Two Speed Motor Amp Draw

[BillyBoy]

We have a two speed motor on several large traveling screens at our cooling water intake. Interestingly the CT measures a LOWER current draw when the speed goes from low to high. In low speed we see about 5.5 amps. In high speed we see about 4.5 amps. This seems just plain wrong to me, but the vendor stated this was typical. One of the guys suggested it might have to do with a delta/wye configuration in the motor. Input is appreciated! Thanks!!

 

[just the cowboy]

different windings

different windings

Remember that a two speed motor uses different windings so it is like two separate motors. Where a vfd is changing both freq and voltage so voltage goes up current goes up like you are thinking.

 

[Jraef]

We have a two speed motor on several large traveling screens at our cooling water intake. Interestingly the CT measures a LOWER current draw when the speed goes from low to high. In low speed we see about 5.5 amps. In high speed we see about 4.5 amps. This seems just plain wrong to me, but the vendor stated this was typical. One of the guys suggested it might have to do with a delta/wye configuration in the motor. Input is appreciated! Thanks!!

There are different TYPES of 2 speed motors, so it will depend on what you have and how the CTs are connected. It's easy to get it wrong, but it might be right.

If it is a 2S2W motor and you only put the CTs on the low speed set of windings, it would not read current at all, so it must be a 2S1W motor. There are 3 different types of those, Costant HP, Constant Torque, and Variable Torque configurations. In the last two types, the higher speed should still read higher amps than the lower speed, but if the CTs are in the wrong place, you might only be reading a portion of the current. In the Constant HP configuration, the higher speed means less torque for the HP to remain the same, so less torque means less current. But without knowing what you have, it's impossible to tell.

Look at the starter. If there are only two contactors, it's 2S2W, so something it wrong if it is reading that way. If there are 3 contactors, it's 2S1W. So then look at the motor nameplate. If it shows two HP ratings, it's CT or VT, so something is wrong with the CT setup. If the nameplate shows one HP rating, it's CHP and it likely will read less amps at the higher speed.

 

[Phil Corso]

BillyBoy...

Are fish the bulk of the entrapped material?

Regards, Phil Corso

 

[BillyBoy]

I dug up some info, pasted below. According to documentation the motor is 2 speed with 2 windings and is constant torque. Both fish and debris are removed by the screens.

 

[Electric-Light]

After giving it some thought, I made sense of it.

What you likely have is a motor that acts as if you have a 1:2 gearbox.
3450 x 1#
1725 x 2#

where # is whatever torque needed to achieve the rated hp.

 

[templdl]

There are different TYPES of 2 speed motors, so it will depend on what you have and how the CTs are connected. It's easy to get it wrong, but it might be right.

If it is a 2S2W motor and you only put the CTs on the low speed set of windings, it would not read current at all, so it must be a 2S1W motor. There are 3 different types of those, Costant HP, Constant Torque, and Variable Torque configurations. In the last two types, the higher speed should still read higher amps than the lower speed, but if the CTs are in the wrong place, you might only be reading a portion of the current. In the Constant HP configuration, the higher speed means less torque for the HP to remain the same, so less torque means less current. But without knowing what you have, it's impossible to tell.

Look at the starter. If there are only two contactors, it's 2S2W, so something it wrong if it is reading that way. If there are 3 contactors, it's 2S1W. So then look at the motor nameplate. If it shows two HP ratings, it's CT or VT, so something is wrong with the CT setup. If the nameplate shows one HP rating, it's CHP and it likely will read less amps at the higher speed.

Excellent explanation!! It was a good review on the differences for me.

 

[Jraef]

I dug up some info, pasted below. According to documentation the motor is 2 speed with 2 windings and is constant torque. Both fish and debris are removed by the screens.

View attachment 12827

View attachment 12826

OK, so from that, and seeing the placement of the CT, it does make sense. It's Constant Torque and 2S2W CT is kind of unusual, but it exists. So inside of that motor, the High Speed windings are 4 pole and rated for 10HP, the Low Speed windings are 8 pole, but rated for 5HP so the torque is the same at either speed. Torque = current on an AC but when you run the screen at the higher speed, it likely requires less load on the motor because at the same flow rate through the screen there is less trash building up on it, therefore the screen requires less torque and the current goes down. But normally, I think you would run the screen at higher speed when there is higher flow, so testing it based on the SAME flow like that would have the screen doing less work.

 

[Phil Corso]

BillyBoy... your schematic is that of a reversing starter. Have you presented it to
just illustrate location of the CT, i.e., it's measuring line-current... regardless of which speed-connection is operating?

Food for thought: Obviously, something is wrong! Typically the HP is designed to accommodate two loads: one large; the other small. The former being based on the mass of the screen itself (say, an order-of-magnitude larger than debris alone); the latter related to entrapment of debris!

Is it possible that doubling speed results in a major reduction of debris entrapment? What is the Ampere draw (at both speeds) without debris?

Regards, Phil

 

[GoldDigger]

The simplest way to get a 2:1 ratio between high and low speed, with constant torque and limited slip is that one of the winding sets is wound with two poles per revolution and the other wound with 4 poles. The coils would be totally independent.

If this is the case, the explanation for lower current at higher speed would come down to a lower power requirement for some reason or a very big difference in the power factor at normal load.

 

[kwired]

The simplest way to get a 2:1 ratio between high and low speed, with constant torque and limited slip is that one of the winding sets is wound with two poles per revolution and the other wound with 4 poles. The coils would be totally independent.

If this is the case, the explanation for lower current at higher speed would come down to a lower power requirement for some reason or a very big difference in the power factor at normal load.

You mentioned what I was going to say - what is power factor at each speed? Two fairly independent windings is like two separate motors - they don't have to operate at same power factor.