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

• souky101
In summary: Yes, the motor is a low-power induction motor and it is designed to have 10% slip at full load, which suggests it is inefficient, light-weight, and low-cost. However, a more efficient small motor would have only about 5% slip at full load. Bigger industrial motors will slip between 1% and 2%.
souky101
TL;DR 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

.

souky101 said:
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

.

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

dlgoff
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.

souky101 said:
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%.

Mark44 said:
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.

Baluncore
Averagesupernova said:
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.

anorlunda said:
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.

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%

#### Attachments

• INDUCTION MOTOR SLIP.jpg
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## 1. What is a century AC motor?

A century AC motor is a type of alternating current (AC) electric motor that is commonly used in industrial and commercial applications. It is a single-phase motor that operates at a constant speed and has a power rating of 1 horsepower (HP) or higher.

## 2. Why does the label on a century AC motor show 1625 RPM?

The label on a century AC motor shows 1625 RPM because this is the nominal speed of the motor. This means that under normal operating conditions, the motor will rotate at a speed of 1625 revolutions per minute (RPM).

## 3. How is the speed of a century AC motor determined?

The speed of a century AC motor is determined by the frequency of the AC power supply and the number of poles in the motor. The speed can be calculated using the formula: RPM = (120 x frequency) / number of poles.

## 4. Can the speed of a century AC motor be changed?

Yes, the speed of a century AC motor can be changed by adjusting the frequency of the AC power supply or by using a variable frequency drive (VFD). However, the speed should not be changed beyond the motor's rated speed, as this can cause damage to the motor.

## 5. Why is it important to investigate the reason behind a century AC motor's label showing 1625 RPM?

It is important to investigate the reason behind a century AC motor's label showing 1625 RPM because it can affect the motor's performance and efficiency. If the motor is operating at a different speed than indicated on the label, it may not be suitable for the intended application and could cause damage or safety hazards. Additionally, understanding the reason for the labeled speed can provide valuable information for troubleshooting and maintenance purposes.

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