newb question on motor selection

[mc02076]

Greetings,

I need to select a motor for an mechanical press application.Its nothing fancy, a 1.5 kW motor will do plus I need variable rpm and a max/min rpm ratio of 60.
So,from what I have read,I can either use a dc motor or an induction motor with motion control,the latter being a better choice as they can go up to an rpm ratio of 1000 (or so I ve heard)
However,while browsing all the manufacturers at global spec,I have found 2 types of induction motors.The ones that are referred to simply as "induction motors" for whom there is no definition over the max and min rpm but only a fixed rpm value for a fixed AC frequency, and the so called "variable speed induction motors" for whom the rpm ratio is 2 to 4.

So I guess my question is this:

Can all induction motors be controlled with an inverter? if so,how can one calculate the rpm ratio that can be achieved?

Thanks in advance!

 

[Skogsgurra]

Yes, all induction motors can be controlled with an inverter, aka VFD (Variable Frequency Drive).

The speed range depends very much on what characteristic the inverter has. There are two main types, the scalar aka U/f type and the vector controlled type.

For a wide control range, the vector type is preferred. And, since modern VFDs almost always have vector mode available, there is no reason not to choose it.

There is a difference also among vector drives. Some can not exert rated torque at low speed and some are better at it. For good control down to zero speed, you need to add an encoder to the motor shaft. But a 60:1 speed range can work without encoder.

So, off you go! Buy an inverter with vector control. Be careful with cooling at the lower speeds. You may need an external fan for your motor if running prolonged times at low speeds. And you may need filters to comply with EMC regulations. All these things a competent salesman can tell you. But with the facts given above, you are in a better position to discuss drives than most "ordinary" customers usually are.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Short answer - no, not all types.

Longer answer - most commodity inverters are designed for use with a 3-phase squirrel cage induction motor. 3-ph squirrel cage motors are the typical general purpose motors you see in most applications, so we'll concentrate on them and ignore the other types of induction motor. The two main problems are:

The insulation of the motor windings should be rated for inverter duty, i.e. it has better breakdown characteristics than 'standard' motor wire, although much of the 'standard' wire is now inverter grade. Go figure!

The second big problem is heat. Most squirrel cage motors are of totally enclosed fan cooled, or TEFC, design. They rely on a shaft-mounted fan to force cooling air over the finned casing to remove heat from the motor. As you reduce speed the fan performance drops rapidly and in high torque applications where the motor draws a relatively high current even at low speed you run in to thermal problems. Motors with a high turndown ratio usually need an auxiliary blower fed from a standard supply to provide cooling at lower speeds.

The third of the two main problems is that some drives perform better at very low speeds than others. To some extent you get what you pay for but if you are looking for good control at the very bottom end of the speed range then you probably should be looking for a sensorless vector drive rather than a scalar volt/Hz drive. It is possible to get even better performance using an encoder in a closed loop system but I doubt you have any need for that quality of control and the additonal expense and complexity would be pointless.


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If we learn from our mistakes I'm getting a great education!

 

[DickDV]

At 1.5kw, I would choose a VFD with sensorless vector capability (most have it these days) and a TENV induction motor.

Since this posting was primarily about selecting a motor, as was mentioned above, you need a motor that has improved insulation for operation on inverter power and you need a motor that will be able to cool itself at the slowest speeds. In the power range you are working with, a Totally Enclosed Non-Vent (TENV) motor would be your most economical choice.

In larger kw ranges, as mentioned above, you would need to choose a motor with a constant speed blower on it.

In general, look for an "Inverter Duty" motor. That will usually give you the improved insulation and the extended speed range.

 

[waross]

The vector drive aside, what kind of a mechanical press do you have? All of the mechanical presses and shears that I have encountered rely heavily on the inertia of a large fly wheel to supply a significant portion of the operating energy.
If this is the main drive motor beware of slow speeds. It may well stall.
These machines mostly use design "D" induction motors. A "standard" induction motor may stall, burn-out or both.
If the motor is related to some auxiliary function on the press then follow the excellent advice of Skogs, Dick and Scotty.
VFDs drive three phase motors. However at 1.5kW most VFDs will accept a single phase input and deliver three phase to a three phase motor.

Bill
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"Why not the best?"
Jimmy Carter

 

[7anoter4]

I don't know what kind of press is yours [eccentric driven, hydraulic driven and so on]
As I am myself an electrical engineer I prefer power electronics [with all its complication as ScottyUK explained very well], but I think a mechanic device for a simple press is more suitable [I saw press in good operation state for 20-30 years]
You may purchase 2 [or 3] speed motors as for instance:

http://www.nyborgfan.com/manuals/Operation_and_Safety_Manual_Mez_Motors.pdf

or Mechanical Variable Speed Reducers see for example:

http://www.winsmith.com/products/variable_speed.html

 

[TurbineGen]

What's wrong with a brushed DC motor and variable controller? These are found in many machine shops and are quite cheap and robust. I have found in large speed ranges such as the ones described here, the brushed DC motor often has better low rpm torque characteristics which may be desirable in a press. Also, you don't need to worry so much about the motor stalling.

There are two types of brushed DC motors that can work: shunt wound motors, and permanent magnet. The permanent magnet motor is strictly regulated by a PWM (pulse width modulation) signal. These are slightly less favorable than the shunt motor where high torque is needed. The shunt motor speed is regulated by increasing or decreasing the current in the shunt winding. An increase in the shunt winding current lowers rpm and increases torque. You will need a variable controller with the shunt motor to get the speed range you require.

All of the above will probably work too. I feel DC motors are simpler and might be more cost effective.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[mc02076]

Thank you for all your replies! they are very informative and I think I now have a way or two to study this problem.

Warros: The pressure plate is driven by ball screws,the press will be used for material testing.You are right,I do intend to use a flywheel and a clutch.I will study the possibility of motor stall and burn out.Thanks for your response!

TurbineGen: I was informed that an induction motor might be cheaper for that kind of power.I have already chosen a brushed DC motor for my application and I am just examining the possibility of an induction motor with motion control.If the cost is not significantly lower I will go with the brushed motor.

However,I feel like I am really left behind on the available motor technology.Can anyone suggest a nice book or site that can give a little "know how" boost?

Thanks again!