, 1 pole pair, nominal speed = 3000 rpm.
4 pole / 2 pole pairs, Ns = 1500 rpm
6 pole / 3, Ns = 1000 rpm
8 pole / 4, Ns = 750 rpm.
10 pole / 5, Ns = 600 rpm.

After 10 poles the motors tend to be non standard. Poles come from the fact that there is a North and South, therefore you can't get 3, 5, 7, 9,......pole motors.

Actual speed is a little lower due to something called slip. Squirrel cage induction motors cannot operate at nominal speed. This is for synchronous machines which a very expensive to buy and control.

For the same power and increasing the number of poles will give you an increase in torque. Related to power, speed. Torque in Nm = (power x 9550) / speed in rpm.

 

[PaulJohnson]

Thanks for the education
The bench grinder is an inductive motor and the drill has brushes so maybe this has something to do with power ?
For this project I'll have to make do with drill power but I have another project on the go which can justify the cost of a decent motor and a proper speed controller

 

[electricpete]

The bench grinder is an inductive motor and the drill has brushes so maybe this has something to do with power ?

If it has brushes, then it's probably a universal motor. That's different than an induction motor. Pretty much all the previous discussion of this thread applied to induction motors.

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(2B)+(2B)' ?

 

[PaulJohnson]

I don't know much about motors and electrics.I can only assume the claimed power of the bench grinder is either fake or most of the power goes to create heat

 

[electricpete]

The meaning of the nameplate power is the power output it will deliver at steady state, near full speed (without exceeding temperature limits). It is not intended to represent the power it will deliver during acceleration. Nor does it represent heat losses in the motor (it is an output mechanical power rating). Reviewing acceleration may require to look at the torque speed curve of the motor and driven equipment among other things. Typically for small pumps, if the motor is rated to drive the load at steady state it will also accelerate it quite well without approaching any limits. Other loads can be more challenging. All that said, I'm surprised about what you reported.

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(2B)+(2B)' ?

 

[waross]

Try an angle grinder and drive your sensor target directly. Some go up to 10,000 RPM (

http://www.makita.ca/index2.php?event=tool&id=219&catid=3).

They use a universal motor that should work with a simple speed controller.

http://www.ebay.ca/sch/i.html?_trks...r&_nkw=4+1/2+angle+grinder&_sacat=0&_from=R40

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

Bench grinders are typically 2 pole motors that run at 3600 rpm, so for a given HP the torque will be half that of a 1800 rpm motor. Also, they are designed to start under no load except inertia, so the starting torque is quite low, by design. Universal motors will put out maximum torque at zero speed.

 

[waross]

Given that the OP is spinning a target to test sensors, I suspect that his problem is not the load but too much loss in his speed step-up mechanism.
Angle grinders use universal motors and run up to 10,000 or 11,000 RPM.
He will also have a useful tool when he is not testing sensors.
Universal motors work with many simple speed reducers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[squeeky]

I agree with Waross. If your pulleys are large and heavy, with large industrial belts, your losses may be very high. As your application requires speed without power, you may get parts from a model shop where toothed belts and pulleys may be what you need.

On a universal motor the light dimmer would work but with a loss in torque as you slowed down. Separating the premature circuit froth field you could run it with a DC drive. You could then go faster using field weakening techniques but again with a loss of torque.

Motors are rated on output (shaft) power but domestic equipment such as lawn mowers claim greater kW (HP) by using input power.

 

[PaulJohnson]

Thanks again
I have put the 8 inch bench grinder back to it's original job ,it was only £60 off ebay so no big deal.
Normally 2900-3000 rpm is OK for testing the VR sensors so i can use my smaller bench grinder (150w) the new device is require to spin an engine distributor so needs a mechanism attached to a 12mm shaft to allow the coupling to slot in.
Using an angle grinder would be OK for direct mounting of a thin target wheel but would be difficult to attach anything else with the thread. I need 4500 shaft speed to simulate 9000rpm engine speed so the drill will be fine and can easily be attached to the 12mm shaft via the chuck.
I'll be building another device soon to drive an engine boring bar so my concerns regarding motor power are more significant
and there will be a stepper motor to drive the feed mechanism-don't want to end up with motors than wouldn't pull the skin off a rice pudding