Motor r.p.m. and voltage change

[jcp]

I had to buy a new motor for my SB9A so I got a 115/220v 1725 rpm 1/2 hp for it, wired for 220v. The rpm seems faster than the original motor so I checked the new motor rpm with a strobe and it's running at 1790 rpm. Would this be because the voltage at the shop is 240v rather than 220v or is this considered within normal variation. Thanks for any enlightenment.

 

[Dabbler]

OK here goes. It is not that simple. Some motors will run a little faster, some will actually run slower when you change voltages. The motor speed (in a simple world) will be governed by the frequency and the load capacity and actual load of the motor.

For the same motor - if it is a 4 pole motor, depending on the way they wound it, you can get more torque at the same RPM at 240V than at 120V (nominal) . This is probably why you are seeing an increase of RPM. Your available torque is higher, so the minimal load is less of a burden on the motor.

 

[jcp]

OK here goes. It is not that simple. Some motors will run a little faster, some will actually run slower when you change voltages. The motor speed (in a simple world) will be governed by the frequency and the load capacity and actual load of the motor.

For the same motor - if it is a 4 pole motor, depending on the way they wound it, you can get more torque at the same RPM at 240V than at 120V (nominal) . This is probably why you are seeing an increase of RPM. Your available torque is higher, so the minimal load is less of a burden on the motor.

Thank you sir! I should have have paid more attention to my father...he was an electrician...mostly motors and their controls.

 

[mksj]

Most AC motors are considered asynchronous, the rotational speed is a function of the number of poles and the line frequency (60 Hz) in the US. Depending on the motor design they have different degrees of slip from the "synchronous speed" which in your case would be 1800 RPM (4 pole motor), a 2 pole would be 3600 RPM, a 6 pole would be 1200 RPM. An asynchronous motor does not have a means to control its synchronous sRPM, so the actual RPM will be lower also will decrease with increasing load. There is some asynchronous speed variations between difference motors that account for different RPMs due to slip. This is what you are seeing. The voltage difference does not account for the speed difference (but it could reduce the power which could cause more slip), but there is a V/Hz ratio so when you go below the motors base speed the ratio stays the same and the voltage drops (more or less) as the Hz decreases. So with a 3 phase motor on a VFD, the motor speed will decrease to half at 30 Hz. VFD's can run in a straight fixed V/Hz ratio or a sensorless vector mode which uses the motor as a feedback device and can typically maintain RPM to 0.1%. Motor controllers have different ways to compensate for load and maintain RPM.

The relative difference between the stator rotational field and the speed of the rotor is called the slip. If the slip of the motor is zero or the rotor has the same rotational speed as the stator rotating field, the motor is called AC Synchronous motor. if an AC motor has a slip or there is a difference between the stator field speed and the rotor, the motor is called asynchronous motor. Synchronous motor typically have some form of motor control (encoder or closed loop feedback) to compensate and adjust the motor to maintain synchronous RPM.

Main Difference Between Synchronous and Asynchronous Motor

Difference Between Synchronous and Asynchronous Motor. Difference Between Synchronous and Induction Motor. Synchronous vs Asynchronous Motor

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