Dahlander Motors 1

[FBW]

Hello All,

Having a few issues here on a dual speed motor running a fan, and i think im getting close to the answer, but i need a bit more insight from some experts.

After a bit of research, I've come to the understanding that what I'm dealing with is a Dahlander motor, which essentially doubles the poles in the motor to change speeds at the same voltage, with the help of a particular type of motor starter (NOT a Delta-Wye motor, which has nothing at all to do with changing speeds, which seems to be causing a lot of confusion for some people).

My question is this: having studied the circuits for both the high and low speed, it seems to me that i should expect to see a substantial current drop in high speed, as what was once 2 windings in series (1 pole) has essentially become 2 windings in parallel (2 pole). Am i correct in assuming this?

If so, that would explain what im seeing, as high speed current is measuring quite a bit lower than low speed current, which seems counter-intuitive to people whom i dont think understand how the motor works.

However, the motor nameplate has FLAs that are near identical for both speeds. So, as you can imagine, its causing some debate on whats going on, not to mention that the motor is not only pulling more that the FLA in low speed, for reasons no one can seem to figure out.

Can anyone shed a bit of light on this for me?

Cheers

 

[ScottyUK]

When you change the number of poles you change the base speed of the motor. Normally that would have some effect upon the driven load: in the case of a fan a doubling of speed it will usually result in a four-fold increase in power required. This means that the line current necessarily increases in order to deliver more power to the load.

I think the place your thinking is going astray is that there aren't really two windings in parallel. They are two electrically similar star-connected windings, but the two star points aren't (usually) tied together to form a true parallel connection. The windings certainly aren't in magnetic parallel because of their angular displacement on the stator core. If you neglect the magnetic circuit you're only doing a partial analysis of the motor, and in a pole-changing design it is essential to consider how the magnetic circuit changes in order to understand its operation.

 

[waross]

A Dahlander motor is sometimes called a consequent pole motor or a subsequent pole motor.
The connections are changed so that all the poles are the same polarity. The opposite poles are formed between the wired poles. Not all winding pitches are suitable for consequent pole use.

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

 

[KuanYau]

To FBW (Industrial)
(OP)
1. " having studied the circuits for both the high and low speed, it seems to me that i should expect to see a substantial current drop in high speed, as what was once 2 windings in series (1 pole) has essentially become 2 windings in parallel (2 pole). Am i correct in assuming this?


2. You have wrong concept of Dahlander connection.
2.1 It is NOT "as what was once 2 windings in series (1 pole) has essentially become 2 windings in parallel (2 pole)".

2.2 For two-speed fan loads, the windings are usually connected in YY/Y formation. There are also YY/D connection for highest output at both speeds etc.

3. "i should expect to see a substantial current drop in high speed"

3.1 NO, in all cases, higher speed connection will result to higher Power(kW) and higher current(A).

4. " the motor nameplate has FLAs that are near identical for both speeds"
4.1 NO, usually the FLAs are NOT near identical for both speeds.
4.2 e.g.1 at 380V YY/Y connection: P1/P2=2/4poles; kW1/kW2=4.5/1kW; I1/I2=8.7/2.3A [suitable for fan load]
4.3 e.g.2 at 380V YY/D connection: P1/P2=2/4poles; kW1/kW2=1.3/1kW; I1/I2=3.2/2.7A
Note: P=No. of poles. 2poles is higher speed than 4poles.

 

[jraef]

If the starter looks kind of like a Wye-Delta, then you must have a 2 speed 1 winding motor. Here in North America, we differentiate the type of 2S1W motor by how it is configured, used and rated. There are three "types"; Constant HP, Constant Torque and Variable Torque. It is only the Constant or Variable Torque versions that is the same as what the IEC world calls a Dahlander motor. The connections look the same on the outside, it is only INSIDE of the motor itself that they differ. In those differences, the easy way to tell is this:

On a Constant Torque version, the Low speed HP or kW rating is 1/2 of the High speed. Torque is the same, therefore current is close to the same in either speed.

On a Variable Torque version, the Low speed is typically 1/4 the HP/kW of the High speed rating, bbecause the torque it develops is typically 1/4 that of the High speed torque. That means the current will (should) be lower as well.

Variable Torque motor version are INTENDED to only be used on centrifugal pumps and fans because the nature of the LOAD profile is such that as the speed reduces, the power required by the load reduces at the cube power of the speed. So at 1/2 speed, the LOAD only requires 1/8 of the power it does at full speed, so having a motor only capable of delivering 1/4 power is still more than adequate. What it sounds like to me, is that you might have a situation wherein someone did not know these subtle differences and applied a Variable Torque version motor on a load that required a Constant Torque version. So because your load requires the same torque regardless of speed, your motor is instantly overloaded as soon as you go to low speed.

Diagrams are shown here, but like I said, the external connections are exactly the same, so there is no way of knowing what you have unless you read the subtle difference in the power rating on the nameplate.

https://www.joliettech.com/easa-ele...winding-easa-electrical-engineering-handbook/

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[zlatkodo]

Hi, FBW,
To start, give us a photo of nameplate or ALL nameplate details.

[URL unfurl="true"]http://winding.wix.com/design[/url]

 

[FBW]

Thanks for all the great replies so far. Ill have more info and nameplate details for you guys today.

 

[FBW]

Nameplate photo attached.

Based on the comments above, i would guess this to be a constant torque motor, given that low speed HP is half of high speed, and little change in current in either speed (28/29).

The motor is drawing around 20 amps in high speed, but in low speed its drawing 31-31 amps, which is over the nameplate FLA and hence, above the overload settings, so we were getting a lot of OL trips. These motors are running supply fans for air handling units. Right now we've adjusted the OL's up a bit, but it's not a solution in my opinion.

There are 5 units doing the same thing, and have been, to my knowledge, for a long time.....

 

[zlatkodo]

On a Constant Torque version, the Low speed HP or kW rating is 1/2 of the High speed.

On a Variable Torque version, the Low speed is typically 1/4 the HP/kW of the High speed rating

Based on the comments above, i would guess this to be a constant torque motor, given that low speed HP is half of high speed

I would not fully agree with that. That's another rule of thumb that often leads to confusion.
I think, it can not be argued that it is a constant torque connection only because the power ratio is approx. 1:2.
Power ratio of two-speed motors can vary much more than the mentioned rule shows.
Unfortunately, manufacturers, for some reason, are not required to mark an internal connection to the nameplate ( as well as they are hiding the NLA).
There is no any appropriate "external" way to determine whether the connection is Y/YY or D/YY. Only reliable answer is inside the winding.
Note that each two-speed winding can be realized in many different winding arrangements. Some of them gives a better winding parameters for low, some for high speed.
( "winding parameters" means : fundamental winding factor, symmetry, differential leakage coefficient, power ratio etc...)
For example, the power ratio will be changed if the winding pitch changes ( but not only power ratio).
Designer selects the winding arrangement that best suits to given application. Consequently, the both the connections are not of "equal quality", because such a design is always a matter of compromise. That could be a reason for your high amps in low speed ( if the load and voltage are within an allowed range). Probably, it is estimated that high speed is more important for such an application.

Winding Design and Calculations

 

[itsmoked]

We'll whatever.. those motors are all going to have shortened lives

before this is straightened out. If those motors came in Engineered air handlers then there is something wrong in the air circuit. Perhaps too low back pressure which is demanding more hp to move the greater amount of air.

If they're just some motors someone stuffed into the hardware then you should probably change pulley sizes or close a damper somewhere to reduce the load.

Could be there is a damper that is supposed to restrict the flow whenever low speed is called for and someone decided it was causing reduced air flow. (duh!)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[KuanYau]

FBW (Industrial)
(OP)
1. "... i would guess this to be a constant torque motor, given that low speed HP is half of high speed, and little change in current in either speed (28/29)".
Yes, where N1/N2 = kW1/kW2 = around 1/2 is usually classified as a "constant torque motor".

2. " The motor is drawing around 20 amps in high speed, but in low speed is drawing 31-31 amps"
No, there is some thing wrong. It is impossible with constant voltage(575V), (say similar P.F. for both speeds)to be operating on high speed(1780rpm) with lower current(20A), while on low speed(880rpm) at higher current(31A).

3. Check the connection diagram and the starter circuitry. They can be very confusing if the terminals are not properly marked.

4. For fan load, the Power(kW) required is proportional to the cube of the Speed(rpm)

5. Check the fan Power requirement curve to establish the Power(kW) required for high and low speeds.
The motor shall fulfils the Speed(rpm) and the Power(kW) per the fan curve.
It shall be N1/N2 = kW1/cube kW1 = I1/cube I1. Also if N1>N2; then Power kW1>kW2; and Current I1>I2.
Attention: Your present motor is NOT having this (fan load) characteristic. It can be used if the Speed and the Power rating fulfil the fan Power requirement curve.

 

[LionelHutz]

Geeze, it really doesn't matter if the motor is constant torque or variable torque. Both are connected the same way. Still, my guess would be constant torque as well. Here are the connections for both types.

High speed - Power to 6,4,5 and short 1,2,3
Low speed - Power to 1,2,3 and open 4,5,6

I would check the connection of the shorting contactor. Does it short 1,2,3 or 4,5,6? My guess would be the starter is configured for the constant HP connection and is shorting 4,5,6 when it should be shorting 1,2,3.

 

[zlatkodo]

Once, when the first motor will be burnt , you need to take the opportunity to record all iron-core measures and winding data & diagram.
Then we can do more: analyze the winding, try to find the right cause and suggest some changes to overcome it.

Winding Analysis

 

[KuanYau]

To FBW (Industrial)
(OP)
1. Your motor name-plate rating shows:
Hp1/Hp2 = 30/15 = 2
N1/N2 = 1780/880 = 2
I1/I2 = 29/28
V= 575V
This is a typical "constant torque application" motor; where N1/N2 = 1780/880 = 2
and Hp1/Hp2 = 30/15 = 2.
These "constant torque" motors can be wound with two separate windings or in Dahlander-connection.

2. Reference to the fan affinity law, when the N1/N2 = 2, Hp1/Hp2 = 2x2x2 =8.
Typical "fan drive application motors" with N1/N2 =2, Hp1/Hp2 = 4...7.7.
These "fan drive" motors can be wound with two separate windings or in Dahlander-connection.

3. Your present "constant torque" motor does NOT match with the "fan drive" application.
Even with N1/N2 is around 1780/880 rpm as required, but the Hp1/Hp2 =2 is a problem. It does NOT match with the "fan drive" application requirement; which needs a Hp1/Hp2 = 8.

 

[ScottyUK]

"It does NOT match with the "fan drive" application requirement; which needs a Hp1/Hp2 = 8."

Actually that doesn't matter in a fan drive application, provided that the power capability in high speed is adequate. True it's not a mathematical match, but it is a perfectly acceptable solution - the motor just has much more capability at the low speed than is required by the load. I agree it could have been specified differently from new, but in this existing application it is ok.

 

[FBW]

Been doing a lot of research on this, and given what i know now, i would agree on the idea that its not the ideal motor for a load like a fan, but as ScottyUK pointed out, it shouldn't be causing any grief so long as the high speed HP is adequate. It certainly shouldn't be causing the kind of current draw I've been seeing.

I found the fan curve for these units, and to my understanding(i've never had to read a fan curve until this started happening), even at free delivery, this fan should not require more than about 5hp at low speed.

I'm going to go have another look at the motor and starter circuit wiring and see if anything looks odd. Learning a lot about fans and motors here, thanks all..

 

[KuanYau]

FBW (Industrial)
(OP)
1. Your present motor is intended to operate on 1780/880rpm. The kW1/kW2=30/15 is a total mismatch.
Assuming 30Hp is rated for 1780rpm, the motor rated 15Hp would be far too high when running
on 880rpm. It does NOT cause any damages physically but financially as it would be running at very low efficiency and power-factor. Running with low efficiency is damaging financially while low power-factor may not, that depends on whether the utilities charges take the Pf into consideration.

2. You had reported that "im seeing, as high speed current is measuring quite a bit lower than low
speed current".
No, there is some thing wrong.

3. A typical 2-speed motor irrespective of whether ["constant torque" or "fan drive"],and with ["two separate windings" or "Dahlander-connection"]:
in general: when N1>N2, kW1>kW2 and I1>I2 (only exceptional cases with very low ratings I2>I1). The Eff1>Eff2 (only exceptional cases Eff2>Eff1) also Pf1>Pf2 (only exceptional cases Pf2>Pf1)

 

[zlatkodo]

Another information (if any), might be useful: no-load Amps in low and high speed-connection.
Note: "no-load" means uncoupled shaft.

 

[KuanYau]

To zlatkodo (Electrical)

1. "Another information (if any), might be useful: no-load Amps in low and high speed-connection".
Usually no-load Amps in low and high speed-connection are NOT useful. These data are not needed for engineering design. That is why these data are usually not listed in the catalogues.

2. The basic rating data are voltage(V)+ Fqc(Hz), N(rpm1/rpm2), current (I1/I2), and less important data are Fff1/Eff2, Pf1/Pf2, Is1/I1 Is2/I2, Torque ratios, moment of inertia and weight etc.

3. Motor ratings:
a) Voltage and Frequency data are to ensure that match with the power source ratings,
b) Speeds N1/N2 match with the delivery volume and static pressure,
c) Current I1/I2 for breaker/fuse, contactors, overloads and conductor sizing,
d) Torque rations, and moment of inertia are for starting-up time study,
e) Eff1/Eff2, Pf1/Pf2, Is1/I1 Is2/I2 and weight are of less important,

 

[LionelHutz]

KuanYau - Do you have proof that the motor at 25% load operates at "very low efficiency"?

Most 3-phase induction motor data I've seen has motor efficiency curve that are reasonably flat from about 15% load to full load. I'd expect the motor might drop about 3% in efficiency at 25% load. I'm just not seeing how that is a very low efficiency or worthy of dire warning about it being the wrong motor. You're possibly looking at about 100W of extra power being used in low speed compared to the same rated efficiency variable torque motor.

I have doubts this would be a 2-winding 2-speed motor because the frame size would likely be larger. It is difficult to fit 2 completely separate windings into the same frame size that a motor with a single winding uses.

Also, the no load current can definitely be a help when troubleshooting a situation such as this. Measuring to much low speed current even without load would eliminate the fan as having any part in causing the high current.