Motor pulley diameter/ amperage

[starsandstripes]

I just installed an exhaust blower motor; 208 volt single phase one hp. Nameplate says 230 volts usable @ 208 volts, 6.5 amps @ 230 volts, SF 1.15. Motor is running @7.4 amps and I have the belts adjusted on the loose side. I know if I install a larger diameter pulley on the motor, it will slow the blower down; what will that do to my running amperage?

 

[gar]

130501-2352 EDT

starsandstripes:

Rather than using memory think about what happens when you change the motor pulley diameter. So what is the result when you use an analytic technique to the question?

If you reduce the the fan blade speed, then you will require less power to rotate the blade. If the motor is an approximately fixed speed device, then reduced load power means less input current to the motor.

.

 

[eHunter]

I just installed an exhaust blower motor; 208 volt single phase one hp. Nameplate says 230 volts usable @ 208 volts, 6.5 amps @ 230 volts, SF 1.15. Motor is running @7.4 amps and I have the belts adjusted on the loose side. I know if I install a larger diameter pulley on the motor, it will slow the blower down; what will that do to my running amperage? ]

130501-2352 EDT

starsandstripes:

Rather than using memory think about what happens when you change the motor pulley diameter. So what is the result when you use an analytic technique to the question?

If you reduce the the fan blade speed, then you will require less power to rotate the blade. If the motor is an approximately fixed speed device, then reduced load power means less input current to the motor.

.

Agreed.
However as a side note, a larger motor drive pulley will increase the driven blower RPM and increase the motor load.

 

[GoldDigger]

I know if I install a larger diameter pulley on the motor, it will slow the blower down;

If you reduce the the fan blade speed, then you will require less power to rotate the blade. If the motor is an approximately fixed speed device, then reduced load power means less input current to the motor.

Didn't anybody notice that a larger pulley at the motor will actually speed the blower up? Causing more load on the motor....

 

[templdl]

I just installed an exhaust blower motor; 208 volt single phase one hp. Nameplate says 230 volts usable @ 208 volts, 6.5 amps @ 230 volts, SF 1.15. Motor is running @7.4 amps and I have the belts adjusted on the loose side. I know if I install a larger diameter pulley on the motor, it will slow the blower down; what will that do to my running amperage?

You bet it will as the load on the motor will change. If you change a sheave it is imperative to check the amperage of the motor.

 

[iwire]

I just installed an exhaust blower motor; 208 volt single phase one hp. Nameplate says 230 volts usable @ 208 volts, 6.5 amps @ 230 volts, SF 1.15. Motor is running @7.4 amps and I have the belts adjusted on the loose side. I know if I install a larger diameter pulley on the motor, it will slow the blower down; what will that do to my running amperage?

Most times the sheave on the motor is adjustable diameter so you can reduce its size slowing the blower and reducing the load on the motor.

Running the belt loose is not a good plan.

 

[gar]

130502-0901 EDT

GoldDigger:

The first sentence in my post was:

Rather than using memory think about what happens when you change the motor pulley diameter. So what is the result when you use an analytic technique to the question?

The implication of my question was that he had expressed the wrong answer.

The purpose in not giving a direct answer was to try to get starsandstripes to think about the mechanics of pulleys or other similar reducers and figure out on his own what the answer is.

Clearly, if his answer was wrong, then there was only one other possibility. But still it makes sense for him to try to reason thru to the answer.

There are too many on the forum that just want an answer, and do not want to think thru in a reasoned fashion the why of an answer where this is possible.

How many can derive the constants 1.414 or 1.732 or something close to the values. What is the difference between and how to determine the RMS and average values of a waveform such as a sine, half-wave rectified, full-wave rectified, square wave, or pulsed wave?

Sometimes one does not have a reference, or memory is incorrect, and a solution is required. Prior experience in attacking a problem by yourself is useful.

.

 

[Gregg Harris]

I just installed an exhaust blower motor; 208 volt single phase one hp. Nameplate says 230 volts usable @ 208 volts, 6.5 amps @ 230 volts, SF 1.15. Motor is running @7.4 amps and I have the belts adjusted on the loose side. I know if I install a larger diameter pulley on the motor, it will slow the blower down; what will that do to my running amperage?

What are you trying to accomplish? If it is to match the nameplate amperage, change the voltage to 230 and your nameplate amperage will match

 

[iwire]

If it is to match the nameplate amperage, change the voltage to 230 and your nameplate amperage will match

Maybe or maybe not.

 

[iwire]

130502-0901 EDT

GoldDigger:

The first sentence in my post was:

The implication of my question was that he had expressed the wrong answer.

The purpose in not giving a direct answer was to try to get starsandstripes to think about the mechanics of pulleys or other similar reducers and figure out on his own what the answer is.

Clearly, if his answer was wrong, then there was only one other possibility. But still it makes sense for him to try to reason thru to the answer.

There are too many on the forum that just want an answer, and do not want to think thru in a reasoned fashion the why of an answer where this is possible.

How many can derive the constants 1.414 or 1.732 or something close to the values. What is the difference between and how to determine the RMS and average values of a waveform such as a sine, half-wave rectified, full-wave rectified, square wave, or pulsed wave?

Sometimes one does not have a reference, or memory is incorrect, and a solution is required. Prior experience in attacking a problem by yourself is useful.

.

That is certainly one view.

Another view is this is a very simple task that many of us deal with often, pretty much anytime a new fan is installed. Most belt driven fans are shipped with an adjustable sheave to simplify air flow adjustments or matching the mechanical load to the motors ability. IMO many here are trying to overly complicate a simple issue.

 

[Jraef]

Air flow = load in a blower. Load = amps on the motor.
Reduce the blower speed and you reduce the flow, thus reduce the load on the motor, thus reduce the amps it will draw.

I think the point gar was trying to make was to get the OP to look more closely at his premise and see where the flaw may be, hiding in plain sight as it was. But now that the cat is out of the bag, the premise of INCREASING the pulley size ON THE MOTOR was the flaw, because that would make the fan spin FASTER, which of course would overload the motor even more. Either decrease the sheave diameter on the motor, or increase it on the fan.

But the other flaw is that now you are taking on responsibility for the air handling design by changing the air flow and that may have serious air balancing consequences that could get you and other people in trouble. Not really your job. Either boost the voltage (the factor by which you are off is roughly equal to the increase due to giving it lower voltage), or tell the HVAC guy that he either needs to put in a bigger motor that will not overload at 208V, or change the air flow design. Your job is to connect the motor and make sure the circuit is correct.

 

[iwire]

But the other flaw is that now you are taking on responsibility for the air handling design by changing the air flow and that may have serious air balancing consequences that could get you and other people in trouble. Not really your job. Either boost the voltage (the factor by which you are off is roughly equal to the increase due to giving it lower voltage), or tell the HVAC guy that he either needs to put in a bigger motor that will not overload at 208V, or change the air flow design. Your job is to connect the motor and make sure the circuit is correct.

There is a whole lot of assuming going on here, assuming there is a true 'design CFM' assuming there will be air balancing, assuming there is a HVAC guy involved etc.

Of course I do my fair share of assuming as well based on what I do.

 

[Jraef]

There is a whole lot of assuming going on here, assuming there is a true 'design CFM' assuming there will be air balancing, assuming there is a HVAC guy involved etc.

Of course I do my fair share of assuming as well based on what I do.

True, but assuming the other way, that HE has to take it all on, is potentially more problematic.

I once did a project of retrofitting HVAC blowers with VFDs on a large office building. The speed ratios between the supply fans and return fans were off by about 2% (we discovered later). During testing we created a vacuum in the building that sucked all the doors closed and nobody could get out! We were on the roof and had no idea, but the building occupants panicked and called the fire dept., who broke the glass doors to let people out. That was expensive for me, I had to cover the cost of the replacement doors. I never tackled an HVAC job after that except as a sub for an HVAC contractor. There is just too much about other trades that I don't know enough about.

 

[Smart $]

Getting back to the OP....

Wouldn't the motor amperes increase when powered 208V, to maintain rated power? The rated current is 6.5A @ 230V.

6.5A ? 230V ? 208V = 7.2A

Also, a typically rated 230V motor is actually for connection to a 240V supply. No such mismatch when referencing 208V operation (i.e. not say 200V). So...

6.5A ? 240V ? 208V = 7.5A

Ultimately I don't see any problem...???? (i.e. other than the loose belts )

 

[growler]

Didn't anybody notice that a larger pulley at the motor will actually speed the blower up? Causing more load on the motor....

Sure I noticed but I thought I would wait and see how far this would go before someone mentioned it.

 

[GoldDigger]

130502-0901 EDT

GoldDigger:

The implication of my question was that he had expressed the wrong answer.

The purpose in not giving a direct answer was to try to get starsandstripes to think about the mechanics of pulleys or other similar reducers and figure out on his own what the answer is.

I admire that intention. My problem was that you then went on to describe the effects of slowing down the blower, which seemed to be a tacit confirmation of the OP's incorrect starting point. If you had stopped with the first part of your answer, I would support it completely.