newb question on motor selection
newb question on motor selection
(OP)
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!
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!





RE: newb question on motor selection
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: newb question on motor selection
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
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If we learn from our mistakes I'm getting a great education!
RE: newb question on motor selection
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.
RE: newb question on motor selection
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
RE: newb question on motor selection
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:
or Mechanical Variable Speed Reducers see for example:
ht
RE: newb question on motor selection
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.
RE: newb question on motor selection
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!