Investigating Why a Century AC Motor's Label Shows 1625 RPM

[souky101]

TL;DR

AC motor RPM

Hi great folk

I have a Century AC motor which is ¼ HP, 115 V, 60 HZ

This is a Permanent Split Capacitor (PSC) motor

It has 4 poles, so the running speed according to the motor equation should be:

.

Speed = 120x60 / p …………….. Where P = number of poles

Therefore RPM = 120 X 60 / 4 = 1800

.

My question now is why the motor label shows 1625 RPM and no 1800

.

Thanks for your investigation in advance

 

[Mark44]

Summary: AC motor RPM

Hi great folk

I have a Century AC motor which is ¼ HP, 115 V, 60 HZ

This is a Permanent Split Capacitor (PSC) motor

It has 4 poles, so the running speed according to the motor equation should be:

.

Speed = 120x60 / p …………….. Where P = number of poles

Therefore RPM = 120 X 60 / 4 = 1800

.

My question now is why the motor label shows 1625 RPM and no 1800

.

Thanks for your investigation in advance

You used 120 V, whereas the motor's running speed was calculated from 115 V.

 

[jrmichler]

Your motor is an AC induction motor. The synchronous speed is the 1800 RPM that you correctly calculated. Induction motors run slower than synchronous speed. The difference between the synchronous speed and the actual speed induces currents in the rotor. Those currents create a rotor magnetic field that works with the stator magnetic field to make the rotor turn. The magnitude of the rotor currents, and thus the strength of the rotor magnetic field, is proportional to the speed difference between the rotor and stator, and also to the rotor resistance.

The motor nameplate RPM is the RPM at which the motor develops full rated power. That same motor, with nothing attached to the shaft, will turn about 1798 RPM. Your motor has a high resistance rotor, so is designed for applications where it needs to run at less than rated speed. Such applications include high inertia loads and certain adjustable speed applications such as furnace blowers. It is a low efficiency motor. Most 1800 RPM induction motor have a full load speed near 1725 to 1750 RPM, and have higher efficiency.

 

[Baluncore]

My question now is why the motor label shows 1625 RPM and no 1800.

jrmichler is correct.
Your motor is a low-power induction motor. It is designed to have 10% slip at full load, which suggests it is inefficient, light-weight, and low-cost.
A more efficient small motor would have only about 5% slip at full load. Bigger industrial motors will slip between 1% and 2%.

 

[Averagesupernova]

You used 120 V, whereas the motor's running speed was calculated from 115 V.

Voltage has nothing to do with it, within reason. The OP's math came up with a synchronous speed that is correct but only by accident. But the math should be: (frequency * 60) / number of pole pairs.
-
Of course as others have pointed out, slip is required for an induction motor to work so the actual speed will be less.

 

[anorlunda]

Voltage has nothing to do with it, within reason.

Correct, but to state it better: changing voltage changes the torque-slip curve. However, nameplate values are not variable; rated voltage; rated frequency; max power and speed at max power.

 

[Averagesupernova]

However, nameplate values are not variable...

I've never seen a three phase motor that lists a range of voltages other than 208-240 and of course the high volt connection of 460. But, those voltages are moved around all the time with variable frequency drives.

 

[Babadag]

The 1800 rpm it is what is called synchronous speed and that is-approximate- the no-load speed-but if it is loaded 1/4 hp a slip will occur then s=(1800-1625)/1800=9.7% [it is very large in my opinion but still possible]
For instance a manufacturer presents in his catalogue for General purpose aluminum motors 460 V 60 Hz and 0.2 hp 1660 rated rpm. (1800-1660)/1800=7.78%