Speed control for 120V drill

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170412-1131 EDT

Shaneyj:

You are an EE student. This means that you should have resources, lab equipment and professors, that could help you on your problem.

What is a universal motor? This usually means a motor with a field winding designed and used in series with a commutated rotor winding. You might initially consider this a DC motor. But if you study what is occurring internally you will see why it will run on AC.

Looking at the motor terminals this device has an AC impedance. But it also has an internal voltage generator. The current thru the motor is a function of speed, mechanical load, and the series impedance. Mechanical load is made up of internal frictional and windage losses plus the external load.

In a DC type motor speed is determined by field intensity, applied source voltage, internal IR voltage drop, and counter EMF (the internally generated voltage). If a DC shunt motor looses it field excitation, then the motor RPM is likely to runaway with destruction of the motor because with less field the motor has to run faster to produce the necessary counter EMF.

In a universal motor the field is proportional to motor current. Because of internal motor losses and windage an unloaded motor current increases as speed increases. Thus, field intensity increases with speed and the motor self limits speed.

For a fixed source voltage speed decreases as load increases because of two factors --- field increases because of load current, and internal IR causes a reduced need for counter EMF.
 
Last edited:
retirede said:
... Look at Jraef's curves. Current and available torque will increase as you lower speed.
Click to expand...
The curves are applicable only to fixed-voltage operation. With a fixed voltage, the speed will decrease as the load demands more torque.

When you reduce voltage on a series-wound brushed motor, you reduce speed. But you also reduce current (constant motor resistance) and torque. (proportional to current)
 
170412-1131 EDT

Shaneyj:

Because of forum timeout and other problems my complete post did not go thru. Following is the complete post.

You are an EE student. This means that you should have resources, lab equipment and professors, that could help you on your problem.

What is a universal motor? This usually means a motor with a field winding designed and used in series with a commutated rotor winding. You might initially consider this a DC motor. But if you study what is occurring internally you will see why it will run on AC.

Looking at the motor terminals this device has an AC impedance. But it also has an internal voltage generator. The current thru the motor is a function of speed, mechanical load, and the series impedance. Mechanical load is made up of internal frictional and windage losses plus the external load.

In a DC type motor speed is determined by field intensity, applied source voltage, internal IR voltage drop, and counter EMF (the internally generated voltage). If a DC shunt motor looses it field excitation, then the motor RPM is likely to runaway with destruction of the motor because with less field the motor has to run faster to produce the necessary counter EMF.

In a universal motor the field is proportional to motor current. Because of internal motor losses and windage an unloaded motor current increases as speed increases. Thus, unloaded externally, the field intensity increases with speed, because the losses increase with speed, and the motor self limits speed.

For a fixed source voltage speed decreases as load increases because of two factors --- field increases because of load current, and internal IR drop, causes a reduced need for counter EMF.

As you reduce source voltage to a universal motor the speed is reduced when unloaded externally. The internal motor impedance remains about constant. As source voltage is reduced the stall torque will decrease about linearly with applied voltage.

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gar,

I take issue with one point that you made. A series wound universal motor will tend to increase speed without limit if unloaded.
The reason is that the normal current flow is, as you say, limited not just by the winding resistance but by the reverse EMF generated by the rotor windings moving in the magnetic field generated by the field winding(s).

As the speed increases, for constant field current, the back EMF increases and this tends to reduce the current. Since in a series wound motor the two currents will be identical this means that the field decreases and the speed required to generate that same back EMF voltage increases.

A series wound motor which is expected to occasionally run unloaded is generally fitted with a centrifugal governor or similar device to limit its speed.

What you describe is not even completely valid for a shunt field motor.
As speed increases the field does NOT increase.

In a universal motor the field is proportional to motor current. Because of internal motor losses and windage an unloaded motor current increases as speed increases. Thus, unloaded externally, the field intensity increases with speed, because the losses increase with speed, and the motor self limits speed.
Click to expand...
 
drcampbell said:
The curves are applicable only to fixed-voltage operation. With a fixed voltage, the speed will decrease as the load demands more torque.

When you reduce voltage on a series-wound brushed motor, you reduce speed. But you also reduce current (constant motor resistance) and torque. (proportional to current)
Click to expand...
That was also my understanding. They do not look like good candidates for speed control.
 
170412-1704 EDT

GoldDigger:

First: On a DC shunt wound motor:

My comment was:
In a DC type motor speed is determined by field intensity, applied source voltage, internal IR voltage drop, and counter EMF (the internally generated voltage). If a DC shunt motor looses it field excitation, then the motor RPM is likely to runaway with destruction of the motor because with less field the motor has to run faster to produce the necessary counter EMF.
Click to expand...

Your comment was:
What you describe is not even completely valid for a shunt field motor.
As speed increases the field does NOT increase.
Click to expand...
I did not say the field increases with speed for a DC shunt motor. What I said was that if you loose field excitation (admittedly the word "it" should have had the letter "s" appended), then the speed increases. Actually field current adjustment is one way to adjust the speed of a DC shunt wound motor above base speed, but with a loss of torque capability.

My comment on a series wound DC motor was:
In a universal motor the field is proportional to motor current. Because of internal motor losses and windage an unloaded motor current increases as speed increases. Thus, unloaded externally, the field intensity increases with speed, because the losses increase with speed, and the motor self limits speed.
Click to expand...
In a DC motor there is a balance between source voltage and the rotor voltage (counter EMF). The difference is roughly related to the mechanical output power. Frictional losses (bearing and windage) rise at a faster rate than linear with respect to speed. Thus, frictional losses cause current to rise faster than speed and this counteracts speed increase in two ways. Increased field intensity that tends to lower rotor speed, and increased internal IR drop that tends to reduce the required counter EMF. This is a self limiting process in contrast to the DC shunt motor loss of field excitation.

An open circuit to the field of a series motor causes the motor to stop. An open c ircuit to a DC shunt motor causes runaway speed.

.

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gar said:
170412-1704 EDT

GoldDigger:

First: On a DC shunt wound motor:

My comment was:


Your comment was:
I did not say the field increases with speed for a DC shunt motor. What I said was that if you loose field excitation (admittedly the word "it" should have had the letter "s" appended), then the speed increases. Actually field current adjustment is one way to adjust the speed of a DC shunt wound motor above base speed, but with a loss of torque capability.

My comment on a series wound DC motor was:

In a DC motor there is a balance between source voltage and the rotor voltage (counter EMF). The difference is roughly related to the mechanical output power. Frictional losses (bearing and windage) rise at a faster rate than linear with respect to speed. Thus, frictional losses cause current to rise faster than speed and this counteracts speed increase in two ways. Increased field intensity that tends to lower rotor speed, and increased internal IR drop that tends to reduce the required counter EMF. This is a self limiting process in contrast to the DC shunt motor loss of field excitation.

An open circuit to the field of a series motor causes the motor to stop. An open c ircuit to a DC shunt motor causes runaway speed.

.

.
Click to expand...

When you refer to a shunt wound DC motor, are you talking about pure shunt or compound (one series and one parallel field coil)?
In a pure shunt motor, it will not spin at all because there will be no field (except possibly for residual magnetism in the pole pieces.)
Whatever magnetic field remains to cause the rotor to move will also cause a back EMF. It is true that the rotor current will be excessive, limited only by the rotor coil resistance.
 
170412-2348 EDT

GoldDigger:

A shunt wound DC motor consists of a commutated armature (rotor), and a field generated with a wound coil (no permanent magnet except for residual flux). This wound field is connected directly in parallel with the armature, and thus has the same voltage applied to it as the armature in the simplest form. If the circuit thru the field opens up, then the field intensity drops greatly and the motor RPM rises to a high level.

If one has a separate controller for excitation of the field, then one can adjust field current to produce overspeed.

In DC machinery lab we had to have one person ready to remove armature excitation if current was lost to the field. In commerical controllers there will be a loss of field current detector to shutdown armature excitation.

In a compound wound DC shunt motor an added coil is added to the field with armature current flowing thru the added coil to make a nearly constant speed motor relative to mechanical load. This compound field winding dates back to the early Edison days where it was used to provide constant generator voltage output under variable load conditions.

Thre DC Machinery book I used was by Bull of the University of Michigan. This was copywritten in 1930. I started this course with Bull as the instructor, but dropped it because I did not like his teaching approach. Later I had Tarboux for both DC and AC machinery. Tarboux was an outstanding teacher with techniques that still stick in my head.

In our lab I believe we used DC motors that were 5 HP or more.

In a test machine for the Ford Aerostar transfer case where I designed and built the electronics we used 3 GE 50 HP shunt wound DC motors for drivers and loads. We did use the overspeed capability by reducing field excitation. These motors also had separately powered fans so that we could operate at full current at zero speed.

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170412-32435 EDT

When I was student at the U of M the vast majority of classes I had were taught by outstanding professors. Many classes would be less than 30 students. Sometimes possibly only 5 students. When there were teaching assistants instead of professors, and I only had a few of these, then I felt that they lacked the insite of the professors. But I also worked part time with a number of my profs.

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You say that when the shunt coil supply opens the magnetic field is reduced. Why does it not just go to zero?
If the only field is the residual flux, why does that not just stay at a constant level, which would lead to a fixed upper speed of the motor? That may be (far?) higher than the design speed, but it would not be unlimited.
 
170413-0831 EDT

GoldDigger:

You are correct that when you loose field excitation that the flux level drops to some residual level. And yes that would define some upper maximum speed, but in real world machines that speed may be high enough to have the rotor fly apart and destroy the machine, and possibly kill someone.

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170413-0838 EDT

ActionDave:

An easy way to look at how torque of a DC motor varies with excitation voltage is to consider locked rotor conditions. At locked rotor the counter EMF is zero, and the rotor resistance alone for a shunt motor is what determines rotor current, and rotor resistance plus field resistance determines the current thru the field and rotor for a series motor.

So basically we are looking at current defined by a simple voltage and resistance circuit where the resistance can be considered constant. As source voltage goes down current goes down.

In a DC motor at any other speed than zero the output torque will be less than at locked rotor. Not true of most AC induction motors.

In a DC shunt motor you have one fixed magnetic field (the field coil) working against a variable field (the rotor) and force or torque is proportional to the rotor current.

In a DC series motor you have two varying magnetic fields working against each other. Here the same current flows in both the field and rotor, and thus the torque is proportional to the square of the rotor-field current.

In summary because we can represent the motor as as a resistance at locked rotor and torque is related to current it is obvious that as source voltage is reduced that torque is reduced.

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ActionDave said:
I don't know much about charts and graphs but I know for sure the same thing happens to me every time I use a variable speed drill.
Click to expand...
Yes, simple speed control that either varies voltage or electronically cuts out part of the voltage waveform, eventually gets to a point where when you "throttle it down" enough you can stop the rotation with your hand. If torque remained the same at this setting you would not be able to do that.
 
Shaneyj said:
I have a Milwaukee 3/4" super hole shooter 350. We would like to be able to vary the speed without compromising rpm. I spoke with a tech from Milwaukee that said they don't make a product but he did not recommend against.
Will a simple voltage control achieve this without loss in torque?
Is there a vfd capable of this?
Thanks for any input!


Sent from my HTC6545LVW using Tapatalk
Click to expand...


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Old school variac from your nearest electronic surplus store.
 
170414-0720 EDT

markebenson:

By the very nature of a series motor you loose maximum torque capability as the applied voltage is reduced.

This is in contrast with a DC shunt motor that can provide full rated torque at 0 RPM with relatively low rotor voltage provided that full field excitation is provided and external cooling is supplied.

These two statements are somewhat misleading, but from a practical perspective reasonably correct.

In the series motor both the rotor and field share the same current. This results in poor speed regulation with load. Full rated torque can be obtained at 0 RPM, but with a much higher input voltage than in the case of rotor (armature) voltage for a DC shunt motor where field excitation is separately held constant.

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