# Speed versus Current



## garfield (Jul 30, 2009)

Does anyone have information on how far below synchronous speed will a three phase design B 40 hp 1800 rpm induction motor go before reaching full load amps or service factor amps?


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## Cow (Jan 16, 2008)

I've read your post three times and still don't know what you're asking?


Maybe I'll learn something here....


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## joebanana (Dec 21, 2010)

Cow said:


> I've read your post three times and still don't know what you're asking?
> 
> 
> Maybe I'll learn something here....


I think he's looking for the point at which the magic smoke comes out.


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## garfield (Jul 30, 2009)

So for example if the name plate says 1765 RPM what would the amp drop be at 1700 RPM or 1730? Assuming the motor has the proper voltage and load is creating the reduction in speed.


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## gpop (May 14, 2018)

I understand the question as.....

A 1800 rpm motor at 60 hertz has a set amount of slip as listed on the name plate..e.g 1756 rpm. Under full load what would the expected rpm measure.


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## micromind (Aug 11, 2007)

As the speed goes down, the current will go up. Given balanced nameplate voltage and a true sine wave, the speed, current and HP will meet at the nameplate values.


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## garfield (Jul 30, 2009)

I’m working on a loop grain system with 2 40 hp motors and one runs about 26 A with the other one at 32 A. The loop basically runs across the top of the bins. Manufacturer says they’re supposed to either be even or one motor slightly ahead of the other on amps. At some point they have traded out one of the original motors for a different motor and I’m wondering if the rpm is a bit different. I’m trying to figure out if I change one sheave to be .2 of an inch larger how much additional current that’s going to draw with the change in rpm.


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## Cow (Jan 16, 2008)

garfield said:


> I’m wondering if the rpm is a bit different.



Sounds like you might need to put a tachometer on it.


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## just the cowboy (Sep 4, 2013)

*Rethink time*

First please tell us what the problem is. Unless you are overloading the system, the motor RPM should not change that much. 


It don't look like you running at full nameplate current, at 480 you should be around 45-55 amps depending on motor? You can change the pulley to whatever you need to match speed if you are not at full load, unless you are fully loaded slip will not mater . From what you said I would assume that the takeaway conveyor is backing up?


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## garfield (Jul 30, 2009)

The belt is slapping every 12 seconds eventually surges into overload condition and the overloads trip and the manufacturer says that the motors are not pulling the same amount and that is the problem.


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## just the cowboy (Sep 4, 2013)

*Any speed controls*



garfield said:


> The belt is slapping every 12 seconds eventually surges into overload condition and the overloads trip and the manufacturer says that the motors are not pulling the same amount and that is the problem.



Any VFD's involved? If so and they changed a motor without a re-tuning of the drive that is more than likely your issue. If no speed controls, you will have to tach the output shafts unloaded and do the math for RPM to linear feet to get the proper size. 


Just guessing and watching current might work.


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## JRaef (Mar 23, 2009)

Assuming it's a Design B torque/speed curve, the slip speed relates to the speed at *100% rated* *load*. As the motor slip increases, the current and torque will increase to attempt to pull it BACK to that speed. If that doesn't happen and the motor can't recover, the PEAK torque, called the Break Down Torque, is attained at roughly 80% of slip speed speed and in that motor design, will represent between 200-220% of rated torque, so therefor the current will be 200-220% of rated full load current. If you then plot that against a NEMA Class 20 Thermal Overload Curve, you can maintain that for approximately 90 seconds. The overload curve is purposely designed to be BELOW the Thermal Damage Curve of a standard motor.










If it is an IEC motor or a submersible pump, they are all Class 10 so you could only maintain that for 30 seconds.


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## frenchelectrican (Mar 15, 2007)

garfield said:


> I’m working on a loop grain system with 2 40 hp motors and one runs about 26 A with the other one at 32 A. The loop basically runs across the top of the bins. Manufacturer says they’re supposed to either be even or one motor slightly ahead of the other on amps. At some point they have traded out one of the original motors for a different motor and I’m wondering if the rpm is a bit different. I’m trying to figure out if I change one sheave to be .2 of an inch larger how much additional current that’s going to draw with the change in rpm.


I am try to get this in my head a little .,

Is both motors is on the same belt or convoyer ? 

If you have speed control like VSD and what it set on for ?


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## gpop (May 14, 2018)

garfield said:


> The belt is slapping every 12 seconds eventually surges into overload condition and the overloads trip and the manufacturer says that the motors are not pulling the same amount and that is the problem.



Would love to see a video of this. Not all problems are electrical.

If the belt is slapping you have to ask what the manufactures spec is on the pre-stretch of the belt.


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## garfield (Jul 30, 2009)

Versaloop from gsi Is the name of the system if you picture like five grain bins in a row there’s a single continuous chain with poly paddles that drag the grain through an 8 inch pipe and they go all the way from the truck unload and they go all the way from the truck unload up a leg then across the top of all five bins each of which has a gate that you can open to drop the grain into and then it goes down a leg at the far side and underneath all of the bins to form a loop. I’m guessing it’s a 700 ft. loop. So at the top of each of the towers at each end is a 40 hp motor. There are soft starts but I think that was just to satisfy the power company. They tell me that the chain is properly tensioned. I am going there Wednesday with a sheave that is 2/10 of an inch smaller and I’ll see if that improves their situation by loading the other motor a bit more. I started out wanting to calculate what would happen when I changed the pully but I guess I’ve given up on that and I’m just gonna put it on and see what happens.


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## garfield (Jul 30, 2009)

Only pic I have


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## gpop (May 14, 2018)

garfield said:


> Versaloop from gsi Is the name of the system if you picture like five grain bins in a row there’s a single continuous chain with poly paddles that drag the grain through an 8 inch pipe and they go all the way from the truck unload and they go all the way from the truck unload up a leg then across the top of all five bins each of which has a gate that you can open to drop the grain into and then it goes down a leg at the far side and underneath all of the bins to form a loop. I’m guessing it’s a 700 ft. loop. So at the top of each of the towers at each end is a 40 hp motor. There are soft starts but I think that was just to satisfy the power company. They tell me that the chain is properly tensioned. I am going there Wednesday with a sheave that is 2/10 of an inch smaller and I’ll see if that improves their situation by loading the other motor a bit more. I started out wanting to calculate what would happen when I changed the pully but I guess I’ve given up on that and I’m just gonna put it on and see what happens.


which motor is having the overload problem the one on the lift side or the one on the return side?


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## gpop (May 14, 2018)

Its possible that the motors are different speeds and the chain hates to be pushed. Even a slightly loose chain will bunch up and bind then pop free. There's also the possibility that the sprockets are worn and have hook teeth causing it to pop. 

This is one of those problems where equipment history real does help. 
Did it ever work, When did it start to play up, What work was done around the time it started to play up, Does it act up worse when loading the first bin or the last bin.


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## paulengr (Oct 8, 2017)

The name plate speed and FLA is the speed it will draw at FLA if the load torque is equal to the horsepower rating (HP = Torque x RPM / 5252 with Torque in foot-pounds). For Design A, B, and E motors it is usually about 3% slip so with an 1800 RPM motor name plate RPM will usually be about 1746 RPM. The "curve" is also quite linear with reasonable amounts of changes in load so at half load you can expect the motor to run at about 1.5% slip or 1773 RPM for instance, and the same is true if you overload it a bit until you reach close to pull out torque which is usually about 250-300% of rated at which point it just stalls. Close to ZERO load the motor load matters so you can never quite reach synchronous speed. But current will not be close to zero as you expect. It is often more like 25% for a 4 pole motor or close to 50% for 6 or more poles. The explanation is quite simple. As the number of poles increases, the size of the magnetic field that provides flux is increasing. However this magnetic field is just that...it's not actually doing any useful work. So it is all reactive power and kvars skyrockets, so naturally amps has to shoot way up there. When the motor is doing work turning a load though that's almost pure work so it's almost all kilowatts, although the current draw you measure (kva) is the vector sum of the two so kva is not the direct A+B result that you expect. It goes up a lot slower than that. But out there where you are close to name plate load torque, it is pretty close to linear.


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## micromind (Aug 11, 2007)

Try rolling the phases on one of the motors.

This is where you advance each phase one step. For example, if A = T1, B = T2 and C = T3, then reconnect A to T2, B to T3 and C to T1. Then go A = T3, B = T1 and C = T2. 

Of course, the starter is the easiest spot to do this, if it's Brown - Orange - Yellow, reconnect for Yellow - Brown - Orange. Then try Orange - Yellow - Brown and see which one results in the best current balance. 

The shaft rotation will stay the same but the currents may very well become more balanced.


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## garfield (Jul 30, 2009)

*update*

We found a motor with a very loud bearing. I don't think that is the problem but it gives me a reason to put a matching motor on. It's been to muddy to get a crane in and replace the motor. I have sheaves that are going to change the motor rpm about 10 rpm i will install if the motor doesn't fix it. I don't have proof of this but i believe that when they replaced the one end gear drive and sprocket the new sprocket is not worn so it's a slightly different diameter.


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