Slow a Motor Down

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Ronnie Barham

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I have a 1 HP single phase 120 volt motor that runs a Dayton blower that is used as a vent-a-hood, it is on a single pole switch so it is full blast on or it is off. It is too loud for the customer. I want to put a DART Mfr. Model # 55AC10-21 (Granger p/n 5JJ61)adjustable AC voltage supply to slow the motor down and lower the noise. Will this work and will it harm the motor?
 
What is the application? Alot of times those hoods are designed to move a certain amount of air. Slowing the motor down may cause it to not move as much air as necessary.
 
The customer understands that, it's for their home so most of the time they just cook for 2 people. So it's ok to move less air to keep the noise down.
 
Without kniowing the exact type of 1 phase motor it is, there is no way to know if it will work. PSC and Shaded Pole motors can be speed controlled, no other type can. Of those two, only Shaded Pole motor can be controlled with simple voltage variation. PSC motor need to have the voltage and frequency changed together.

Dropping only the voltage will not change the speed, but it lowers the motor torque. If the load doesn't change, the loss of torque allows the load to slow the motor town. But on anything but a Shaded Pole motor, the loss of torque and lower speed results in an increase in slip, so the motor pulls MORE current and does LESS work, which means it overloads.
 
I recently replaced a Dayton hood fan motor for a customer (double squirrel cage design). It originally had a speed control built in, but the motor whined like crazy on any setting. I called Grainger, they put me through to someone from the motor division, and he explained that none of their new (replacement) motors are compatible with any kind of speed control due to efficiency mandates and I would have to replace the speed control with an on-off switch. I don't know if this will affect you or not, but it may be worth a quick call to verify.
 
From my understanding you may be able t reduce the speed f the motor by making is slip by applying a lower voltage.
This 60hz motor may have a synchronous speed of 3600 or 1800RPM with an actual speed of 3450 or 1775 RPM. An induction motor will always try to reach that speed regardless of the voltage that is applied unless the voltage is reduced to such a point to cause the motor to slip below that speed.
In my pinion it is not possible for you to practically reduce the speed of this motor without using a VFD to change the frequency which may not be doable.
Multispeed furnace blower motors have a separate winding configuration for each of the speeds. I would strongly suggest that they bit the bullet and purchase a multi speed motor.
 
EdwardSWard said:
Why wouldn't a triac fan speed controller work?
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This is the way that I understand it:
A triac that would control a bathroom fan would be similar to a light dimmer switch which pulses the voltage. It switched the voltage of the sign wave on and off which provides essentially a lower voltage to the load. It is actually a chopped of sign wave which normally can be adjusted from the 0 volts to the maximum volts of the sign wave.
The frequency doesn't change but the voltage does.
As I indicated in a previous post an induction motor always want to accelerate to its synchronous speed. When voltage is reduced as would be done with a triac, the motor starts to slip and even though the motor speed will be reduced as I recall starting and max torque be reduce about 20% with even a 10% voltage drop, slip 20-30% etc.
On small fractional HP fan motors it is tolerated. But a 1hp motor is not something that just isn't done.
And, having brought up using a triac on a 1hp motor have you identified one that would handle a 1 hp motor?
 
Ronnie Barham said:
It is a Shaded Pole motor, should I use the DART that I mention or one from Variac.
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Either one. The Dart controller will make the motor whine, the Variac will not. Most likely a Variac will cost you more though. A quick check looks as though a properly enclosed variac will cost you around $100-$150, the Dart controller from Grainger looks like it is under $40. If you can live with the noise, you could go with the Dart controller.
 
Ronnie... the caveat I alluded to follows:

For any motor equipped with a speed-controller? type is relatively unimportant? one has to be concerned about its response to a power-interruption. Upon power restoration, the voltage will be limited so that current-draw is limited, i.e., ?typically? less than rated!
Thus, at the lower starting-voltage available torque is insufficient to accelerate the motor. Then it heats up because there is no fan cooling. Of course, thermal overload protection is nil, so damage ensues.
I investigated such an incident in a large HPI plant. It went unnoticed because operators were dealing with recovery efforts! The resultant motor fire caused considerable damage! Fortunately no-one was seriously injured!
I know nothing about the Dart?s response to a power interruption, but it should be thoroughly investigated!
Whenever I recommend a Variac for speed control I recommend a minimum current setting, thereby precluding the scenario described above!
Regards, Phil Corso
 
Slowing motor down is going to decrease airflow, is this acceptable for the application?

Is ducting sufficient size for the application?

Possibly have poor selection of equipment to start with. A need for more air flow comes with either the cost of more noise or you can oversize blower and ducting but run at lower speed as a way of reducing noise.

If any length to the duct you will want the blower away from the user end - it does help with noise - and also size of duct needs increased if there is length and worse yet elbows. One elbow creates restriction similar to 10+ feet of duct.
 
The only proper way to control the speed of an AC motor is a VFD. In this case, is it possible to swap the original motor with a 3PH motor with same frame, HP and speed ratings then install a VFD?

Shaded pole motors can somewhat be controlled with variable voltage source but it is very inefficient and can be unstable if the load varies much.

If the motor is a cap start (most likely is) as you slow it down, you will reach a point when the start winding will switch on and off as the motor speeds up and slows down. This will likely burn the motor out after a short time unless it has a thermal cut out.

My personal opinion is the best solution is to install a 3ph motor and install a VFD.
 
stevephillips said:
The only proper way to control the speed of an AC motor is a VFD. In this case, is it possible to swap the original motor with a 3PH motor with same frame, HP and speed ratings then install a VFD?

Shaded pole motors can somewhat be controlled with variable voltage source but it is very inefficient and can be unstable if the load varies much.

If the motor is a cap start (most likely is) as you slow it down, you will reach a point when the start winding will switch on and off as the motor speeds up and slows down. This will likely burn the motor out after a short time unless it has a thermal cut out.

My personal opinion is the best solution is to install a 3ph motor and install a VFD.
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When it comes to speed control and blowers, PSC motors are speed controlled quite often. Reducing the speed reduces the necessary torque, and works well with blowers. If you tried to slow down a PSC motor for a constant torque application it may not do as well.

As mentioned shaded pole motors are also easily speed controlled, but they become difficult to find much larger than 1/2 HP.
 
He already confirmed that it is a Shaded Pole motor. Voltage control of a shaded pole motor works and although it may not be as efficient as other technologies, the motors are typically so small that the difference is almost imperceptible.
 
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