Understanding Const. Tq, Var Tq, & Const. HP Induction motors 4

[amptramp]

I am familiar with constant torque, variable torque, and constant horsepower loads. It has been brought to my attention there are constant torque, variable torque, and constant horsepower induction motors. These motor designs seem to be associated with multispeed motors and consequent pole designs. How are the different characteristics (constant torque, variable torque, etc.) achieved by design? Is it the consequent pole design that achieves these differing characteristics or something else in the motor design? Here is a link to Leeson Electric that illustrates the motor types I am curious about.

http://www.leeson.com/Literature/pdf/1050/AC2SpeedCustomPDQ.pdf

 

[DickDV]

In the case of multi (not variable) speed motors, when you change speeds you have a choice in designing the machine.

Since HP=TxRPM/5250, if you want a constant hp motor and you drop the speed by half, clearly you will need to double the torque, etc. So, you would design the motor poles and magnetics that way.

On the other hand, if it's constant torque you are after, then dropping the speed by half and holding the torque constant, will result in a drop in hp of one-half, etc.

I haven't seen a multispeed motor designed this way but, if you were running a centrifugal pump, for example, and wanted to drop the speed by one half, the load hp would drop by 7/8. You could design the motor so at one-half speed The motor has only one-quarter of the torque to match the characteristic of the pump. At that lower speed, the motor would develop only one-eighth hp to match the pump loading requirement. That would probably be called a variable torque motor.

 

[jraef]

Nicely put Dick.


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[waross]

From the Cowern Papers on the Baldor web site;
At

http://baldor.com/support/literature_load.asp?LitNumber=PR2525

A PRIMER ON TWO SPEED MOTORS
There seems to be a lot of mystery involved in two speed motors but they are really quite simple. They
can first be divided into two different winding types:
TWO SPEED, TWO WINDING
The two winding motor is made in such a manner that it is really two motors wound into one stator. One
winding, when energized, gives one of the speeds. When the second winding is energized, the motor
takes on the speed that is determined by the second winding. The two speed, two winding motor can
be used to get virtually any combination of normal motor speeds and the two different speeds need not
be related to each other by a 2:1 speed factor. Thus, a two speed motor requiring 1750 RPM and 1140
RPM would, of necessity, have to be a two winding motor.
TWO SPEED, ONE WINDING
The second type of motor is the two speed, single winding motor. In this type of motor, a 2:1 relationship
between the low and high speed must exist. Two speed, single winding motors are of the design that is
called consequent pole. These motors are wound for one speed but when the winding is reconnected,
the number of magnetic poles within the stator is doubled and the motor speed is reduced to one-half of
the original speed. The two speed, one winding motor is, by nature, more economical to manufacture
than the two speed, two winding motor. This is because the same winding is used for both speeds and
the slots in which the conductors are placed within the motor do not have to be nearly as large as they
would have to be to accommodate two separate windings that work independently. Thus, the frame size
on the two speed, single winding motor can usually be smaller than on an equivalent two winding motor.
LOAD CLASSIFICATION
A second item that generates a good deal of confusion in selecting two speed motors is the load
classification for which these motors are to be used. In this case, the type of load to be driven must be
defined and the motor is selected to match the load requirement.
The three types that are available are: Constant Torque, Variable Torque, and Constant Horsepower.
For more details on load types please refer to “Understanding Torque” in this booklet.
CONSTANT TORQUE
Constant torque loads are those types of loads where the torque requirement is independent of speed.
this type of load is the normally occurring load on such things as conveyors, positive displacement
pumps, extruders, hydraulic pumps, packaging machinery, and other similar types of loads.
VARIABLE TORQUE
A second load type that is very different from Constant Torque is the kind of load presented to a motor
by centrifugal pumps and blowers. In this case, the load torque requirement changes from a low value at
low speed to a very high value at high speed. On a typical variable torque load, doubling the speed will
increase the torque requirement by 4 times and the horsepower requirement by 8 times. Thus, on this
type load, brute force must be supplied at the high speed and much reduced levels of horsepower and
torque are required at the low speed. A typical two speed, variable torque motor might have a rating of 1
HP at 1725 and .25 HP at 850 RPM.
The characteristics of many pumps, fans, and blowers are such that a speed reduction to one-half
results in an output at the low speed which may be unacceptable. Thus, many two speed, variable
torque motors are made with a speed combination of 1725/1140 RPM. This combination gives an output
from the fan or pump of roughly one-half when the low speed is utilized.
16
CONSTANT HORSEPOWER
The final type of two speed motor that is utilized is the two speed, constant horsepower motor. In this
case, the motor is designed so that the horsepower stays constant when the speed is reduced to the
low value. In order to do this, it is necessary for the motor’s torque to double when it is operating in the
low speed mode. The normal application for this type of motor is on metal working processes such as
drill presses, lathes, milling machines, and other similar metal removing machines.
The requirement for constant horsepower can perhaps be best visualized when you consider the
requirements of a simple machine like a drill press. In this case, when drilling a large hole with a large drill,
the speed is low but the torque requirement is very high. Compare that to the opposite extreme of drilling
a small hole when the drill speed must be high but the torque requirement is low. Thus, there is a
requirement for torque to be high when speed is low and torque to be low when speed is high. This is
the Constant Horsepower situation.
The Constant Horsepower motor is the most expensive two speed motor. Three phase, two speed
motors are quite readily available in constant torque and variable torque. Two speed, constant
horsepower motors are usually only available on a special order basis.
TWO SPEED, SINGLE PHASE MOTORS
Two speed, single phase motors for constant torque requirements are more difficult to supply since there
is a problem of providing a starting switch that will operate at the proper time for both speeds. Thus, the
normal two speed, single phase motor is offered as a variable torque motor in a permanent split
capacitor configuration. The permanent split capacitor motor has very low starting torque but is suitable
for use on small centrifugal pumps and fans.
SUMMARY
The use of two speed motors in the future will grow quite rapidly as industrial motor users begin to realize
the desirability of using this type of motor on exhaust fans and circulating pumps so that the air flow and
water flow can be optimized to suit the conditions that exist in a plant or a process. Very dramatic
savings in energy can be achieved by utilizing the two speed approach.

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

 

[amptramp]

Hello all:

Thanks for the replies.

waross:

I found the Cowern papers also. Some good info. Since I made my post I have learned a lot by Googleing multispeed motors, consequent pole windings, and Dahlander windings. I suppose I should have done my research first before I bothered all of you; however, your insights are always enlightening. I have found the motor lead connections, and now understand them. I am still investigating the physics of the three different connections - constant torque, variable torque, and constant horsepower. I may be over complicating things as I feel the differing torque-speed-horsepower interactions must be due to the distribution of the pole pairs. I have pulled my Rotating Machines texts but they do not discuss multispeed asynch motors.

 

[edison123]

Well done, Bill.

amptramp - Try to get hold of 'performance and design of ac machines' by M G Say. I consider that as my bible.

Muthu

http://www.edison.co.in

 

[desertfox]

hi waross

a star for your excellent post.

desertfox

 

[waross]

I'll accept that star in absentia for Mr. Cowern PE. He has a gift for explaining motor concepts in simple terms.
Thank you on behalf of Mr. Cowern PE.

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

 

[electricpete]

amptramp has some questions about consequent poles. I looked that up a little bit and here is a discussion:

http://www.uiitraining.com/b51a/100/15104ac_motor_rewind_intro7.htm

http://www.uiitraining.com/b51a/100/15104ac_motor_rewind_intro8.htm

Each type of consequent pole motor winding will have two separate connections. One is for the high-speed number of poles (4, 6, etc.) and one is for the low-speed number of poles (8, 12, etc.). A specially designed motor controller is used to supply power to the connections (speed) desired. The poles in the high-speed connection will be connected for alternate polarities. Poles in the low-speed connection will be connected for the same polarity, and an equal number of poles of opposite polarity will result as a consequence. That is, the opposing same poles flux create a virtual opposite pole between them. The name consequent pole comes from this consequence. The core and frame must be made from a special high permiability steel to provide the flux path for these virtual poles.

So it’s clear the high speed winding is typical winding construction. That makes sense to me.

For example if we look at the pole phase groups moving around the circumference for a 4-pole motor, 60hz motor (1800rpm) we might have something like:
A, B’, C, A’, B, C’ A, B’, C, A’, B, C’

And now when we get to slow speed (900rpm), something different is required. So the pole phase groups in the slow speed diagram are arranged as follows:
A B C A B C A B C A B C

It makes perfect sense to me that this slow speed arrangement yields 900rpm = half the speed of the high speed connection, but not for the reasons stated in the quote above.

To my way of thinking:
1 - the high-speed (typical) pattern A, B’, C, A’, B, C’ A, B’, C, A’, B, C’ has 60 electrical degrees between pole phase groups. So for the field to go once around the stator = 12 pole phase groups will require 12*60 degrees = 720 degrees = 2 cycles = (2/60) sec = (1/30) sec => 1800rpm.
2 - the low-speed pattern A, B, C, A, B, C A, B, C, A, B, C has 120 electrical degrees between pole phase groups. So for the field to go once around the stator = 12 pole phase groups will require 12*120 degrees = 1440 degrees = 4 cycles = (4/60) sec = (1/15) sec => 900rpm.

I don’t understand the need for any “induced poles” to accomplish this... in my mind it is simply a re-definition of what constitutes a pole (or more specifically the time interval between poles). There is no need for another pole between ppg A and ppg B to provide return path for flux, because A and B have different phase. And their discussion of induced pole implies a return path involving through the frame involving homopolar flux (“The core and frame must be made from a special high permiability steel to provide the flux path for these virtual poles”) – I don’t see the need for that: the fluxes from A+B+C sum to 0 in the same way that the fluxes from A+B’+C+A’+B+C’ sum to 0.

That doesn’t make sense to me. I think they are off-base. I am going to stick with my way of thinking (this is simply a redefinition of degrees between pole as 120 instead of 60) unless someone else can explain it better to me.


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[electricpete]

By the way, I am not focusing on the constant HP, constant T, etc type connections, which affect how the pole phase groups are wound (in series with each other, in parallel, wye delta etc). Only the speed which does not require examination of connection, but only what polarity of winding is in each pole phase group position.

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[electricpete]

If one were of the opinion that the flux from A must return through A’, then one would conclude that an "induced" pole is necessary. However imo there is no reason the flux from A cannot return thru B and C.

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[electricpete]

Maybe it should be a new thread: how does the consequent pole connection accomplish change in speed.

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[electricpete]

I will post a new thread on this.

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[edison123]

pete - One pole cannot exist without another. What happens in low speed winding is that all the pole groups are connected a manner to produce only one pole (i.e. the current going in the same direction in all the pole groups) and the other pole, by physics, is automatically produced.

Muthu

http://www.edison.co.in